--- /dev/null
+============================
+A block layer cache (bcache)
+============================
+
+Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be
+nice if you could use them as cache... Hence bcache.
+
+Wiki and git repositories are at:
+
+ - http://bcache.evilpiepirate.org
+ - http://evilpiepirate.org/git/linux-bcache.git
+ - http://evilpiepirate.org/git/bcache-tools.git
+
+It's designed around the performance characteristics of SSDs - it only allocates
+in erase block sized buckets, and it uses a hybrid btree/log to track cached
+extents (which can be anywhere from a single sector to the bucket size). It's
+designed to avoid random writes at all costs; it fills up an erase block
+sequentially, then issues a discard before reusing it.
+
+Both writethrough and writeback caching are supported. Writeback defaults to
+off, but can be switched on and off arbitrarily at runtime. Bcache goes to
+great lengths to protect your data - it reliably handles unclean shutdown. (It
+doesn't even have a notion of a clean shutdown; bcache simply doesn't return
+writes as completed until they're on stable storage).
+
+Writeback caching can use most of the cache for buffering writes - writing
+dirty data to the backing device is always done sequentially, scanning from the
+start to the end of the index.
+
+Since random IO is what SSDs excel at, there generally won't be much benefit
+to caching large sequential IO. Bcache detects sequential IO and skips it;
+it also keeps a rolling average of the IO sizes per task, and as long as the
+average is above the cutoff it will skip all IO from that task - instead of
+caching the first 512k after every seek. Backups and large file copies should
+thus entirely bypass the cache.
+
+In the event of a data IO error on the flash it will try to recover by reading
+from disk or invalidating cache entries. For unrecoverable errors (meta data
+or dirty data), caching is automatically disabled; if dirty data was present
+in the cache it first disables writeback caching and waits for all dirty data
+to be flushed.
+
+Getting started:
+You'll need make-bcache from the bcache-tools repository. Both the cache device
+and backing device must be formatted before use::
+
+ make-bcache -B /dev/sdb
+ make-bcache -C /dev/sdc
+
+make-bcache has the ability to format multiple devices at the same time - if
+you format your backing devices and cache device at the same time, you won't
+have to manually attach::
+
+ make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
+
+bcache-tools now ships udev rules, and bcache devices are known to the kernel
+immediately. Without udev, you can manually register devices like this::
+
+ echo /dev/sdb > /sys/fs/bcache/register
+ echo /dev/sdc > /sys/fs/bcache/register
+
+Registering the backing device makes the bcache device show up in /dev; you can
+now format it and use it as normal. But the first time using a new bcache
+device, it'll be running in passthrough mode until you attach it to a cache.
+If you are thinking about using bcache later, it is recommended to setup all your
+slow devices as bcache backing devices without a cache, and you can choose to add
+a caching device later.
+See 'ATTACHING' section below.
+
+The devices show up as::
+
+ /dev/bcache<N>
+
+As well as (with udev)::
+
+ /dev/bcache/by-uuid/<uuid>
+ /dev/bcache/by-label/<label>
+
+To get started::
+
+ mkfs.ext4 /dev/bcache0
+ mount /dev/bcache0 /mnt
+
+You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
+You can also control them through /sys/fs//bcache/<cset-uuid>/ .
+
+Cache devices are managed as sets; multiple caches per set isn't supported yet
+but will allow for mirroring of metadata and dirty data in the future. Your new
+cache set shows up as /sys/fs/bcache/<UUID>
+
+Attaching
+---------
+
+After your cache device and backing device are registered, the backing device
+must be attached to your cache set to enable caching. Attaching a backing
+device to a cache set is done thusly, with the UUID of the cache set in
+/sys/fs/bcache::
+
+ echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
+
+This only has to be done once. The next time you reboot, just reregister all
+your bcache devices. If a backing device has data in a cache somewhere, the
+/dev/bcache<N> device won't be created until the cache shows up - particularly
+important if you have writeback caching turned on.
+
+If you're booting up and your cache device is gone and never coming back, you
+can force run the backing device::
+
+ echo 1 > /sys/block/sdb/bcache/running
+
+(You need to use /sys/block/sdb (or whatever your backing device is called), not
+/sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a
+partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache)
+
+The backing device will still use that cache set if it shows up in the future,
+but all the cached data will be invalidated. If there was dirty data in the
+cache, don't expect the filesystem to be recoverable - you will have massive
+filesystem corruption, though ext4's fsck does work miracles.
+
+Error Handling
+--------------
+
+Bcache tries to transparently handle IO errors to/from the cache device without
+affecting normal operation; if it sees too many errors (the threshold is
+configurable, and defaults to 0) it shuts down the cache device and switches all
+the backing devices to passthrough mode.
+
+ - For reads from the cache, if they error we just retry the read from the
+ backing device.
+
+ - For writethrough writes, if the write to the cache errors we just switch to
+ invalidating the data at that lba in the cache (i.e. the same thing we do for
+ a write that bypasses the cache)
+
+ - For writeback writes, we currently pass that error back up to the
+ filesystem/userspace. This could be improved - we could retry it as a write
+ that skips the cache so we don't have to error the write.
+
+ - When we detach, we first try to flush any dirty data (if we were running in
+ writeback mode). It currently doesn't do anything intelligent if it fails to
+ read some of the dirty data, though.
+
+
+Howto/cookbook
+--------------
+
+A) Starting a bcache with a missing caching device
+
+If registering the backing device doesn't help, it's already there, you just need
+to force it to run without the cache::
+
+ host:~# echo /dev/sdb1 > /sys/fs/bcache/register
+ [ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
+
+Next, you try to register your caching device if it's present. However
+if it's absent, or registration fails for some reason, you can still
+start your bcache without its cache, like so::
+
+ host:/sys/block/sdb/sdb1/bcache# echo 1 > running
+
+Note that this may cause data loss if you were running in writeback mode.
+
+
+B) Bcache does not find its cache::
+
+ host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
+ [ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
+ [ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
+ [ 1933.478179] : cache set not found
+
+In this case, the caching device was simply not registered at boot
+or disappeared and came back, and needs to be (re-)registered::
+
+ host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
+
+
+C) Corrupt bcache crashes the kernel at device registration time:
+
+This should never happen. If it does happen, then you have found a bug!
+Please report it to the bcache development list: linux-bcache@vger.kernel.org
+
+Be sure to provide as much information that you can including kernel dmesg
+output if available so that we may assist.
+
+
+D) Recovering data without bcache:
+
+If bcache is not available in the kernel, a filesystem on the backing
+device is still available at an 8KiB offset. So either via a loopdev
+of the backing device created with --offset 8K, or any value defined by
+--data-offset when you originally formatted bcache with `make-bcache`.
+
+For example::
+
+ losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
+
+This should present your unmodified backing device data in /dev/loop0
+
+If your cache is in writethrough mode, then you can safely discard the
+cache device without loosing data.
