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-Memory Resource Controller
-
-NOTE: The Memory Resource Controller has generically been referred to as the
- memory controller in this document. Do not confuse memory controller
- used here with the memory controller that is used in hardware.
-
-(For editors)
-In this document:
- When we mention a cgroup (cgroupfs's directory) with memory controller,
- we call it "memory cgroup". When you see git-log and source code, you'll
- see patch's title and function names tend to use "memcg".
- In this document, we avoid using it.
-
-Benefits and Purpose of the memory controller
-
-The memory controller isolates the memory behaviour of a group of tasks
-from the rest of the system. The article on LWN [12] mentions some probable
-uses of the memory controller. The memory controller can be used to
-
-a. Isolate an application or a group of applications
- Memory hungry applications can be isolated and limited to a smaller
- amount of memory.
-b. Create a cgroup with limited amount of memory, this can be used
- as a good alternative to booting with mem=XXXX.
-c. Virtualization solutions can control the amount of memory they want
- to assign to a virtual machine instance.
-d. A CD/DVD burner could control the amount of memory used by the
- rest of the system to ensure that burning does not fail due to lack
- of available memory.
-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 limiting them.
- - pages are linked to per-memcg LRU exclusively, and there is no global LRU.
- - 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 support is work in progress, and the current version provides
- basically functionality. (See Section 2.7)
-
-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 res_counter usage for memory
- (See 5.5 for details)
- memory.memsw.usage_in_bytes # show current res_counter usage for memory+Swap
- (See 5.5 for details)
- 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.max_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.
- memory.numa_stat # show the number of memory usage per numa node
-
- memory.kmem.tcp.limit_in_bytes # set/show hard limit for tcp buf memory
- memory.kmem.tcp.usage_in_bytes # show current tcp buf memory allocation
-
-1. History
-
-The memory controller has a long history. A request for comments for the memory
-controller was posted by Balbir Singh [1]. At the time the RFC was posted
-there were several implementations for memory control. The goal of the
-RFC was to build consensus and agreement for the minimal features required
-for memory control. The first RSS controller was posted by Balbir Singh[2]
-in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
-RSS controller. At OLS, at the resource management BoF, everyone suggested
-that we handle both page cache and RSS together. Another request was raised
-to allow user space handling of OOM. The current memory controller is
-at version 6; it combines both mapped (RSS) and unmapped Page
-Cache Control [11].
-
-2. Memory Control
-
-Memory is a unique resource in the sense that it is present in a limited
-amount. If a task requires a lot of CPU processing, the task can spread
-its processing over a period of hours, days, months or years, but with
-memory, the same physical memory needs to be reused to accomplish the task.
-
-The memory controller implementation has been divided into phases. These
-are:
-
-1. Memory controller
-2. mlock(2) controller
-3. Kernel user memory accounting and slab control
-4. user mappings length controller
-
-The memory controller is the first controller developed.
-
-2.1. Design
-
-The core of the design is a counter called the res_counter. The res_counter
-tracks the current memory usage and limit of the group of processes associated
-with the controller. Each cgroup has a memory controller specific data
-structure (mem_cgroup) associated with it.
-
-2.2. Accounting
-
- +--------------------+
- | mem_cgroup |
- | (res_counter) |
- +--------------------+
- / ^ \
- / | \
- +---------------+ | +---------------+
- | mm_struct | |.... | mm_struct |
- | | | | |
- +---------------+ | +---------------+
- |
- + --------------+
- |
- +---------------+ +------+--------+
- | page +----------> page_cgroup|
- | | | |
- +---------------+ +---------------+
-
- (Figure 1: Hierarchy of Accounting)
-
-
-Figure 1 shows the important aspects of the controller
-
-1. Accounting happens per cgroup
-2. Each mm_struct knows about which cgroup it belongs to
-3. Each page has a pointer to the page_cgroup, which in turn knows the
- cgroup it belongs to
-
-The accounting is done as follows: mem_cgroup_charge() is invoked to setup
-the necessary data structures and check if the cgroup that is being charged
-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
-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 are never reclaimable and will not be on the LRU
-are not accounted. We just account 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
-inserted into inode (radix-tree). While it's mapped into the page tables of
-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. 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 tend to be out-of-control from VM view.
