| 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| NOTE: This document is hopelessly outdated and it asks for a complete |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 |
| the i_pages 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 accounting is enabled for all memory cgroups by default. But |
| it can be disabled system-wide by passing cgroup.memory=nokmem to the kernel |
| at boot time. In this case, kernel memory will not be accounted at all. |
| |
| 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 |
| dirty - # of bytes that are waiting to get written back to the disk. |
| 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 + mapped_file" will give you resident set size of cgroup. |
| (Note: file and shmem may be shared among other cgroups. In that case, |
| mapped_file 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. |
| |
| By default, events are propagated upward until the event is handled, i.e. the |
| events are not pass-through. 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. Group B, will receive |
| notification only if there are no event listers for group C. |
| |
| There are three optional modes that specify different propagation behavior: |
| |
| - "default": this is the default behavior specified above. This mode is the |
| same as omitting the optional mode parameter, preserved by backwards |
| compatibility. |
| |
| - "hierarchy": events always propagate up to the root, similar to the default |
| behavior, except that propagation continues regardless of whether there are |
| event listeners at each level, with the "hierarchy" mode. In the above |
| example, groups A, B, and C will receive notification of memory pressure. |
| |
| - "local": events are pass-through, i.e. they only receive notifications when |
| memory pressure is experienced in the memcg for which the notification is |
| registered. In the above example, group C will receive notification if |
| registered for "local" notification and the group experiences memory |
| pressure. However, group B will never receive notification, regardless if |
| there is an event listener for group C or not, if group B is registered for |
| local notification. |
| |
| The level and event notification mode ("hierarchy" or "local", if necessary) are |
| specified by a comma-delimited string, i.e. "low,hierarchy" specifies |
| hierarchical, pass-through, notification for all ancestor memcgs. Notification |
| that is the default, non pass-through behavior, does not specify a mode. |
| "medium,local" specifies pass-through notification for the medium level. |
| |
| 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 as "<event_fd> <fd of memory.pressure_level> <level[,mode]>" |
| 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,hierarchy & |
| # 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/ |