+
+
+E) Wiping a cache device
+
+::
+
+ host:~# wipefs -a /dev/sdh2
+ 16 bytes were erased at offset 0x1018 (bcache)
+ they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+
+After you boot back with bcache enabled, you recreate the cache and attach it::
+
+ host:~# make-bcache -C /dev/sdh2
+ UUID: 7be7e175-8f4c-4f99-94b2-9c904d227045
+ Set UUID: 5bc072a8-ab17-446d-9744-e247949913c1
+ version: 0
+ nbuckets: 106874
+ block_size: 1
+ bucket_size: 1024
+ nr_in_set: 1
+ nr_this_dev: 0
+ first_bucket: 1
+ [ 650.511912] bcache: run_cache_set() invalidating existing data
+ [ 650.549228] bcache: register_cache() registered cache device sdh2
+
+start backing device with missing cache::
+
+ host:/sys/block/md5/bcache# echo 1 > running
+
+attach new cache::
+
+ host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
+ [ 865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
+
+
+F) Remove or replace a caching device::
+
+ host:/sys/block/sda/sda7/bcache# echo 1 > detach
+ [ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
+
+ host:~# wipefs -a /dev/nvme0n1p4
+ wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
+ Ooops, it's disabled, but not unregistered, so it's still protected
+
+We need to go and unregister it::
+
+ host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
+ lrwxrwxrwx 1 root root 0 Feb 25 18:33 cache0 -> ../../../devices/pci0000:00/0000:00:1d.0/0000:70:00.0/nvme/nvme0/nvme0n1/nvme0n1p4/bcache/
+ host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
+ kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
+
+Now we can wipe it::
+
+ host:~# wipefs -a /dev/nvme0n1p4
+ /dev/nvme0n1p4: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+
+
+G) dm-crypt and bcache
+
+First setup bcache unencrypted and then install dmcrypt on top of
+/dev/bcache<N> This will work faster than if you dmcrypt both the backing
+and caching devices and then install bcache on top. [benchmarks?]
+
+
+H) Stop/free a registered bcache to wipe and/or recreate it
+
+Suppose that you need to free up all bcache references so that you can
+fdisk run and re-register a changed partition table, which won't work
+if there are any active backing or caching devices left on it:
+
+1) Is it present in /dev/bcache* ? (there are times where it won't be)
+
+ If so, it's easy::
+
+ host:/sys/block/bcache0/bcache# echo 1 > stop
+
+2) But if your backing device is gone, this won't work::
+
+ host:/sys/block/bcache0# cd bcache
+ bash: cd: bcache: No such file or directory
+
+ In this case, you may have to unregister the dmcrypt block device that
+ references this bcache to free it up::
+
+ host:~# dmsetup remove oldds1
+ bcache: bcache_device_free() bcache0 stopped
+ bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
+
+ This causes the backing bcache to be removed from /sys/fs/bcache and
+ then it can be reused. This would be true of any block device stacking
+ where bcache is a lower device.
+
+3) In other cases, you can also look in /sys/fs/bcache/::
+
+ host:/sys/fs/bcache# ls -l */{cache?,bdev?}
+ lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/
+ lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/
+ lrwxrwxrwx 1 root root 0 Mar 5 09:39 5bc072a8-ab17-446d-9744-e247949913c1/cache0 -> ../../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/ata10/host9/target9:0:0/9:0:0:0/block/sdl/sdl2/bcache/
+
+ The device names will show which UUID is relevant, cd in that directory
+ and stop the cache::
+
+ host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
+
+ This will free up bcache references and let you reuse the partition for
+ other purposes.
+
+
+
+Troubleshooting performance
+---------------------------
+
+Bcache has a bunch of config options and tunables. The defaults are intended to
+be reasonable for typical desktop and server workloads, but they're not what you
+want for getting the best possible numbers when benchmarking.
+
+ - Backing device alignment
+
+ The default metadata size in bcache is 8k. If your backing device is
+ RAID based, then be sure to align this by a multiple of your stride
+ width using `make-bcache --data-offset`. If you intend to expand your
+ disk array in the future, then multiply a series of primes by your
+ raid stripe size to get the disk multiples that you would like.
+
+ For example: If you have a 64k stripe size, then the following offset
+ would provide alignment for many common RAID5 data spindle counts::
+
+ 64k * 2*2*2*3*3*5*7 bytes = 161280k
+
+ That space is wasted, but for only 157.5MB you can grow your RAID 5
+ volume to the following data-spindle counts without re-aligning::
+
+ 3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
+
+ - Bad write performance
+
+ If write performance is not what you expected, you probably wanted to be
+ running in writeback mode, which isn't the default (not due to a lack of
+ maturity, but simply because in writeback mode you'll lose data if something
+ happens to your SSD)::
+
+ # echo writeback > /sys/block/bcache0/bcache/cache_mode
+
+ - Bad performance, or traffic not going to the SSD that you'd expect
+
+ By default, bcache doesn't cache everything. It tries to skip sequential IO -
+ because you really want to be caching the random IO, and if you copy a 10
+ gigabyte file you probably don't want that pushing 10 gigabytes of randomly
+ accessed data out of your cache.
+
+ But if you want to benchmark reads from cache, and you start out with fio
+ writing an 8 gigabyte test file - so you want to disable that::
+
+ # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
+
+ To set it back to the default (4 mb), do::
+
+ # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
+
+ - Traffic's still going to the spindle/still getting cache misses
+
+ In the real world, SSDs don't always keep up with disks - particularly with
+ slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
+ you want to avoid being bottlenecked by the SSD and having it slow everything
+ down.
+
+ To avoid that bcache tracks latency to the cache device, and gradually
+ throttles traffic if the latency exceeds a threshold (it does this by
+ cranking down the sequential bypass).
+
+ You can disable this if you need to by setting the thresholds to 0::
+
+ # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
+ # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
+
+ The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
+
+ - Still getting cache misses, of the same data
+
+ One last issue that sometimes trips people up is actually an old bug, due to
+ the way cache coherency is handled for cache misses. If a btree node is full,
+ a cache miss won't be able to insert a key for the new data and the data
+ won't be written to the cache.
+
+ In practice this isn't an issue because as soon as a write comes along it'll
+ cause the btree node to be split, and you need almost no write traffic for
+ this to not show up enough to be noticeable (especially since bcache's btree
+ nodes are huge and index large regions of the device). But when you're
+ benchmarking, if you're trying to warm the cache by reading a bunch of data
+ and there's no other traffic - that can be a problem.
+
+ Solution: warm the cache by doing writes, or use the testing branch (there's
+ a fix for the issue there).
+
+
+Sysfs - backing device
+----------------------
+
+Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
+(if attached) /sys/fs/bcache/<cset-uuid>/bdev*
+
+attach
+ Echo the UUID of a cache set to this file to enable caching.
+
+cache_mode
+ Can be one of either writethrough, writeback, writearound or none.
+
+clear_stats
+ Writing to this file resets the running total stats (not the day/hour/5 minute
+ decaying versions).
+
+detach
+ Write to this file to detach from a cache set. If there is dirty data in the
+ cache, it will be flushed first.
+
+dirty_data
+ Amount of dirty data for this backing device in the cache. Continuously
+ updated unlike the cache set's version, but may be slightly off.
+
+label
+ Name of underlying device.
+
+readahead
+ Size of readahead that should be performed. Defaults to 0. If set to e.g.
+ 1M, it will round cache miss reads up to that size, but without overlapping
+ existing cache entries.
+
+running
+ 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether
+ it's in passthrough mode or caching).
+
+sequential_cutoff
+ A sequential IO will bypass the cache once it passes this threshold; the
+ most recent 128 IOs are tracked so sequential IO can be detected even when
+ it isn't all done at once.
+
+sequential_merge
+ If non zero, bcache keeps a list of the last 128 requests submitted to compare
+ against all new requests to determine which new requests are sequential
+ continuations of previous requests for the purpose of determining sequential
+ cutoff. This is necessary if the sequential cutoff value is greater than the
+ maximum acceptable sequential size for any single request.
+
+state
+ The backing device can be in one of four different states:
+
+ no cache: Has never been attached to a cache set.
+
+ clean: Part of a cache set, and there is no cached dirty data.
+
+ dirty: Part of a cache set, and there is cached dirty data.
+
+ inconsistent: The backing device was forcibly run by the user when there was
+ dirty data cached but the cache set was unavailable; whatever data was on the
+ backing device has likely been corrupted.
+
+stop
+ Write to this file to shut down the bcache device and close the backing
+ device.
+
+writeback_delay
+ When dirty data is written to the cache and it previously did not contain
+ any, waits some number of seconds before initiating writeback. Defaults to
+ 30.
