Message-Id: <20100409145005.a7d001ca.nishimura@mxp.nes.nec.co.jp>
Date:	Fri, 9 Apr 2010 14:50:05 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	randy.dunlap@...cle.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: update documentation v4

On Fri, 9 Apr 2010 13:45:53 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> Thank you all.
> ==
> 
> Documentation update.
> 
> Some information are old, and  I think current documentation doesn't work
> as "a guide for users".
> We need summary of all of our controls, at least.
> 
> Changelog: 2010/04/09
> * replace 'lru' with 'LRU' and 'oom' with 'OOM'
> * fixed double-space breakage
> * applied all comments and fixed wrong parts pointed out.
> * fixed cgroup.procs
> 
> Changelog: 2009/04/07
> * fixed tons of typos.
> * replaced "memcg" with "memory cgroup" AMAP.
> * replaced "mem+swap" with "memory+swap"
> 
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Looks good to me.

Reviewed-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>

Thanks,
Daisuke Nishimura.
> ---
>  Documentation/cgroups/memory.txt |  277 ++++++++++++++++++++++++++-------------
>  1 file changed, 188 insertions(+), 89 deletions(-)
> 
> Index: mmotm-temp/Documentation/cgroups/memory.txt
> ===================================================================
> --- mmotm-temp.orig/Documentation/cgroups/memory.txt
> +++ mmotm-temp/Documentation/cgroups/memory.txt
> @@ -4,16 +4,6 @@ NOTE: The Memory Resource Controller has
>  to as the memory controller in this document. Do not confuse memory controller
>  used here with the memory controller that is used in hardware.
>  
> -Salient features
> -
> -a. Enable control of Anonymous, Page Cache (mapped and unmapped) and
> -   Swap Cache memory pages.
> -b. The infrastructure allows easy addition of other types of memory to control
> -c. Provides *zero overhead* for non memory controller users
> -d. Provides a double LRU: global memory pressure causes reclaim from the
> -   global LRU; a cgroup on hitting a limit, reclaims from the per
> -   cgroup LRU
> -
>  Benefits and Purpose of the memory controller
>  
>  The memory controller isolates the memory behaviour of a group of tasks
> @@ -33,6 +23,45 @@ d. A CD/DVD burner could control the amo
>  e. There are several other use cases, find one or use the controller just
>     for fun (to learn and hack on the VM subsystem).
>  
> +Current Status: linux-2.6.34-mmotm(development version of 2010/April)
> +
> +Features:
> + - accounting anonymous pages, file caches, swap caches usage and limit them.
> + - private LRU and reclaim routine. (system's global LRU and private LRU
> +   work independently from each other)
> + - optionally, memory+swap usage can be accounted and limited.
> + - hierarchical accounting
> + - soft limit
> + - moving(recharging) account at moving a task is selectable.
> + - usage threshold notifier
> + - oom-killer disable knob and oom-notifier
> + - Root cgroup has no limit controls.
> +
> + Kernel memory and Hugepages are not under control yet. We just manage
> + pages on LRU. To add more controls, we have to take care of performance.
> +
> +Brief summary of control files.
> +
> + tasks				 # attach a task(thread) and show list of threads
> + cgroup.procs			 # show list of processes
> + cgroup.event_control		 # an interface for event_fd()
> + memory.usage_in_bytes		 # show current memory(RSS+Cache) usage.
> + memory.memsw.usage_in_bytes	 # show current memory+Swap usage
> + memory.limit_in_bytes		 # set/show limit of memory usage
> + memory.memsw.limit_in_bytes	 # set/show limit of memory+Swap usage
> + memory.failcnt			 # show the number of memory usage hits limits
> + memory.memsw.failcnt		 # show the number of memory+Swap hits limits
> + memory.max_usage_in_bytes	 # show max memory usage recorded
> + memory.memsw.usage_in_bytes	 # show max memory+Swap usage recorded
> + memory.soft_limit_in_bytes	 # set/show soft limit of memory usage
> + memory.stat			 # show various statistics
> + memory.use_hierarchy		 # set/show hierarchical account enabled
> + memory.force_empty		 # trigger forced move charge to parent
> + memory.swappiness		 # set/show swappiness parameter of vmscan
> + 				 (See sysctl's vm.swappiness)
> + memory.move_charge_at_immigrate # set/show controls of moving charges
> + memory.oom_control		 # set/show oom controls.