-
-2.3 Shared Page Accounting
-
-Shared pages are accounted on the basis of the first touch approach. The
-cgroup that first touches a page is accounted for the page. The principle
-behind this approach is that a cgroup that aggressively uses a shared
-page will eventually get charged for it (once it is uncharged from
-the cgroup that brought it in -- this will happen on memory pressure).
-
-But see section 8.2: when moving a task to another cgroup, its pages may
-be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
-
-Exception: If CONFIG_CGROUP_CGROUP_MEM_RES_CTLR_SWAP is not used.
-When you do swapoff and make swapped-out pages of shmem(tmpfs) to
-be backed into memory in force, charges for pages are accounted against the
-caller of swapoff rather than the users of shmem.
-
-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.
-
-When swap is accounted, following files are added.
- - memory.memsw.usage_in_bytes.
- - memory.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 '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
-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
-When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
-in this cgroup. Then, swap-out will not be done by cgroup routine and file
-caches are dropped. But as mentioned above, global LRU can do swapout memory
-from it for sanity of the system's memory management state. You can't forbid
-it by cgroup.
-
-2.5 Reclaim
-
-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. (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
-list.
-
-NOTE: Reclaim does not work for the root cgroup, since we cannot set any
-limits on the root cgroup.
-
-Note2: When panic_on_oom is set to "2", the whole system will panic.
-
-When oom event notifier is registered, event will be delivered.
-(See oom_control section)
-
-2.6 Locking
-
- lock_page_cgroup()/unlock_page_cgroup() should not be called under
- mapping->tree_lock.
-
- 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.
-
-2.7 Kernel Memory Extension (CONFIG_CGROUP_MEM_RES_CTLR_KMEM)
-
-With the Kernel memory extension, the Memory Controller is able to limit
-the amount of kernel memory used by the system. Kernel memory is fundamentally
-different than user memory, since it can't be swapped out, which makes it
-possible to DoS the system by consuming too much of this precious resource.
-
-Kernel memory limits are not imposed for the root cgroup. Usage for the root
-cgroup may or may not be accounted.
-
-Currently no soft limit is implemented for kernel memory. It is future work
-to trigger slab reclaim when those limits are reached.
-
-2.7.1 Current Kernel Memory resources accounted
-
-* sockets memory pressure: some sockets protocols have memory pressure
-thresholds. The Memory Controller allows them to be controlled individually
-per cgroup, instead of globally.
-
-* tcp memory pressure: sockets memory pressure for the tcp protocol.
-
-3. User Interface
-
-0. Configuration
-
-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 (see cgroups.txt, Why are cgroups needed?)
-# mount -t tmpfs none /sys/fs/cgroup
-# mkdir /sys/fs/cgroup/memory
-# mount -t cgroup none /sys/fs/cgroup/memory -o memory
-
-2. Make the new group and move bash into it
-# mkdir /sys/fs/cgroup/memory/0
-# echo $$ > /sys/fs/cgroup/memory/0/tasks
-
-Since now we're in the 0 cgroup, we can alter the memory limit:
-# echo 4M > /sys/fs/cgroup/memory/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. (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 /sys/fs/cgroup/memory/0/memory.limit_in_bytes
-4194304
-
-We can check the usage:
-# cat /sys/fs/cgroup/memory/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
-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
-this file after a write to guarantee the value committed by the kernel.
-
-# echo 1 > memory.limit_in_bytes
-# cat memory.limit_in_bytes
-4096
-
-The memory.failcnt field gives the number of times that the cgroup limit was
-exceeded.
-
-The memory.stat file gives accounting information. Now, the number of
-caches, RSS and Active pages/Inactive pages are shown.
-
-4. Testing
-
-For testing features and implementation, see memcg_test.txt.
-
-Performance test is also important. To see pure memory controller'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 the above two are testing extreme situations.
-Trying usual test under memory controller is always helpful.
-
-4.1 Troubleshooting
-
-Sometimes a user might find that the application under a cgroup is
-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
-
-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, its charge is not
-carried forward by default. The pages allocated from the original cgroup still
-remain charged to it, the charge is dropped when the page is freed or
-reclaimed.
-
-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. (because we charge against pages, not
-against tasks.)
-
-We move the stats to root (if use_hierarchy==0) or parent (if
-use_hierarchy==1), and no change on the charge except uncharging
-from the child.