+
+writeback_percent
+ If nonzero, bcache tries to keep around this percentage of the cache dirty by
+ throttling background writeback and using a PD controller to smoothly adjust
+ the rate.
+
+writeback_rate
+ Rate in sectors per second - if writeback_percent is nonzero, background
+ writeback is throttled to this rate. Continuously adjusted by bcache but may
+ also be set by the user.
+
+writeback_running
+ If off, writeback of dirty data will not take place at all. Dirty data will
+ still be added to the cache until it is mostly full; only meant for
+ benchmarking. Defaults to on.
+
+Sysfs - backing device stats
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are directories with these numbers for a running total, as well as
+versions that decay over the past day, hour and 5 minutes; they're also
+aggregated in the cache set directory as well.
+
+bypassed
+ Amount of IO (both reads and writes) that has bypassed the cache
+
+cache_hits, cache_misses, cache_hit_ratio
+ Hits and misses are counted per individual IO as bcache sees them; a
+ partial hit is counted as a miss.
+
+cache_bypass_hits, cache_bypass_misses
+ Hits and misses for IO that is intended to skip the cache are still counted,
+ but broken out here.
+
+cache_miss_collisions
+ Counts instances where data was going to be inserted into the cache from a
+ cache miss, but raced with a write and data was already present (usually 0
+ since the synchronization for cache misses was rewritten)
+
+cache_readaheads
+ Count of times readahead occurred.
+
+Sysfs - cache set
+~~~~~~~~~~~~~~~~~
+
+Available at /sys/fs/bcache/<cset-uuid>
+
+average_key_size
+ Average data per key in the btree.
+
+bdev<0..n>
+ Symlink to each of the attached backing devices.
+
+block_size
+ Block size of the cache devices.
+
+btree_cache_size
+ Amount of memory currently used by the btree cache
+
+bucket_size
+ Size of buckets
+
+cache<0..n>
+ Symlink to each of the cache devices comprising this cache set.
+
+cache_available_percent
+ Percentage of cache device which doesn't contain dirty data, and could
+ potentially be used for writeback. This doesn't mean this space isn't used
+ for clean cached data; the unused statistic (in priority_stats) is typically
+ much lower.
+
+clear_stats
+ Clears the statistics associated with this cache
+
+dirty_data
+ Amount of dirty data is in the cache (updated when garbage collection runs).
+
+flash_vol_create
+ Echoing a size to this file (in human readable units, k/M/G) creates a thinly
+ provisioned volume backed by the cache set.
+
+io_error_halflife, io_error_limit
+ These determines how many errors we accept before disabling the cache.
+ Each error is decayed by the half life (in # ios). If the decaying count
+ reaches io_error_limit dirty data is written out and the cache is disabled.
+
+journal_delay_ms
+ Journal writes will delay for up to this many milliseconds, unless a cache
+ flush happens sooner. Defaults to 100.
+
+root_usage_percent
+ Percentage of the root btree node in use. If this gets too high the node
+ will split, increasing the tree depth.
+
+stop
+ Write to this file to shut down the cache set - waits until all attached
+ backing devices have been shut down.
+
+tree_depth
+ Depth of the btree (A single node btree has depth 0).
+
+unregister
+ Detaches all backing devices and closes the cache devices; if dirty data is
+ present it will disable writeback caching and wait for it to be flushed.
+
+Sysfs - cache set internal
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This directory also exposes timings for a number of internal operations, with
+separate files for average duration, average frequency, last occurrence and max
+duration: garbage collection, btree read, btree node sorts and btree splits.
+
+active_journal_entries
+ Number of journal entries that are newer than the index.
+
+btree_nodes
+ Total nodes in the btree.
+
+btree_used_percent
+ Average fraction of btree in use.
+
+bset_tree_stats
+ Statistics about the auxiliary search trees
+
+btree_cache_max_chain
+ Longest chain in the btree node cache's hash table
+
+cache_read_races
+ Counts instances where while data was being read from the cache, the bucket
+ was reused and invalidated - i.e. where the pointer was stale after the read
+ completed. When this occurs the data is reread from the backing device.
+
+trigger_gc
+ Writing to this file forces garbage collection to run.
+
+Sysfs - Cache device
+~~~~~~~~~~~~~~~~~~~~
+
+Available at /sys/block/<cdev>/bcache
+
+block_size
+ Minimum granularity of writes - should match hardware sector size.
+
+btree_written
+ Sum of all btree writes, in (kilo/mega/giga) bytes
+
+bucket_size
+ Size of buckets
+
+cache_replacement_policy
+ One of either lru, fifo or random.
+
+discard
+ Boolean; if on a discard/TRIM will be issued to each bucket before it is
+ reused. Defaults to off, since SATA TRIM is an unqueued command (and thus
+ slow).
+
+freelist_percent
+ Size of the freelist as a percentage of nbuckets. Can be written to to
+ increase the number of buckets kept on the freelist, which lets you
+ artificially reduce the size of the cache at runtime. Mostly for testing
+ purposes (i.e. testing how different size caches affect your hit rate), but
+ since buckets are discarded when they move on to the freelist will also make
+ the SSD's garbage collection easier by effectively giving it more reserved
+ space.
+
+io_errors
+ Number of errors that have occurred, decayed by io_error_halflife.
+
+metadata_written
+ Sum of all non data writes (btree writes and all other metadata).
+
+nbuckets
+ Total buckets in this cache
+
+priority_stats
+ Statistics about how recently data in the cache has been accessed.
+ This can reveal your working set size. Unused is the percentage of
+ the cache that doesn't contain any data. Metadata is bcache's
+ metadata overhead. Average is the average priority of cache buckets.
+ Next is a list of quantiles with the priority threshold of each.
+
+written
+ Sum of all data that has been written to the cache; comparison with
+ btree_written gives the amount of write inflation in bcache.
--- /dev/null
+================
+Control Group v2
+================
+
+:Date: October, 2015
+:Author: Tejun Heo <tj@kernel.org>
+
+This is the authoritative documentation on the design, interface and
+conventions of cgroup v2. It describes all userland-visible aspects
+of cgroup including core and specific controller behaviors. All
+future changes must be reflected in this document. Documentation for
+v1 is available under Documentation/cgroup-v1/.
+
+.. CONTENTS
+
+ 1. Introduction
+ 1-1. Terminology
+ 1-2. What is cgroup?
+ 2. Basic Operations
+ 2-1. Mounting
+ 2-2. Organizing Processes and Threads
+ 2-2-1. Processes
+ 2-2-2. Threads
+ 2-3. [Un]populated Notification
+ 2-4. Controlling Controllers
+ 2-4-1. Enabling and Disabling
+ 2-4-2. Top-down Constraint
+ 2-4-3. No Internal Process Constraint
+ 2-5. Delegation
+ 2-5-1. Model of Delegation
+ 2-5-2. Delegation Containment
+ 2-6. Guidelines
+ 2-6-1. Organize Once and Control
+ 2-6-2. Avoid Name Collisions
+ 3. Resource Distribution Models
+ 3-1. Weights
+ 3-2. Limits
+ 3-3. Protections
+ 3-4. Allocations
+ 4. Interface Files
+ 4-1. Format
+ 4-2. Conventions
+ 4-3. Core Interface Files
+ 5. Controllers
+ 5-1. CPU
+ 5-1-1. CPU Interface Files
+ 5-2. Memory
+ 5-2-1. Memory Interface Files
+ 5-2-2. Usage Guidelines
+ 5-2-3. Memory Ownership
+ 5-3. IO
+ 5-3-1. IO Interface Files
+ 5-3-2. Writeback
+ 5-4. PID
+ 5-4-1. PID Interface Files
+ 5-5. Device
+ 5-6. RDMA
+ 5-6-1. RDMA Interface Files
+ 5-7. Misc
+ 5-7-1. perf_event
+ 5-N. Non-normative information
+ 5-N-1. CPU controller root cgroup process behaviour
+ 5-N-2. IO controller root cgroup process behaviour
+ 6. Namespace
+ 6-1. Basics
+ 6-2. The Root and Views
+ 6-3. Migration and setns(2)
+ 6-4. Interaction with Other Namespaces
+ P. Information on Kernel Programming
+ P-1. Filesystem Support for Writeback
+ D. Deprecated v1 Core Features
+ R. Issues with v1 and Rationales for v2
+ R-1. Multiple Hierarchies
+ R-2. Thread Granularity
+ R-3. Competition Between Inner Nodes and Threads
+ R-4. Other Interface Issues
+ R-5. Controller Issues and Remedies
+ R-5-1. Memory
+
+
+Introduction
+============
+
+Terminology
+-----------
+
+"cgroup" stands for "control group" and is never capitalized. The
+singular form is used to designate the whole feature and also as a
+qualifier as in "cgroup controllers". When explicitly referring to
+multiple individual control groups, the plural form "cgroups" is used.