> +
>  1. History
>  
>  The memory controller has a long history. A request for comments for the memory
> @@ -106,14 +135,14 @@ the necessary data structures and check 
>  is over its limit. If it is then reclaim is invoked on the cgroup.
>  More details can be found in the reclaim section of this document.
>  If everything goes well, a page meta-data-structure called page_cgroup is
> -allocated and associated with the page.  This routine also adds the page to
> -the per cgroup LRU.
> +updated. page_cgroup has its own LRU on cgroup.
> +(*) page_cgroup structure is allocated at boot/memory-hotplug time.
>  
>  2.2.1 Accounting details
>  
>  All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
> -(some pages which never be reclaimable and will not be on global LRU
> - are not accounted. we just accounts pages under usual vm management.)
> +Some pages which are never reclaimable and will not be on the global LRU
> +are not accounted. We just accounts pages under usual VM management.
>  
>  RSS pages are accounted at page_fault unless they've already been accounted
>  for earlier. A file page will be accounted for as Page Cache when it's
> @@ -121,12 +150,19 @@ inserted into inode (radix-tree). While 
>  processes, duplicate accounting is carefully avoided.
>  
>  A RSS page is unaccounted when it's fully unmapped. A PageCache page is
> -unaccounted when it's removed from radix-tree.
> +unaccounted when it's removed from radix-tree. Even if RSS pages are fully
> +unmapped (by kswapd), they may exist as SwapCache in the system until they
> +are really freed. Such SwapCaches also also accounted.
> +A swapped-in page is not accounted until it's mapped.
> +
> +Note: The kernel does swapin-readahead and read multiple swaps at once.
> +This means swapped-in pages may contain pages for other tasks than a task
> +causing page fault. So, we avoid accounting at swap-in I/O.
>  
>  At page migration, accounting information is kept.
>  
> -Note: we just account pages-on-lru because our purpose is to control amount
> -of used pages. not-on-lru pages are tend to be out-of-control from vm view.
> +Note: we just account pages-on-LRU because our purpose is to control amount
> +of used pages. not-on-LRU pages tend to be out-of-control from VM view.
>  
>  2.3 Shared Page Accounting
>  
> @@ -143,6 +179,7 @@ caller of swapoff rather than the users 
>  
>  
>  2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)
> +
>  Swap Extension allows you to record charge for swap. A swapped-in page is
>  charged back to original page allocator if possible.
>  
> @@ -150,13 +187,20 @@ When swap is accounted, following files 
>   - memory.memsw.usage_in_bytes.
>   - memory.memsw.limit_in_bytes.
>  
> -usage of mem+swap is limited by memsw.limit_in_bytes.
> +memsw means memory+swap. Usage of memory+swap is limited by
> +memsw.limit_in_bytes.
> +
> +Example: Assume a system with 4G of swap. A task which allocates 6G of memory
> +(by mistake) under 2G memory limitation will use all swap.
> +In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
> +By using memsw limit, you can avoid system OOM which can be caused by swap
> +shortage.
>  
> -* why 'mem+swap' rather than swap.
> +* why 'memory+swap' rather than swap.
>  The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
>  to move account from memory to swap...there is no change in usage of
> -mem+swap. In other words, when we want to limit the usage of swap without
> -affecting global LRU, mem+swap limit is better than just limiting swap from
> +memory+swap. In other words, when we want to limit the usage of swap without
> +affecting global LRU, memory+swap limit is better than just limiting swap from
>  OS point of view.
>  
>  * What happens when a cgroup hits memory.memsw.limit_in_bytes
> @@ -168,12 +212,12 @@ it by cgroup.
