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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 a 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 a 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.limit_in_bytes # set/show hard limit for kernel memory
+ memory.kmem.usage_in_bytes # show current kernel memory allocation
+ memory.kmem.failcnt # show the number of kernel memory usage hits limits
+ memory.kmem.max_usage_in_bytes # show max kernel memory usage recorded
+
+ 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
+ memory.kmem.tcp.failcnt # show the number of tcp buf memory usage hits limits
+ memory.kmem.tcp.max_usage_in_bytes # show max tcp buf memory usage recorded
+
+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_common() is invoked to
+set up 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.
+
+An 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 are also accounted.
+A swapped-in page is not accounted until it's mapped.
+
+Note: The kernel does swapin-readahead and reads 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_MEMCG_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_MEMCG_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 the 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
+an 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_MEMCG_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 won't be accounted at all until limit on a group is set. This
+allows for existing setups to continue working without disruption. The limit
+cannot be set if the cgroup have children, or if there are already tasks in the
+cgroup. Attempting to set the limit under those conditions will return -EBUSY.
+When use_hierarchy == 1 and a group is accounted, its children will
+automatically be accounted regardless of their limit value.
+
+After a group is first limited, it will be kept being accounted until it
+is removed. The memory limitation itself, can of course be removed by writing
+-1 to memory.kmem.limit_in_bytes. In this case, kmem will be accounted, but not
+limited.
+
+Kernel memory limits are not imposed for the root cgroup. Usage for the root
+cgroup may or may not be accounted. The memory used is accumulated into
+memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
+(currently only for tcp).
+The main "kmem" counter is fed into the main counter, so kmem charges will
+also be visible from the user counter.
+
+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
+
+* stack pages: every process consumes some stack pages. By accounting into
+kernel memory, we prevent new processes from being created when the kernel
+memory usage is too high.
+
+* slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy
+of each kmem_cache is created everytime the cache is touched by the first time
+from inside the memcg. The creation is done lazily, so some objects can still be
+skipped while the cache is being created. All objects in a slab page should
+belong to the same memcg. This only fails to hold when a task is migrated to a
+different memcg during the page allocation by the cache.
+
+* 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.
+
+2.7.3 Common use cases
+
+Because the "kmem" counter is fed to the main user counter, kernel memory can
+never be limited completely independently of user memory. Say "U" is the user
+limit, and "K" the kernel limit. There are three possible ways limits can be
+set:
+
+ U != 0, K = unlimited:
+ This is the standard memcg limitation mechanism already present before kmem
+ accounting. Kernel memory is completely ignored.
+
+ U != 0, K < U:
+ Kernel memory is a subset of the user memory. This setup is useful in
+ deployments where the total amount of memory per-cgroup is overcommited.
+ Overcommiting kernel memory limits is definitely not recommended, since the
+ box can still run out of non-reclaimable memory.
+ In this case, the admin could set up K so that the sum of all groups is
+ never greater than the total memory, and freely set U at the cost of his
+ QoS.
+
+ U != 0, K >= U:
+ Since kmem charges will also be fed to the user counter and reclaim will be
+ triggered for the cgroup for both kinds of memory. This setup gives the
+ admin a unified view of memory, and it is also useful for people who just
+ want to track kernel memory usage.
+
+3. User Interface
+
+0. Configuration
+
+a. Enable CONFIG_CGROUPS
+b. Enable CONFIG_RESOURCE_COUNTERS
+c. Enable CONFIG_MEMCG
+d. Enable CONFIG_MEMCG_SWAP (to use swap extension)
+d. Enable CONFIG_MEMCG_KMEM (to use kmem 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 setting 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 the 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, disabling OOM_Kill as per "10. OOM Control" (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.
+
+ The typical use case for this interface is before calling 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.
+
+ Also, note that when memory.kmem.limit_in_bytes is set the charges due to
+ kernel pages will still be seen. This is not considered a failure and the
+ write will still return success. In this case, it is expected that
+ memory.kmem.usage_in_bytes == memory.usage_in_bytes.
+
+ 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.
+Please note that unlike the global swappiness, memcg knob set to 0
+really prevents from any swapping even if there is a swap storage
+available. This might lead to memcg OOM killer if there are no file
+pages to reclaim.
+
+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 a 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 use cases 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 set up 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 enabledi (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, in other words,
+ 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 moved
+
+Each bit in move_charge_at_immigrate has its own meaning about what type of
+charges should be moved. But in any case, 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 the 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, an application must:
+- 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 the cgroup notification
+API (See cgroups.txt). It allows to register multiple OOM notification
+delivery and gets notification when OOM happens.
+
+To register a notifier, an application must:
+ - 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
+
+The application will be notified through eventfd when OOM happens.
+OOM notification doesn't work for the root cgroup.
+
+You can disable the 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 a 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/