+
+
+What is cgroup?
+---------------
+
+cgroup is a mechanism to organize processes hierarchically and
+distribute system resources along the hierarchy in a controlled and
+configurable manner.
+
+cgroup is largely composed of two parts - the core and controllers.
+cgroup core is primarily responsible for hierarchically organizing
+processes. A cgroup controller is usually responsible for
+distributing a specific type of system resource along the hierarchy
+although there are utility controllers which serve purposes other than
+resource distribution.
+
+cgroups form a tree structure and every process in the system belongs
+to one and only one cgroup. All threads of a process belong to the
+same cgroup. On creation, all processes are put in the cgroup that
+the parent process belongs to at the time. A process can be migrated
+to another cgroup. Migration of a process doesn't affect already
+existing descendant processes.
+
+Following certain structural constraints, controllers may be enabled or
+disabled selectively on a cgroup. All controller behaviors are
+hierarchical - if a controller is enabled on a cgroup, it affects all
+processes which belong to the cgroups consisting the inclusive
+sub-hierarchy of the cgroup. When a controller is enabled on a nested
+cgroup, it always restricts the resource distribution further. The
+restrictions set closer to the root in the hierarchy can not be
+overridden from further away.
+
+
+Basic Operations
+================
+
+Mounting
+--------
+
+Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
+hierarchy can be mounted with the following mount command::
+
+ # mount -t cgroup2 none $MOUNT_POINT
+
+cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All
+controllers which support v2 and are not bound to a v1 hierarchy are
+automatically bound to the v2 hierarchy and show up at the root.
+Controllers which are not in active use in the v2 hierarchy can be
+bound to other hierarchies. This allows mixing v2 hierarchy with the
+legacy v1 multiple hierarchies in a fully backward compatible way.
+
+A controller can be moved across hierarchies only after the controller
+is no longer referenced in its current hierarchy. Because per-cgroup
+controller states are destroyed asynchronously and controllers may
+have lingering references, a controller may not show up immediately on
+the v2 hierarchy after the final umount of the previous hierarchy.
+Similarly, a controller should be fully disabled to be moved out of
+the unified hierarchy and it may take some time for the disabled
+controller to become available for other hierarchies; furthermore, due
+to inter-controller dependencies, other controllers may need to be
+disabled too.
+
+While useful for development and manual configurations, moving
+controllers dynamically between the v2 and other hierarchies is
+strongly discouraged for production use. It is recommended to decide
+the hierarchies and controller associations before starting using the
+controllers after system boot.
+
+During transition to v2, system management software might still
+automount the v1 cgroup filesystem and so hijack all controllers
+during boot, before manual intervention is possible. To make testing
+and experimenting easier, the kernel parameter cgroup_no_v1= allows
+disabling controllers in v1 and make them always available in v2.
+
+cgroup v2 currently supports the following mount options.
+
+ nsdelegate
+
+ Consider cgroup namespaces as delegation boundaries. This
+ option is system wide and can only be set on mount or modified
+ through remount from the init namespace. The mount option is
+ ignored on non-init namespace mounts. Please refer to the
+ Delegation section for details.
+
+
+Organizing Processes and Threads
+--------------------------------
+
+Processes
+~~~~~~~~~
+
+Initially, only the root cgroup exists to which all processes belong.
+A child cgroup can be created by creating a sub-directory::
+
+ # mkdir $CGROUP_NAME
+
+A given cgroup may have multiple child cgroups forming a tree
+structure. Each cgroup has a read-writable interface file
+"cgroup.procs". When read, it lists the PIDs of all processes which
+belong to the cgroup one-per-line. The PIDs are not ordered and the
+same PID may show up more than once if the process got moved to
+another cgroup and then back or the PID got recycled while reading.
+
+A process can be migrated into a cgroup by writing its PID to the
+target cgroup's "cgroup.procs" file. Only one process can be migrated
+on a single write(2) call. If a process is composed of multiple
+threads, writing the PID of any thread migrates all threads of the
+process.
+
+When a process forks a child process, the new process is born into the
+cgroup that the forking process belongs to at the time of the
+operation. After exit, a process stays associated with the cgroup
+that it belonged to at the time of exit until it's reaped; however, a
+zombie process does not appear in "cgroup.procs" and thus can't be
+moved to another cgroup.
+
+A cgroup which doesn't have any children or live processes can be
+destroyed by removing the directory. Note that a cgroup which doesn't
+have any children and is associated only with zombie processes is
+considered empty and can be removed::
+
+ # rmdir $CGROUP_NAME
+
+"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy
+cgroup is in use in the system, this file may contain multiple lines,
+one for each hierarchy. The entry for cgroup v2 is always in the
+format "0::$PATH"::
+
+ # cat /proc/842/cgroup
+ ...
+ 0::/test-cgroup/test-cgroup-nested
+
+If the process becomes a zombie and the cgroup it was associated with
+is removed subsequently, " (deleted)" is appended to the path::
+
+ # cat /proc/842/cgroup
+ ...
+ 0::/test-cgroup/test-cgroup-nested (deleted)
+
+
+Threads
+~~~~~~~
+
+cgroup v2 supports thread granularity for a subset of controllers to
+support use cases requiring hierarchical resource distribution across
+the threads of a group of processes. By default, all threads of a
+process belong to the same cgroup, which also serves as the resource
+domain to host resource consumptions which are not specific to a
+process or thread. The thread mode allows threads to be spread across
+a subtree while still maintaining the common resource domain for them.
+
+Controllers which support thread mode are called threaded controllers.
+The ones which don't are called domain controllers.
+
+Marking a cgroup threaded makes it join the resource domain of its
+parent as a threaded cgroup. The parent may be another threaded
+cgroup whose resource domain is further up in the hierarchy. The root
+of a threaded subtree, that is, the nearest ancestor which is not
+threaded, is called threaded domain or thread root interchangeably and
+serves as the resource domain for the entire subtree.
+
+Inside a threaded subtree, threads of a process can be put in
+different cgroups and are not subject to the no internal process
+constraint - threaded controllers can be enabled on non-leaf cgroups
+whether they have threads in them or not.
+
+As the threaded domain cgroup hosts all the domain resource
+consumptions of the subtree, it is considered to have internal
+resource consumptions whether there are processes in it or not and
+can't have populated child cgroups which aren't threaded. Because the
+root cgroup is not subject to no internal process constraint, it can
+serve both as a threaded domain and a parent to domain cgroups.
+
+The current operation mode or type of the cgroup is shown in the
+"cgroup.type" file which indicates whether the cgroup is a normal
+domain, a domain which is serving as the domain of a threaded subtree,
+or a threaded cgroup.