>  
>  2.5 Reclaim
>  
> -Each cgroup maintains a per cgroup LRU that consists of an active
> -and inactive list. When a cgroup goes over its limit, we first try
> +Each cgroup maintains a per cgroup LRU which has the same structure as
> +global VM. When a cgroup goes over its limit, we first try
>  to reclaim memory from the cgroup so as to make space for the new
>  pages that the cgroup has touched. If the reclaim is unsuccessful,
>  an OOM routine is invoked to select and kill the bulkiest task in the
> -cgroup.
> +cgroup. (See 10. OOM Control below.)
>  
>  The reclaim algorithm has not been modified for cgroups, except that
>  pages that are selected for reclaiming come from the per cgroup LRU
> @@ -187,13 +231,19 @@ Note2: When panic_on_oom is set to "2", 
>  When oom event notifier is registered, event will be delivered.
>  (See oom_control section)
>  
> -2. Locking
> +2.6 Locking
>  
> -The memory controller uses the following hierarchy
> +   lock_page_cgroup()/unlock_page_cgroup() should not be called under
> +   mapping->tree_lock.
>  
> -1. zone->lru_lock is used for selecting pages to be isolated
> -2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone)
> -3. lock_page_cgroup() is used to protect page->page_cgroup
> +   Other lock order is following:
> +   PG_locked.
> +   mm->page_table_lock
> +       zone->lru_lock
> +	  lock_page_cgroup.
> +  In many cases, just lock_page_cgroup() is called.
> +  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
> +  zone->lru_lock, it has no lock of its own.
>  
>  3. User Interface
>  
> @@ -202,6 +252,7 @@ The memory controller uses the following
>  a. Enable CONFIG_CGROUPS
>  b. Enable CONFIG_RESOURCE_COUNTERS
>  c. Enable CONFIG_CGROUP_MEM_RES_CTLR
> +d. Enable CONFIG_CGROUP_MEM_RES_CTLR_SWAP (to use swap extension)
>  
>  1. Prepare the cgroups
>  # mkdir -p /cgroups
> @@ -209,31 +260,29 @@ c. Enable CONFIG_CGROUP_MEM_RES_CTLR
>  
>  2. Make the new group and move bash into it
>  # mkdir /cgroups/0
> -# echo $$ >  /cgroups/0/tasks
> +# echo $$ > /cgroups/0/tasks
>  
>  Since now we're in the 0 cgroup,
>  We can alter the memory limit:
>  # echo 4M > /cgroups/0/memory.limit_in_bytes
>  
>  NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
> -mega or gigabytes.
> +mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
> +
>  NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
>  NOTE: We cannot set limits on the root cgroup any more.
>  
>  # cat /cgroups/0/memory.limit_in_bytes
>  4194304
>  
> -NOTE: The interface has now changed to display the usage in bytes
> -instead of pages
> -
>  We can check the usage:
>  # cat /cgroups/0/memory.usage_in_bytes
>  1216512
>  
>  A successful write to this file does not guarantee a successful set of
> -this limit to the value written into the file.  This can be due to a
> +this limit to the value written into the file. This can be due to a
>  number of factors, such as rounding up to page boundaries or the total
> -availability of memory on the system.  The user is required to re-read
> +availability of memory on the system. The user is required to re-read
>  this file after a write to guarantee the value committed by the kernel.
>  
>  # echo 1 > memory.limit_in_bytes
> @@ -248,15 +297,24 @@ caches, RSS and Active pages/Inactive pa
>  
>  4. Testing
>  
> -Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].
> -Apart from that v6 has been tested with several applications and regular
> -daily use. The controller has also been tested on the PPC64, x86_64 and
> -UML platforms.
> +For testing features and implementation, see memcg_test.txt.
> +
> +Performance test is also important. To see pure memory cgroup's overhead,
> +testing on tmpfs will give you good numbers of small overheads.
> +Example: do kernel make on tmpfs.
> +
> +Page-fault scalability is also important. At measuring parallel
> +page fault test, multi-process test may be better than multi-thread
> +test because it has noise of shared objects/status.