-
-Charges recorded in swap information is not updated at removal of cgroup.
-Recorded information is discarded and a cgroup which uses swap (swapcache)
-will be charged as a new owner of it.
-
-About use_hierarchy, see Section 6.
-
-5. Misc. interfaces.
-
-5.1 force_empty
- memory.force_empty interface is provided to make cgroup's memory usage empty.
- You can use this interface only when the cgroup has no tasks.
- When writing anything to this
-
- # echo 0 > memory.force_empty
-
- 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(if use_hierarchy==1) or root (if use_hierarchy==0) and this
- cgroup will be empty.
-
- 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
- moved to the parent. If you want to avoid that, force_empty will be useful.
-
- About use_hierarchy, see Section 6.
-
-5.2 stat file
-
-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 charging events to the memory cgroup. The charging
- event happens each time a page is accounted as either mapped
- anon page(RSS) or cache page(Page Cache) to the cgroup.
-pgpgout - # of uncharging events to the memory cgroup. The uncharging
- event happens each time a page is unaccounted from the cgroup.
-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.
-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).
-
-# 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_<counter> - # hierarchical version of <counter>, which in
- addition to the cgroup's own value includes the
- sum of all hierarchical children's values of
- <counter>, i.e. total_cache
-
-# The following additional stats are dependent on CONFIG_DEBUG_VM.
-
-recent_rotated_anon - VM internal parameter. (see mm/vmscan.c)
-recent_rotated_file - VM internal parameter. (see mm/vmscan.c)
-recent_scanned_anon - VM internal parameter. (see mm/vmscan.c)
-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.
- 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.
- '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 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
-
-5.5 usage_in_bytes
-
-For efficiency, as other kernel components, memory cgroup uses some optimization
-to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
-method and doesn't show 'exact' value of memory(and swap) usage, it's an fuzz
-value for efficient access. (Of course, when necessary, it's synchronized.)
-If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
-value in memory.stat(see 5.2).
-
-5.6 numa_stat
-
-This is similar to numa_maps but operates on a per-memcg basis. This is
-useful for providing visibility into the numa locality information within
-an memcg since the pages are allowed to be allocated from any physical
-node. One of the usecases is evaluating application performance by
-combining this information with the application's cpu allocation.
-
-We export "total", "file", "anon" and "unevictable" pages per-node for
-each memcg. The ouput format of memory.numa_stat is:
-
-total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
-file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
-anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
-unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
-
-And we have total = file + anon + unevictable.
-
-6. Hierarchy support
-
-The memory controller supports a deep hierarchy and hierarchical accounting.
-The hierarchy is created by creating the appropriate cgroups in the
-cgroup filesystem. Consider for example, the following cgroup filesystem
-hierarchy
-
- root
- / | \
- / | \
- a b c
- | \
- | \
- d e
-
-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
-limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
-children of the ancestor.
-
-6.1 Enabling hierarchical accounting and reclaim
-
-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
-
-The feature can be disabled by
-
-# echo 0 > memory.use_hierarchy
-
-NOTE1: Enabling/disabling will fail if either the cgroup already has other
- cgroups created below it, or if the parent cgroup has use_hierarchy
- enabled.
-
-NOTE2: When panic_on_oom is set to "2", the whole system will panic in
- case of an OOM event in any cgroup.
-
-7. Soft limits
-
-Soft limits allow for greater sharing of memory. The idea behind soft limits
-is to allow control groups to use as much of the memory as needed, provided
-
-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
-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.
-
-Please note that soft limits is a best effort feature, it comes with
-no guarantees, but it does its best to make sure that when memory is
-heavily contended for, memory is allocated based on the soft limit
-hints/setup. Currently soft limit based reclaim is setup such that
-it gets invoked from balance_pgdat (kswapd).
-
-7.1 Interface
-
-Soft limits can be setup by using the following commands (in this example we
-assume a soft limit of 256 MiB)
-
-# echo 256M > memory.soft_limit_in_bytes
-
-If we want to change this to 1G, we can at any time use
-
-# 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
-NOTE2: It is recommended to set the soft limit always below the hard limit,
- otherwise the hard limit will take precedence.