+
+On creation, a cgroup is always a domain cgroup and can be made
+threaded by writing "threaded" to the "cgroup.type" file. The
+operation is single direction::
+
+ # echo threaded > cgroup.type
+
+Once threaded, the cgroup can't be made a domain again. To enable the
+thread mode, the following conditions must be met.
+
+- As the cgroup will join the parent's resource domain. The parent
+ must either be a valid (threaded) domain or a threaded cgroup.
+
+- When the parent is an unthreaded domain, it must not have any domain
+ controllers enabled or populated domain children. The root is
+ exempt from this requirement.
+
+Topology-wise, a cgroup can be in an invalid state. Please consider
+the following topology::
+
+ A (threaded domain) - B (threaded) - C (domain, just created)
+
+C is created as a domain but isn't connected to a parent which can
+host child domains. C can't be used until it is turned into a
+threaded cgroup. "cgroup.type" file will report "domain (invalid)" in
+these cases. Operations which fail due to invalid topology use
+EOPNOTSUPP as the errno.
+
+A domain cgroup is turned into a threaded domain when one of its child
+cgroup becomes threaded or threaded controllers are enabled in the
+"cgroup.subtree_control" file while there are processes in the cgroup.
+A threaded domain reverts to a normal domain when the conditions
+clear.
+
+When read, "cgroup.threads" contains the list of the thread IDs of all
+threads in the cgroup. Except that the operations are per-thread
+instead of per-process, "cgroup.threads" has the same format and
+behaves the same way as "cgroup.procs". While "cgroup.threads" can be
+written to in any cgroup, as it can only move threads inside the same
+threaded domain, its operations are confined inside each threaded
+subtree.
+
+The threaded domain cgroup serves as the resource domain for the whole
+subtree, and, while the threads can be scattered across the subtree,
+all the processes are considered to be in the threaded domain cgroup.
+"cgroup.procs" in a threaded domain cgroup contains the PIDs of all
+processes in the subtree and is not readable in the subtree proper.
+However, "cgroup.procs" can be written to from anywhere in the subtree
+to migrate all threads of the matching process to the cgroup.
+
+Only threaded controllers can be enabled in a threaded subtree. When
+a threaded controller is enabled inside a threaded subtree, it only
+accounts for and controls resource consumptions associated with the
+threads in the cgroup and its descendants. All consumptions which
+aren't tied to a specific thread belong to the threaded domain cgroup.
+
+Because a threaded subtree is exempt from no internal process
+constraint, a threaded controller must be able to handle competition
+between threads in a non-leaf cgroup and its child cgroups. Each
+threaded controller defines how such competitions are handled.
+
+
+[Un]populated Notification
+--------------------------
+
+Each non-root cgroup has a "cgroup.events" file which contains
+"populated" field indicating whether the cgroup's sub-hierarchy has
+live processes in it. Its value is 0 if there is no live process in
+the cgroup and its descendants; otherwise, 1. poll and [id]notify
+events are triggered when the value changes. This can be used, for
+example, to start a clean-up operation after all processes of a given
+sub-hierarchy have exited. The populated state updates and
+notifications are recursive. Consider the following sub-hierarchy
+where the numbers in the parentheses represent the numbers of processes
+in each cgroup::
+
+ A(4) - B(0) - C(1)
+ \ D(0)
+
+A, B and C's "populated" fields would be 1 while D's 0. After the one
+process in C exits, B and C's "populated" fields would flip to "0" and
+file modified events will be generated on the "cgroup.events" files of
+both cgroups.
+
+
+Controlling Controllers
+-----------------------
+
+Enabling and Disabling
+~~~~~~~~~~~~~~~~~~~~~~
+
+Each cgroup has a "cgroup.controllers" file which lists all
+controllers available for the cgroup to enable::
+
+ # cat cgroup.controllers
+ cpu io memory
+
+No controller is enabled by default. Controllers can be enabled and
+disabled by writing to the "cgroup.subtree_control" file::
+
+ # echo "+cpu +memory -io" > cgroup.subtree_control
+
+Only controllers which are listed in "cgroup.controllers" can be
+enabled. When multiple operations are specified as above, either they
+all succeed or fail. If multiple operations on the same controller
+are specified, the last one is effective.
+
+Enabling a controller in a cgroup indicates that the distribution of
+the target resource across its immediate children will be controlled.
+Consider the following sub-hierarchy. The enabled controllers are
+listed in parentheses::
+
+ A(cpu,memory) - B(memory) - C()
+ \ D()
+
+As A has "cpu" and "memory" enabled, A will control the distribution
+of CPU cycles and memory to its children, in this case, B. As B has
+"memory" enabled but not "CPU", C and D will compete freely on CPU
+cycles but their division of memory available to B will be controlled.
+
+As a controller regulates the distribution of the target resource to
+the cgroup's children, enabling it creates the controller's interface
+files in the child cgroups. In the above example, enabling "cpu" on B
+would create the "cpu." prefixed controller interface files in C and
+D. Likewise, disabling "memory" from B would remove the "memory."
+prefixed controller interface files from C and D. This means that the
+controller interface files - anything which doesn't start with
+"cgroup." are owned by the parent rather than the cgroup itself.
+
+
+Top-down Constraint
+~~~~~~~~~~~~~~~~~~~
+
+Resources are distributed top-down and a cgroup can further distribute
+a resource only if the resource has been distributed to it from the
+parent. This means that all non-root "cgroup.subtree_control" files
+can only contain controllers which are enabled in the parent's
+"cgroup.subtree_control" file. A controller can be enabled only if
+the parent has the controller enabled and a controller can't be
+disabled if one or more children have it enabled.
+
+
+No Internal Process Constraint
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Non-root cgroups can distribute domain resources to their children
+only when they don't have any processes of their own. In other words,
+only domain cgroups which don't contain any processes can have domain
+controllers enabled in their "cgroup.subtree_control" files.
+
+This guarantees that, when a domain controller is looking at the part
+of the hierarchy which has it enabled, processes are always only on
+the leaves. This rules out situations where child cgroups compete
+against internal processes of the parent.
+
+The root cgroup is exempt from this restriction. Root contains
+processes and anonymous resource consumption which can't be associated
+with any other cgroups and requires special treatment from most
+controllers. How resource consumption in the root cgroup is governed
+is up to each controller (for more information on this topic please
+refer to the Non-normative information section in the Controllers
+chapter).
+
+Note that the restriction doesn't get in the way if there is no
+enabled controller in the cgroup's "cgroup.subtree_control". This is
+important as otherwise it wouldn't be possible to create children of a
+populated cgroup. To control resource distribution of a cgroup, the
+cgroup must create children and transfer all its processes to the
+children before enabling controllers in its "cgroup.subtree_control"
+file.
+
+
+Delegation
+----------
+
+Model of Delegation
+~~~~~~~~~~~~~~~~~~~
+
+A cgroup can be delegated in two ways. First, to a less privileged
+user by granting write access of the directory and its "cgroup.procs",
+"cgroup.threads" and "cgroup.subtree_control" files to the user.
+Second, if the "nsdelegate" mount option is set, automatically to a
+cgroup namespace on namespace creation.
+
+Because the resource control interface files in a given directory
+control the distribution of the parent's resources, the delegatee
+shouldn't be allowed to write to them. For the first method, this is
+achieved by not granting access to these files. For the second, the
+kernel rejects writes to all files other than "cgroup.procs" and
+"cgroup.subtree_control" on a namespace root from inside the
+namespace.
+
+The end results are equivalent for both delegation types. Once
+delegated, the user can build sub-hierarchy under the directory,
+organize processes inside it as it sees fit and further distribute the
+resources it received from the parent. The limits and other settings
+of all resource controllers are hierarchical and regardless of what
+happens in the delegated sub-hierarchy, nothing can escape the
+resource restrictions imposed by the parent.