> +
> +But above 2 is testing extreme situation. Trying usual test under memory cgroup
> +is always helpful.
> +
>  
>  4.1 Troubleshooting
>  
>  Sometimes a user might find that the application under a cgroup is
> -terminated. There are several causes for this:
> +terminated by OOM killer. There are several causes for this:
>  
>  1. The cgroup limit is too low (just too low to do anything useful)
>  2. The user is using anonymous memory and swap is turned off or too low
> @@ -264,6 +322,9 @@ terminated. There are several causes for
>  A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
>  some of the pages cached in the cgroup (page cache pages).
>  
> +To know what happens, disable OOM_Kill by 10. OOM Control(see below) and
> +seeing what happens will be helpful.
> +
>  4.2 Task migration
>  
>  When a task migrates from one cgroup to another, it's charge is not
> @@ -271,16 +332,19 @@ carried forward by default. The pages al
>  remain charged to it, the charge is dropped when the page is freed or
>  reclaimed.
>  
> -Note: You can move charges of a task along with task migration. See 8.
> +You can move charges of a task along with task migration.
> +See 8. "Move charges at task migration"
>  
>  4.3 Removing a cgroup
>  
>  A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
>  cgroup might have some charge associated with it, even though all
> -tasks have migrated away from it.
> -Such charges are freed(at default) or moved to its parent. When moved,
> -both of RSS and CACHES are moved to parent.
> -If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also.
> +tasks have migrated away from it. (because we charge against pages, not
> +against tasks.)
> +
> +Such charges are freed or moved to their parent. At moving, both of RSS
> +and CACHES are moved to parent.
> +rmdir() may return -EBUSY if freeing/moving fails. See 5.1 also.
>  
>  Charges recorded in swap information is not updated at removal of cgroup.
>  Recorded information is discarded and a cgroup which uses swap (swapcache)
> @@ -296,10 +360,10 @@ will be charged as a new owner of it.
>  
>    # echo 0 > memory.force_empty
>  
> -  Almost all pages tracked by this memcg will be unmapped and freed. Some of
> -  pages cannot be freed because it's locked or in-use. Such pages are moved
> -  to parent and this cgroup will be empty. But this may return -EBUSY in
> -  some too busy case.
> +  Almost all pages tracked by this memory cgroup will be unmapped and freed.
> +  Some pages cannot be freed because they are locked or in-use. Such pages are
> +  moved to parent and this cgroup will be empty. This may return -EBUSY if
> +  VM is too busy to free/move all pages immediately.
>  
>    Typical use case of this interface is that calling this before rmdir().
>    Because rmdir() moves all pages to parent, some out-of-use page caches can be
> @@ -309,19 +373,41 @@ will be charged as a new owner of it.
>  
>  memory.stat file includes following statistics
>  
> +# per-memory cgroup local status
>  cache		- # of bytes of page cache memory.
>  rss		- # of bytes of anonymous and swap cache memory.
> +mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
>  pgpgin		- # of pages paged in (equivalent to # of charging events).
>  pgpgout		- # of pages paged out (equivalent to # of uncharging events).
> -active_anon	- # of bytes of anonymous and  swap cache memory on active
> -		  lru list.
> +swap		- # of bytes of swap usage
>  inactive_anon	- # of bytes of anonymous memory and swap cache memory on
> -		  inactive lru list.
> -active_file	- # of bytes of file-backed memory on active lru list.
> -inactive_file	- # of bytes of file-backed memory on inactive lru list.
> +		LRU list.
> +active_anon	- # of bytes of anonymous and swap cache memory on active
> +		inactive LRU list.
> +inactive_file	- # of bytes of file-backed memory on inactive LRU list.
> +active_file	- # of bytes of file-backed memory on active LRU list.
>  unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).
>  
> -The following additional stats are dependent on CONFIG_DEBUG_VM.
> +# status considering hierarchy (see memory.use_hierarchy settings)
> +
> +hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
> +			under which the memory cgroup is
> +hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
> +			hierarchy under which memory cgroup is.