-
-8. Move charges at task migration
-
-Users can move charges associated with a task along with task migration, that
-is, uncharge task's pages from the old cgroup and charge them to the new cgroup.
-This feature is not supported in !CONFIG_MMU environments because of lack of
-page tables.
-
-8.1 Interface
-
-This feature is disabled by default. It can be enabled(and disabled again) by
-writing to memory.move_charge_at_immigrate of the destination cgroup.
-
-If you want to enable it:
-
-# echo (some positive value) > memory.move_charge_at_immigrate
-
-Note: Each bits of move_charge_at_immigrate has its own meaning about what type
- of charges should be moved. See 8.2 for details.
-Note: Charges are moved only when you move mm->owner, IOW, a leader of a thread
- group.
-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 much.
-
-And if you want disable it again:
-
-# echo 0 > memory.move_charge_at_immigrate
-
-8.2 Type of charges which can be move
-
-Each bits of move_charge_at_immigrate has its own meaning about what type of
-charges should be moved. But in any cases, it must be noted that an account of
-a page or a swap can be moved only when it is charged to the task's current(old)
-memory cgroup.
-
- bit | what type of charges would be moved ?
- -----+------------------------------------------------------------------------
- 0 | A charge of an anonymous page(or swap of it) used by the target task.
- | You must enable Swap Extension(see 2.4) to enable move of swap charges.
- -----+------------------------------------------------------------------------
- 1 | A charge of file pages(normal file, tmpfs file(e.g. ipc shared memory)
- | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
- | anonymous pages, file pages(and swaps) in the range mmapped by the task
- | will be moved even if the task hasn't done page fault, i.e. they might
- | not be the task's "RSS", but other task's "RSS" that maps the same file.
- | And mapcount of the page is ignored(the page can be moved even if
- | page_mapcount(page) > 1). You must enable Swap Extension(see 2.4) to
- | enable move of swap charges.
-
-8.3 TODO
-
-- All of moving charge operations are done under cgroup_mutex. It's not good
- behavior to hold the mutex too long, so we may need some trick.
-
-9. Memory thresholds
-
-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> <fd of memory.usage_in_bytes> <threshold>" to
- cgroup.event_control.
-
-Application will be notified through eventfd when memory usage crosses
-threshold in any direction.
-
-It's applicable for root and non-root cgroup.
-
-10. OOM Control
-
-memory.oom_control file is for OOM notification and other controls.
-
-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> <fd of memory.oom_control>" to
- cgroup.event_control
-
-Application will be notified 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, as:
-
- #echo 1 > memory.oom_control
-
-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 memory cgroup's OOM status by
- * enlarge limit or reduce usage.
-To reduce usage,
- * kill some tasks.
- * move some tasks to other group with account migration.
- * remove some files (on tmpfs?)
-
-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 memory cgroup is under OOM, tasks may
- be stopped.)
-
-11. TODO
-
-1. Add support for accounting huge pages (as a separate controller)
-2. Make per-cgroup scanner reclaim not-shared pages first
-3. Teach controller to account for shared-pages
-4. Start reclamation in the background when the limit is
- not yet hit but the usage is getting closer
-
-Summary
-
-Overall, the memory controller has been a stable controller and has been
-commented and discussed quite extensively in the community.
-
-References
-
-1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
-2. Singh, Balbir. Memory Controller (RSS Control),
- http://lwn.net/Articles/222762/
-3. Emelianov, Pavel. Resource controllers based on process cgroups
- http://lkml.org/lkml/2007/3/6/198
-4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
- http://lkml.org/lkml/2007/4/9/78
-5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
- http://lkml.org/lkml/2007/5/30/244
-6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
-7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
- subsystem (v3), http://lwn.net/Articles/235534/
-8. Singh, Balbir. RSS controller v2 test results (lmbench),
- http://lkml.org/lkml/2007/5/17/232
-9. Singh, Balbir. RSS controller v2 AIM9 results
- http://lkml.org/lkml/2007/5/18/1
-10. Singh, Balbir. Memory controller v6 test results,
- http://lkml.org/lkml/2007/8/19/36
-11. Singh, Balbir. Memory controller introduction (v6),
- http://lkml.org/lkml/2007/8/17/69
-12. Corbet, Jonathan, Controlling memory use in cgroups,
- http://lwn.net/Articles/243795/