+
+Currently, cgroup doesn't impose any restrictions on the number of
+cgroups in or nesting depth of a delegated sub-hierarchy; however,
+this may be limited explicitly in the future.
+
+
+Delegation Containment
+~~~~~~~~~~~~~~~~~~~~~~
+
+A delegated sub-hierarchy is contained in the sense that processes
+can't be moved into or out of the sub-hierarchy by the delegatee.
+
+For delegations to a less privileged user, this is achieved by
+requiring the following conditions for a process with a non-root euid
+to migrate a target process into a cgroup by writing its PID to the
+"cgroup.procs" file.
+
+- The writer must have write access to the "cgroup.procs" file.
+
+- The writer must have write access to the "cgroup.procs" file of the
+ common ancestor of the source and destination cgroups.
+
+The above two constraints ensure that while a delegatee may migrate
+processes around freely in the delegated sub-hierarchy it can't pull
+in from or push out to outside the sub-hierarchy.
+
+For an example, let's assume cgroups C0 and C1 have been delegated to
+user U0 who created C00, C01 under C0 and C10 under C1 as follows and
+all processes under C0 and C1 belong to U0::
+
+ ~~~~~~~~~~~~~ - C0 - C00
+ ~ cgroup ~ \ C01
+ ~ hierarchy ~
+ ~~~~~~~~~~~~~ - C1 - C10
+
+Let's also say U0 wants to write the PID of a process which is
+currently in C10 into "C00/cgroup.procs". U0 has write access to the
+file; however, the common ancestor of the source cgroup C10 and the
+destination cgroup C00 is above the points of delegation and U0 would
+not have write access to its "cgroup.procs" files and thus the write
+will be denied with -EACCES.
+
+For delegations to namespaces, containment is achieved by requiring
+that both the source and destination cgroups are reachable from the
+namespace of the process which is attempting the migration. If either
+is not reachable, the migration is rejected with -ENOENT.
+
+
+Guidelines
+----------
+
+Organize Once and Control
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Migrating a process across cgroups is a relatively expensive operation
+and stateful resources such as memory are not moved together with the
+process. This is an explicit design decision as there often exist
+inherent trade-offs between migration and various hot paths in terms
+of synchronization cost.
+
+As such, migrating processes across cgroups frequently as a means to
+apply different resource restrictions is discouraged. A workload
+should be assigned to a cgroup according to the system's logical and
+resource structure once on start-up. Dynamic adjustments to resource
+distribution can be made by changing controller configuration through
+the interface files.
+
+
+Avoid Name Collisions
+~~~~~~~~~~~~~~~~~~~~~
+
+Interface files for a cgroup and its children cgroups occupy the same
+directory and it is possible to create children cgroups which collide
+with interface files.
+
+All cgroup core interface files are prefixed with "cgroup." and each
+controller's interface files are prefixed with the controller name and
+a dot. A controller's name is composed of lower case alphabets and
+'_'s but never begins with an '_' so it can be used as the prefix
+character for collision avoidance. Also, interface file names won't
+start or end with terms which are often used in categorizing workloads
+such as job, service, slice, unit or workload.
+
+cgroup doesn't do anything to prevent name collisions and it's the
+user's responsibility to avoid them.
+
+
+Resource Distribution Models
+============================
+
+cgroup controllers implement several resource distribution schemes
+depending on the resource type and expected use cases. This section
+describes major schemes in use along with their expected behaviors.
+
+
+Weights
+-------
+
+A parent's resource is distributed by adding up the weights of all
+active children and giving each the fraction matching the ratio of its
+weight against the sum. As only children which can make use of the
+resource at the moment participate in the distribution, this is
+work-conserving. Due to the dynamic nature, this model is usually
+used for stateless resources.
+
+All weights are in the range [1, 10000] with the default at 100. This
+allows symmetric multiplicative biases in both directions at fine
+enough granularity while staying in the intuitive range.
+
+As long as the weight is in range, all configuration combinations are
+valid and there is no reason to reject configuration changes or
+process migrations.
+
+"cpu.weight" proportionally distributes CPU cycles to active children
+and is an example of this type.
+
+
+Limits
+------
+
+A child can only consume upto the configured amount of the resource.
+Limits can be over-committed - the sum of the limits of children can
+exceed the amount of resource available to the parent.
+
+Limits are in the range [0, max] and defaults to "max", which is noop.
+
+As limits can be over-committed, all configuration combinations are
+valid and there is no reason to reject configuration changes or
+process migrations.
+
+"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
+on an IO device and is an example of this type.
+
+
+Protections
+-----------
+
+A cgroup is protected to be allocated upto the configured amount of
+the resource if the usages of all its ancestors are under their
+protected levels. Protections can be hard guarantees or best effort
+soft boundaries. Protections can also be over-committed in which case
+only upto the amount available to the parent is protected among
+children.
+
+Protections are in the range [0, max] and defaults to 0, which is
+noop.
+
+As protections can be over-committed, all configuration combinations
+are valid and there is no reason to reject configuration changes or
+process migrations.
+
+"memory.low" implements best-effort memory protection and is an
+example of this type.
+
+
+Allocations
+-----------
+
+A cgroup is exclusively allocated a certain amount of a finite
+resource. Allocations can't be over-committed - the sum of the
+allocations of children can not exceed the amount of resource
+available to the parent.
+
+Allocations are in the range [0, max] and defaults to 0, which is no
+resource.
+
+As allocations can't be over-committed, some configuration
+combinations are invalid and should be rejected. Also, if the
+resource is mandatory for execution of processes, process migrations
+may be rejected.
+
+"cpu.rt.max" hard-allocates realtime slices and is an example of this
+type.
+
+
+Interface Files
+===============
+
+Format
+------
+
+All interface files should be in one of the following formats whenever
+possible::
+
+ New-line separated values
+ (when only one value can be written at once)
+
+ VAL0\n
+ VAL1\n
+ ...
+
+ Space separated values
+ (when read-only or multiple values can be written at once)
+
+ VAL0 VAL1 ...\n
+
+ Flat keyed
+
+ KEY0 VAL0\n
+ KEY1 VAL1\n
+ ...
+
+ Nested keyed
+
+ KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
+ KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
+ ...
+
+For a writable file, the format for writing should generally match
+reading; however, controllers may allow omitting later fields or
+implement restricted shortcuts for most common use cases.
+
+For both flat and nested keyed files, only the values for a single key
+can be written at a time. For nested keyed files, the sub key pairs
+may be specified in any order and not all pairs have to be specified.
+
+
+Conventions
+-----------
+
+- Settings for a single feature should be contained in a single file.
+
+- The root cgroup should be exempt from resource control and thus
+ shouldn't have resource control interface files. Also,
+ informational files on the root cgroup which end up showing global
+ information available elsewhere shouldn't exist.
+
+- If a controller implements weight based resource distribution, its
+ interface file should be named "weight" and have the range [1,
+ 10000] with 100 as the default. The values are chosen to allow
+ enough and symmetric bias in both directions while keeping it
+ intuitive (the default is 100%).
+
+- If a controller implements an absolute resource guarantee and/or
+ limit, the interface files should be named "min" and "max"
+ respectively. If a controller implements best effort resource
+ guarantee and/or limit, the interface files should be named "low"
+ and "high" respectively.
+
+ In the above four control files, the special token "max" should be
+ used to represent upward infinity for both reading and writing.
+
+- If a setting has a configurable default value and keyed specific
+ overrides, the default entry should be keyed with "default" and
+ appear as the first entry in the file.
+
+ The default value can be updated by writing either "default $VAL" or
+ "$VAL".
+
+ When writing to update a specific override, "default" can be used as
+ the value to indicate removal of the override. Override entries
+ with "default" as the value must not appear when read.