> +
> +total_cache		- sum of all children's "cache"
> +total_rss		- sum of all children's "rss"
> +total_mapped_file	- sum of all children's "cache"
> +total_pgpgin		- sum of all children's "pgpgin"
> +total_pgpgout		- sum of all children's "pgpgout"
> +total_swap		- sum of all children's "swap"
> +total_inactive_anon	- sum of all children's "inactive_anon"
> +total_active_anon	- sum of all children's "active_anon"
> +total_inactive_file	- sum of all children's "inactive_file"
> +total_active_file	- sum of all children's "active_file"
> +total_unevictable	- sum of all children's "unevictable"
> +
> +# The following additional stats are dependent on CONFIG_DEBUG_VM.
>  
>  inactive_ratio		- VM internal parameter. (see mm/page_alloc.c)
>  recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)
> @@ -330,24 +416,37 @@ recent_scanned_anon	- VM internal parame
>  recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)
>  
>  Memo:
> -	recent_rotated means recent frequency of lru rotation.
> -	recent_scanned means recent # of scans to lru.
> +	recent_rotated means recent frequency of LRU rotation.
> +	recent_scanned means recent # of scans to LRU.
>  	showing for better debug please see the code for meanings.
>  
>  Note:
>  	Only anonymous and swap cache memory is listed as part of 'rss' stat.
>  	This should not be confused with the true 'resident set size' or the
> -	amount of physical memory used by the cgroup. Per-cgroup rss
> -	accounting is not done yet.
> +	amount of physical memory used by the cgroup.
> +	'rss + file_mapped" will give you resident set size of cgroup.
> +	(Note: file and shmem may be shared among other cgroups. In that case,
> +	 file_mapped is accounted only when the memory cgroup is owner of page
> +	 cache.)
>  
>  5.3 swappiness
> -  Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
>  
> -  Following cgroups' swappiness can't be changed.
> -  - root cgroup (uses /proc/sys/vm/swappiness).
> -  - a cgroup which uses hierarchy and it has child cgroup.
> -  - a cgroup which uses hierarchy and not the root of hierarchy.
> +Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
> +
> +Following cgroups' swappiness can't be changed.
> +- root cgroup (uses /proc/sys/vm/swappiness).
> +- a cgroup which uses hierarchy and it has other cgroup(s) below it.
> +- a cgroup which uses hierarchy and not the root of hierarchy.
> +
> +5.4 failcnt
> +
> +A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
> +This failcnt(== failure count) shows the number of times that a usage counter
> +hit its limit. When a memory cgroup hits a limit, failcnt increases and
> +memory under it will be reclaimed.
>  
> +You can reset failcnt by writing 0 to failcnt file.
> +# echo 0 > .../memory.failcnt
>  
>  6. Hierarchy support
>  
> @@ -366,13 +465,13 @@ hierarchy
>  
>  In the diagram above, with hierarchical accounting enabled, all memory
>  usage of e, is accounted to its ancestors up until the root (i.e, c and root),
> -that has memory.use_hierarchy enabled.  If one of the ancestors goes over its
> +that has memory.use_hierarchy enabled. If one of the ancestors goes over its
>  limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
>  children of the ancestor.
>  
>  6.1 Enabling hierarchical accounting and reclaim
>  
> -The memory controller by default disables the hierarchy feature. Support
> +A memory cgroup by default disables the hierarchy feature. Support
>  can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
>  
>  # echo 1 > memory.use_hierarchy
> @@ -385,7 +484,7 @@ NOTE1: Enabling/disabling will fail if t
>  cgroups created below it.
>  
>  NOTE2: When panic_on_oom is set to "2", the whole system will panic in
> -case of an oom event in any cgroup.
> +case of an OOM event in any cgroup.
>  
>  7. Soft limits
>  
> @@ -395,7 +494,7 @@ is to allow control groups to use as muc
>  a. There is no memory contention
>  b. They do not exceed their hard limit
>  
> -When the system detects memory contention or low memory control groups
> +When the system detects memory contention or low memory, control groups
>  are pushed back to their soft limits. If the soft limit of each control
>  group is very high, they are pushed back as much as possible to make
>  sure that one control group does not starve the others of memory.
