Re: [PATCH] memcg: update documentation v3

[Daisuke Nishimura]

It seems nice overall.

Thank you for your work.

I have a few minor comments.

On Thu, 8 Apr 2010 14:58:00 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> 
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Documentation update for memory cgroup
> 
> Some informations are old, and  I think current document doesn't work
> as "a guide for users".
> We need summary of all of our controls, at least.
> 
> This patch updates information for current implementations and add a
> summary of interfaces. etc...
> 
> Changelog:
>  - fixed tons of typos.
>  - replaced "memcg" with "memory cgroup" AMAP.
>  - replaced "mem+swap" with "memory+swap"
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  Documentation/cgroups/memory.txt |  210 ++++++++++++++++++++++++++++-----------
>  1 file changed, 152 insertions(+), 58 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)
> + cgroup.procs			 # attach a process(all threads under it)
IIUC, writing to cgroup.procs isn't supported yet. So, I think we don't have to
bother explaining cgroup.procs here.

> + 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 hit limits
> + memory.memsw.failcnt		 # show the number of memory+Swap hit 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 +150,18 @@ 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. It's bacause we can't
> +know a page will be finaly mapped at swapin-readahead happens.
> +
> +A Cache pages is unaccounted when it's removed from inode (radix-tree).
>  
Is this line necessary ? We say the similar thing above("A PageCache page is
unaccounted when it's removed from radix-tree.").

>  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.
> +of used pages. not-on-lru pages are tend to be out-of-control from VM view.
>  
>  2.3 Shared Page Accounting
>  
> @@ -143,6 +178,7 @@ 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 +186,20 @@ 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.
>  
> -* why 'mem+swap' rather than swap.
> +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.
> +(Bad process will cause OOM under the memory cgroup. By using memsw limit,
> +you can avoid system OOM which can be caused by swap shortage.)
> +
> +* 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 +211,12 @@ 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
> @@ -189,11 +232,17 @@ When oom event notifier is registered, e
>  
>  2. Locking
>  
hmm, chapter 2 is used as "2. Memory Control" already :(
"2.6 Locking" is better ?

> -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 +251,7 @@ 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
> @@ -216,16 +266,14 @@ 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
> @@ -248,15 +296,24 @@ 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 +321,9 @@ 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 +331,19 @@ 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 +359,10 @@ 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 of pages cannot be freed because it's 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 +372,41 @@ 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
> +		  lru list.
> +active_anon	- # of bytes of anonymous and  swap cache memory on active
>  		  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.
> +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)
> @@ -337,17 +422,26 @@ Memo:
>  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 amoung 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 it has other cgroup(s) below it.
>    - a cgroup which uses hierarchy and not the root of hierarchy.
>  
> +5.4 failcnt
> +
> +The memory controller 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 controller hit a limit, failcnt increases and
> +memory under it will be reclaimed.
>  
I think it would be better to explain we can reset it by writing to the file.

>  6. Hierarchy support
>  
> @@ -395,7 +489,7 @@ 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 +503,7 @@ 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
>  
> @@ -445,7 +539,7 @@ 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:
>  
> @@ -513,9 +607,9 @@ As.
>  
>  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.
> +in memory cgroup's oom-waitq 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 sitaution by
>  	* enlarge limit or reduce usage.
>  To reduce usage,
>  	* kill some tasks.
> @@ -526,7 +620,7 @@ 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
> 

Otherwise, it looks good to me.


Thanks,
Daisuke Nishimura.
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