+
+ For example, a setting which is keyed by major:minor device numbers
+ with integer values may look like the following::
+
+ # cat cgroup-example-interface-file
+ default 150
+ 8:0 300
+
+ The default value can be updated by::
+
+ # echo 125 > cgroup-example-interface-file
+
+ or::
+
+ # echo "default 125" > cgroup-example-interface-file
+
+ An override can be set by::
+
+ # echo "8:16 170" > cgroup-example-interface-file
+
+ and cleared by::
+
+ # echo "8:0 default" > cgroup-example-interface-file
+ # cat cgroup-example-interface-file
+ default 125
+ 8:16 170
+
+- For events which are not very high frequency, an interface file
+ "events" should be created which lists event key value pairs.
+ Whenever a notifiable event happens, file modified event should be
+ generated on the file.
+
+
+Core Interface Files
+--------------------
+
+All cgroup core files are prefixed with "cgroup."
+
+ cgroup.type
+
+ A read-write single value file which exists on non-root
+ cgroups.
+
+ When read, it indicates the current type of the cgroup, which
+ can be one of the following values.
+
+ - "domain" : A normal valid domain cgroup.
+
+ - "domain threaded" : A threaded domain cgroup which is
+ serving as the root of a threaded subtree.
+
+ - "domain invalid" : A cgroup which is in an invalid state.
+ It can't be populated or have controllers enabled. It may
+ be allowed to become a threaded cgroup.
+
+ - "threaded" : A threaded cgroup which is a member of a
+ threaded subtree.
+
+ A cgroup can be turned into a threaded cgroup by writing
+ "threaded" to this file.
+
+ cgroup.procs
+ A read-write new-line separated values file which exists on
+ all cgroups.
+
+ When read, it lists the PIDs of all processes which belong to
+ the cgroup one-per-line. The PIDs are not ordered and the
+ same PID may show up more than once if the process got moved
+ to another cgroup and then back or the PID got recycled while
+ reading.
+
+ A PID can be written to migrate the process associated with
+ the PID to the cgroup. The writer should match all of the
+ following conditions.
+
+ - It must have write access to the "cgroup.procs" file.
+
+ - It must have write access to the "cgroup.procs" file of the
+ common ancestor of the source and destination cgroups.
+
+ When delegating a sub-hierarchy, write access to this file
+ should be granted along with the containing directory.
+
+ In a threaded cgroup, reading this file fails with EOPNOTSUPP
+ as all the processes belong to the thread root. Writing is
+ supported and moves every thread of the process to the cgroup.
+
+ cgroup.threads
+ A read-write new-line separated values file which exists on
+ all cgroups.
+
+ When read, it lists the TIDs of all threads which belong to
+ the cgroup one-per-line. The TIDs are not ordered and the
+ same TID may show up more than once if the thread got moved to
+ another cgroup and then back or the TID got recycled while
+ reading.
+
+ A TID can be written to migrate the thread associated with the
+ TID to the cgroup. The writer should match all of the
+ following conditions.
+
+ - It must have write access to the "cgroup.threads" file.
+
+ - The cgroup that the thread is currently in must be in the
+ same resource domain as the destination cgroup.
+
+ - It must have write access to the "cgroup.procs" file of the
+ common ancestor of the source and destination cgroups.
+
+ When delegating a sub-hierarchy, write access to this file
+ should be granted along with the containing directory.
+
+ cgroup.controllers
+ A read-only space separated values file which exists on all
+ cgroups.
+
+ It shows space separated list of all controllers available to
+ the cgroup. The controllers are not ordered.
+
+ cgroup.subtree_control
+ A read-write space separated values file which exists on all
+ cgroups. Starts out empty.
+
+ When read, it shows space separated list of the controllers
+ which are enabled to control resource distribution from the
+ cgroup to its children.
+
+ Space separated list of controllers prefixed with '+' or '-'
+ can be written to enable or disable controllers. A controller
+ name prefixed with '+' enables the controller and '-'
+ disables. If a controller appears more than once on the list,
+ the last one is effective. When multiple enable and disable
+ operations are specified, either all succeed or all fail.
+
+ cgroup.events
+ A read-only flat-keyed file which exists on non-root cgroups.
+ The following entries are defined. Unless specified
+ otherwise, a value change in this file generates a file
+ modified event.
+
+ populated
+ 1 if the cgroup or its descendants contains any live
+ processes; otherwise, 0.
+
+ cgroup.max.descendants
+ A read-write single value files. The default is "max".
+
+ Maximum allowed number of descent cgroups.
+ If the actual number of descendants is equal or larger,
+ an attempt to create a new cgroup in the hierarchy will fail.
+
+ cgroup.max.depth
+ A read-write single value files. The default is "max".
+
+ Maximum allowed descent depth below the current cgroup.
+ If the actual descent depth is equal or larger,
+ an attempt to create a new child cgroup will fail.
+
+ cgroup.stat
+ A read-only flat-keyed file with the following entries:
+
+ nr_descendants
+ Total number of visible descendant cgroups.
+
+ nr_dying_descendants
+ Total number of dying descendant cgroups. A cgroup becomes
+ dying after being deleted by a user. The cgroup will remain
+ in dying state for some time undefined time (which can depend
+ on system load) before being completely destroyed.
+
+ A process can't enter a dying cgroup under any circumstances,
+ a dying cgroup can't revive.
+
+ A dying cgroup can consume system resources not exceeding
+ limits, which were active at the moment of cgroup deletion.
+
+
+Controllers
+===========
+
+CPU
+---
+
+The "cpu" controllers regulates distribution of CPU cycles. This
+controller implements weight and absolute bandwidth limit models for
+normal scheduling policy and absolute bandwidth allocation model for
+realtime scheduling policy.
+
+WARNING: cgroup2 doesn't yet support control of realtime processes and
+the cpu controller can only be enabled when all RT processes are in
+the root cgroup. Be aware that system management software may already
+have placed RT processes into nonroot cgroups during the system boot
+process, and these processes may need to be moved to the root cgroup
+before the cpu controller can be enabled.
+
+
+CPU Interface Files
+~~~~~~~~~~~~~~~~~~~
+
+All time durations are in microseconds.
+
+ cpu.stat
+ A read-only flat-keyed file which exists on non-root cgroups.
+ This file exists whether the controller is enabled or not.
+
+ It always reports the following three stats:
+
+ - usage_usec
+ - user_usec
+ - system_usec
+
+ and the following three when the controller is enabled:
+
+ - nr_periods
+ - nr_throttled
+ - throttled_usec
+
+ cpu.weight
+ A read-write single value file which exists on non-root
+ cgroups. The default is "100".
+
+ The weight in the range [1, 10000].
+
+ cpu.weight.nice
+ A read-write single value file which exists on non-root
+ cgroups. The default is "0".
+
+ The nice value is in the range [-20, 19].
+
+ This interface file is an alternative interface for
+ "cpu.weight" and allows reading and setting weight using the
+ same values used by nice(2). Because the range is smaller and
+ granularity is coarser for the nice values, the read value is
+ the closest approximation of the current weight.
+
+ cpu.max
+ A read-write two value file which exists on non-root cgroups.
+ The default is "max 100000".
+
+ The maximum bandwidth limit. It's in the following format::
+
+ $MAX $PERIOD
+
+ which indicates that the group may consume upto $MAX in each
+ $PERIOD duration. "max" for $MAX indicates no limit. If only
+ one number is written, $MAX is updated.
+
+
+Memory
+------
+
+The "memory" controller regulates distribution of memory. Memory is
+stateful and implements both limit and protection models. Due to the
+intertwining between memory usage and reclaim pressure and the
+stateful nature of memory, the distribution model is relatively
+complex.