> @@ -409,7 +508,7 @@ it gets invoked from balance_pgdat (kswa
>  7.1 Interface
>  
>  Soft limits can be setup by using the following commands (in this example we
> -assume a soft limit of 256 megabytes)
> +assume a soft limit of 256 MiB)
>  
>  # echo 256M > memory.soft_limit_in_bytes
>  
> @@ -418,7 +517,7 @@ If we want to change this to 1G, we can 
>  # echo 1G > memory.soft_limit_in_bytes
>  
>  NOTE1: Soft limits take effect over a long period of time, since they involve
> -       reclaiming memory for balancing between memory cgroups
> +reclaiming memory for balancing between memory cgroups
>  NOTE2: It is recommended to set the soft limit always below the hard limit,
>         otherwise the hard limit will take precedence.
>  
> @@ -445,7 +544,7 @@ Note: Charges are moved only when you mo
>  Note: If we cannot find enough space for the task in the destination cgroup, we
>        try to make space by reclaiming memory. Task migration may fail if we
>        cannot make enough space.
> -Note: It can take several seconds if you move charges in giga bytes order.
> +Note: It can take several seconds if you move charges much.
>  
>  And if you want disable it again:
>  
> @@ -476,15 +575,15 @@ Note: More type of pages(e.g. file cache
>  
>  9. Memory thresholds
>  
> -Memory controler implements memory thresholds using cgroups notification
> +Memory cgroup implements memory thresholds using cgroups notification
>  API (see cgroups.txt). It allows to register multiple memory and memsw
>  thresholds and gets notifications when it crosses.
>  
>  To register a threshold application need:
> - - create an eventfd using eventfd(2);
> - - open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
> - - write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
> -   cgroup.event_control.
> +- create an eventfd using eventfd(2);
> +- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
> +- write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
> +  cgroup.event_control.
>  
>  Application will be notified through eventfd when memory usage crosses
>  threshold in any direction.
> @@ -495,27 +594,27 @@ It's applicable for root and non-root cg
>  
>  memory.oom_control file is for OOM notification and other controls.
>  
> -Memory controler implements oom notifier using cgroup notification
> -API (See cgroups.txt). It allows to register multiple oom notification
> -delivery and gets notification when oom happens.
> +Memory cgroup implements OOM notifier using cgroup notification
> +API (See cgroups.txt). It allows to register multiple OOM notification
> +delivery and gets notification when OOM happens.
>  
>  To register a notifier, application need:
>   - create an eventfd using eventfd(2)
>   - open memory.oom_control file
>   - write string like "<event_fd> <memory.oom_control>" to cgroup.event_control
>  
> -Application will be notifier through eventfd when oom happens.
> +Application will be notifier through eventfd when OOM happens.
>  OOM notification doesn't work for root cgroup.
>  
> -You can disable oom-killer by writing "1" to memory.oom_control file.
> +You can disable OOM-killer by writing "1" to memory.oom_control file.
>  As.
>  	#echo 1 > memory.oom_control
>  
> -This operation is only allowed to the top cgroup of subhierarchy.
> -If oom-killer is disabled, tasks under cgroup will hang/sleep
> -in memcg's oom-waitq when they request accountable memory.
> +This operation is only allowed to the top cgroup of sub-hierarchy.
> +If OOM-killer is disabled, tasks under cgroup will hang/sleep
> +in memory cgroup's OOM-waitqueue when they request accountable memory.
>  
> -For running them, you have to relax the memcg's oom sitaution by
> +For running them, you have to relax the memory cgroup's OOM status by
>  	* enlarge limit or reduce usage.
>  To reduce usage,
>  	* kill some tasks.
> @@ -526,7 +625,7 @@ Then, stopped tasks will work again.
>  
>  At reading, current status of OOM is shown.
>  	oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
> -	under_oom	 0 or 1 (if 1, the memcg is under OOM,tasks may
> +	under_oom	 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
>  				 be stopped.)
>  
>  11. TODO
> 
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