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diff --git a/kernel/Documentation/cgroups/memory.txt b/kernel/Documentation/cgroups/memory.txt new file mode 100644 index 000000000..f456b4315 --- /dev/null +++ b/kernel/Documentation/cgroups/memory.txt @@ -0,0 +1,875 @@ +Memory Resource Controller + +NOTE: This document is hopelessly outdated and it asks for a complete + rewrite. It still contains a useful information so we are keeping it + here but make sure to check the current code if you need a deeper + understanding. + +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 + - memory pressure 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 usage for memory + (See 5.5 for details) + memory.memsw.usage_in_bytes # show current 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.pressure_level # set memory pressure notifications + 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 page_counter. The +page_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 | + | (page_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_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 every time 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. + +2.7.2 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. + WARNING: In the current implementation, memory reclaim will NOT be + triggered for a cgroup when it hits K while staying below U, which makes + this setup impractical. + + 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 + +3.0. Configuration + +a. Enable CONFIG_CGROUPS +b. Enable CONFIG_MEMCG +c. Enable CONFIG_MEMCG_SWAP (to use swap extension) +d. Enable CONFIG_MEMCG_KMEM (to use kmem extension) + +3.1. 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 + +3.2. 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. + When writing anything to this + + # echo 0 > memory.force_empty + + the cgroup will be reclaimed and as many pages reclaimed as possible. + + 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 (includes + transparent hugepages). +rss_huge - # of bytes of anonymous transparent hugepages. +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 +writeback - # of bytes of file/anon cache that are queued for syncing to + disk. +inactive_anon - # of bytes of anonymous and swap cache memory on inactive + LRU list. +active_anon - # of bytes of anonymous and swap cache 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). + +# 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 + +Overrides /proc/sys/vm/swappiness for the particular group. The tunable +in the root cgroup corresponds to the global swappiness setting. + +Please note that unlike during the global reclaim, limit reclaim +enforces that 0 swappiness 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. + +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. + +Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable" +per-node page counts including "hierarchical_<counter>" which sums up all +hierarchical children's values in addition to the memcg's own value. + +The output 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> ... +hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ... + +The "total" count is sum of 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 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, 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 + +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. Memory Pressure + +The pressure level notifications can be used to monitor the memory +allocation cost; based on the pressure, applications can implement +different strategies of managing their memory resources. The pressure +levels are defined as following: + +The "low" level means that the system is reclaiming memory for new +allocations. Monitoring this reclaiming activity might be useful for +maintaining cache level. Upon notification, the program (typically +"Activity Manager") might analyze vmstat and act in advance (i.e. +prematurely shutdown unimportant services). + +The "medium" level means that the system is experiencing medium memory +pressure, the system might be making swap, paging out active file caches, +etc. Upon this event applications may decide to further analyze +vmstat/zoneinfo/memcg or internal memory usage statistics and free any +resources that can be easily reconstructed or re-read from a disk. + +The "critical" level means that the system is actively thrashing, it is +about to out of memory (OOM) or even the in-kernel OOM killer is on its +way to trigger. Applications should do whatever they can to help the +system. It might be too late to consult with vmstat or any other +statistics, so it's advisable to take an immediate action. + +The events are propagated upward until the event is handled, i.e. the +events are not pass-through. Here is what this means: for example you have +three cgroups: A->B->C. Now you set up an event listener on cgroups A, B +and C, and suppose group C experiences some pressure. In this situation, +only group C will receive the notification, i.e. groups A and B will not +receive it. This is done to avoid excessive "broadcasting" of messages, +which disturbs the system and which is especially bad if we are low on +memory or thrashing. So, organize the cgroups wisely, or propagate the +events manually (or, ask us to implement the pass-through events, +explaining why would you need them.) + +The file memory.pressure_level is only used to setup an eventfd. To +register a notification, an application must: + +- create an eventfd using eventfd(2); +- open memory.pressure_level; +- write string like "<event_fd> <fd of memory.pressure_level> <level>" + to cgroup.event_control. + +Application will be notified through eventfd when memory pressure is at +the specific level (or higher). Read/write operations to +memory.pressure_level are no implemented. + +Test: + + Here is a small script example that makes a new cgroup, sets up a + memory limit, sets up a notification in the cgroup and then makes child + cgroup experience a critical pressure: + + # cd /sys/fs/cgroup/memory/ + # mkdir foo + # cd foo + # cgroup_event_listener memory.pressure_level low & + # echo 8000000 > memory.limit_in_bytes + # echo 8000000 > memory.memsw.limit_in_bytes + # echo $$ > tasks + # dd if=/dev/zero | read x + + (Expect a bunch of notifications, and eventually, the oom-killer will + trigger.) + +12. TODO + +1. Make per-cgroup scanner reclaim not-shared pages first +2. Teach controller to account for shared-pages +3. 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/ |