+
+While not completely water-tight, all major memory usages by a given
+cgroup are tracked so that the total memory consumption can be
+accounted and controlled to a reasonable extent. Currently, the
+following types of memory usages are tracked.
+
+- Userland memory - page cache and anonymous memory.
+
+- Kernel data structures such as dentries and inodes.
+
+- TCP socket buffers.
+
+The above list may expand in the future for better coverage.
+
+
+Memory Interface Files
+~~~~~~~~~~~~~~~~~~~~~~
+
+All memory amounts are in bytes. If a value which is not aligned to
+PAGE_SIZE is written, the value may be rounded up to the closest
+PAGE_SIZE multiple when read back.
+
+ memory.current
+ A read-only single value file which exists on non-root
+ cgroups.
+
+ The total amount of memory currently being used by the cgroup
+ and its descendants.
+
+ memory.low
+ A read-write single value file which exists on non-root
+ cgroups. The default is "0".
+
+ Best-effort memory protection. If the memory usages of a
+ cgroup and all its ancestors are below their low boundaries,
+ the cgroup's memory won't be reclaimed unless memory can be
+ reclaimed from unprotected cgroups.
+
+ Putting more memory than generally available under this
+ protection is discouraged.
+
+ memory.high
+ A read-write single value file which exists on non-root
+ cgroups. The default is "max".
+
+ Memory usage throttle limit. This is the main mechanism to
+ control memory usage of a cgroup. If a cgroup's usage goes
+ over the high boundary, the processes of the cgroup are
+ throttled and put under heavy reclaim pressure.
+
+ Going over the high limit never invokes the OOM killer and
+ under extreme conditions the limit may be breached.
+
+ memory.max
+ A read-write single value file which exists on non-root
+ cgroups. The default is "max".
+
+ Memory usage hard limit. This is the final protection
+ mechanism. If a cgroup's memory usage reaches this limit and
+ can't be reduced, the OOM killer is invoked in the cgroup.
+ Under certain circumstances, the usage may go over the limit
+ temporarily.
+
+ This is the ultimate protection mechanism. As long as the
+ high limit is used and monitored properly, this limit's
+ utility is limited to providing the final safety net.
+
+ memory.events
+ A read-only flat-keyed file which exists on non-root cgroups.
+ The following entries are defined. Unless specified
+ otherwise, a value change in this file generates a file
+ modified event.
+
+ low
+ The number of times the cgroup is reclaimed due to
+ high memory pressure even though its usage is under
+ the low boundary. This usually indicates that the low
+ boundary is over-committed.
+
+ high
+ The number of times processes of the cgroup are
+ throttled and routed to perform direct memory reclaim
+ because the high memory boundary was exceeded. For a
+ cgroup whose memory usage is capped by the high limit
+ rather than global memory pressure, this event's
+ occurrences are expected.
+
+ max
+ The number of times the cgroup's memory usage was
+ about to go over the max boundary. If direct reclaim
+ fails to bring it down, the cgroup goes to OOM state.
+
+ oom
+ The number of time the cgroup's memory usage was
+ reached the limit and allocation was about to fail.
+
+ Depending on context result could be invocation of OOM
+ killer and retrying allocation or failing allocation.
+
+ Failed allocation in its turn could be returned into
+ userspace as -ENOMEM or silently ignored in cases like
+ disk readahead. For now OOM in memory cgroup kills
+ tasks iff shortage has happened inside page fault.
+
+ oom_kill
+ The number of processes belonging to this cgroup
+ killed by any kind of OOM killer.
+
+ memory.stat
+ A read-only flat-keyed file which exists on non-root cgroups.
+
+ This breaks down the cgroup's memory footprint into different
+ types of memory, type-specific details, and other information
+ on the state and past events of the memory management system.
+
+ All memory amounts are in bytes.
+
+ The entries are ordered to be human readable, and new entries
+ can show up in the middle. Don't rely on items remaining in a
+ fixed position; use the keys to look up specific values!
+
+ anon
+ Amount of memory used in anonymous mappings such as
+ brk(), sbrk(), and mmap(MAP_ANONYMOUS)
+
+ file
+ Amount of memory used to cache filesystem data,
+ including tmpfs and shared memory.
+
+ kernel_stack
+ Amount of memory allocated to kernel stacks.
+
+ slab
+ Amount of memory used for storing in-kernel data
+ structures.
+
+ sock
+ Amount of memory used in network transmission buffers
+
+ shmem
+ Amount of cached filesystem data that is swap-backed,
+ such as tmpfs, shm segments, shared anonymous mmap()s
+
+ file_mapped
+ Amount of cached filesystem data mapped with mmap()
+
+ file_dirty
+ Amount of cached filesystem data that was modified but
+ not yet written back to disk
+
+ file_writeback
+ Amount of cached filesystem data that was modified and
+ is currently being written back to disk
+
+ inactive_anon, active_anon, inactive_file, active_file, unevictable
+ Amount of memory, swap-backed and filesystem-backed,
+ on the internal memory management lists used by the
+ page reclaim algorithm
+
+ slab_reclaimable
+ Part of "slab" that might be reclaimed, such as
+ dentries and inodes.
+
+ slab_unreclaimable
+ Part of "slab" that cannot be reclaimed on memory
+ pressure.
+
+ pgfault
+ Total number of page faults incurred
+
+ pgmajfault
+ Number of major page faults incurred
+
+ workingset_refault
+
+ Number of refaults of previously evicted pages
+
+ workingset_activate
+
+ Number of refaulted pages that were immediately activated
+
+ workingset_nodereclaim
+
+ Number of times a shadow node has been reclaimed
+
+ pgrefill
+
+ Amount of scanned pages (in an active LRU list)
+
+ pgscan
+
+ Amount of scanned pages (in an inactive LRU list)
+
+ pgsteal
+
+ Amount of reclaimed pages
+
+ pgactivate
+
+ Amount of pages moved to the active LRU list
+
+ pgdeactivate
+
+ Amount of pages moved to the inactive LRU lis
+
+ pglazyfree
+
+ Amount of pages postponed to be freed under memory pressure
+
+ pglazyfreed
+
+ Amount of reclaimed lazyfree pages
+
+ memory.swap.current
+ A read-only single value file which exists on non-root
+ cgroups.
+
+ The total amount of swap currently being used by the cgroup
+ and its descendants.
+
+ memory.swap.max
+ A read-write single value file which exists on non-root
+ cgroups. The default is "max".
+
+ Swap usage hard limit. If a cgroup's swap usage reaches this
+ limit, anonymous memory of the cgroup will not be swapped out.
+
+
+Usage Guidelines
+~~~~~~~~~~~~~~~~
+
+"memory.high" is the main mechanism to control memory usage.
+Over-committing on high limit (sum of high limits > available memory)
+and letting global memory pressure to distribute memory according to
+usage is a viable strategy.
+
+Because breach of the high limit doesn't trigger the OOM killer but
+throttles the offending cgroup, a management agent has ample
+opportunities to monitor and take appropriate actions such as granting
+more memory or terminating the workload.
+
+Determining whether a cgroup has enough memory is not trivial as
+memory usage doesn't indicate whether the workload can benefit from
+more memory. For example, a workload which writes data received from
+network to a file can use all available memory but can also operate as
+performant with a small amount of memory. A measure of memory
+pressure - how much the workload is being impacted due to lack of
+memory - is necessary to determine whether a workload needs more
+memory; unfortunately, memory pressure monitoring mechanism isn't
+implemented yet.
+
+
+Memory Ownership
+~~~~~~~~~~~~~~~~
+
+A memory area is charged to the cgroup which instantiated it and stays
+charged to the cgroup until the area is released. Migrating a process
+to a different cgroup doesn't move the memory usages that it
+insta
git.codelabs.ch / muen / linux.git / commitdiff