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 tasksfrom the rest of the system. The article on LWN [12] mentions some probableuses 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 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.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 memorycontroller was posted by Balbir Singh [1]. At the time the RFC was postedthere were several implementations for memory control. The goal of theRFC was to build consensus and agreement for the minimal features requiredfor 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 theRSS controller. At OLS, at the resource management BoF, everyone suggestedthat we handle both page cache and RSS together. Another request was raisedto allow user space handling of OOM. The current memory controller isat version 6; it combines both mapped (RSS) and unmapped PageCache Control [11]. 2. Memory Control Memory is a unique resource in the sense that it is present in a limitedamount. If a task requires a lot of CPU processing, the task can spreadits processing over a period of hours, days, months or years, but withmemory, the same physical memory needs to be reused to accomplish the task. The memory controller implementation has been divided into phases. Theseare: 1. Memory controller2. mlock(2) controller3. Kernel user memory accounting and slab control4. 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_countertracks the current memory usage and limit of the group of processes associatedwith the controller. Each cgroup has a memory controller specific datastructure (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 cgroup2. Each mm_struct knows about which cgroup it belongs to3. 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 toset up the necessary data structures and check if the cgroup that is beingcharged 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 isupdated. 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 LRUare not accounted. We just account pages under usual VM management. RSS pages are accounted at page_fault unless they've already been accountedfor earlier. A file page will be accounted for as Page Cache when it'sinserted into inode (radix-tree). While it's mapped into the page tables ofprocesses, duplicate accounting is carefully avoided. An RSS page is unaccounted when it's fully unmapped. A PageCache page isunaccounted when it's removed from radix-tree. Even if RSS pages are fullyunmapped (by kswapd), they may exist as SwapCache in the system until theyare 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 taskcausing 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 amountof 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. Thecgroup that first touches a page is accounted for the page. The principlebehind this approach is that a cgroup that aggressively uses a sharedpage will eventually get charged for it (once it is uncharged fromthe 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 maybe 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) tobe backed into memory in force, charges for pages are accounted against thecaller 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 ischarged 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 bymemsw.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 swapshortage. * why 'memory+swap' rather than swap.The global LRU(kswapd) can swap out arbitrary pages. Swap-out meansto move account from memory to swap...there is no change in usage ofmemory+swap. In other words, when we want to limit the usage of swap withoutaffecting global LRU, memory+swap limit is better than just limiting swap froman OS point of view. * What happens when a cgroup hits memory.memsw.limit_in_bytesWhen a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-outin this cgroup. Then, swap-out will not be done by cgroup routine and filecaches are dropped. But as mentioned above, global LRU can do swapout memoryfrom it for sanity of the system's memory management state. You can't forbidit by cgroup. 2.5 Reclaim Each cgroup maintains a per cgroup LRU which has the same structure asglobal VM. When a cgroup goes over its limit, we first tryto reclaim memory from the cgroup so as to make space for the newpages that the cgroup has touched. If the reclaim is unsuccessful,an OOM routine is invoked to select and kill the bulkiest task in thecgroup. (See 10. OOM Control below.) The reclaim algorithm has not been modified for cgroups, except thatpages that are selected for reclaiming come from the per-cgroup LRUlist. NOTE: Reclaim does not work for the root cgroup, since we cannot set anylimits 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 limitthe amount of kernel memory used by the system. Kernel memory is fundamentallydifferent than user memory, since it can't be swapped out, which makes itpossible 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. Thisallows for existing setups to continue working without disruption. The limitcannot be set if the cgroup have children, or if there are already tasks in thecgroup. Attempting to set the limit under those conditions will return -EBUSY.When use_hierarchy == 1 and a group is accounted, its children willautomatically be accounted regardless of their limit value. After a group is first limited, it will be kept being accounted until itis 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 notlimited. Kernel memory limits are not imposed for the root cgroup. Usage for the rootcgroup may or may not be accounted. The memory used is accumulated intomemory.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 willalso be visible from the user counter. Currently no soft limit is implemented for kernel memory. It is future workto 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 intokernel memory, we prevent new processes from being created when the kernelmemory usage is too high. * slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copyof each kmem_cache is created every time the cache is touched by the first timefrom inside the memcg. The creation is done lazily, so some objects can still beskipped while the cache is being created. All objects in a slab page shouldbelong to the same memcg. This only fails to hold when a task is migrated to adifferent memcg during the page allocation by the cache. * sockets memory pressure: some sockets protocols have memory pressurethresholds. The Memory Controller allows them to be controlled individuallyper 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 cannever be limited completely independently of user memory. Say "U" is the userlimit, and "K" the kernel limit. There are three possible ways limits can beset: 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_CGROUPSb. Enable CONFIG_RESOURCE_COUNTERSc. Enable CONFIG_MEMCGd. 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_bytes4194304 We can check the usage:# cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes1216512 A successful write to this file does not guarantee a successful setting ofthis limit to the value written into the file. This can be due to anumber of factors, such as rounding up to page boundaries or the totalavailability of memory on the system. The user is required to re-readthis file after a write to guarantee the value committed by the kernel. # echo 1 > memory.limit_in_bytes# cat memory.limit_in_bytes4096 The memory.failcnt field gives the number of times that the cgroup limit wasexceeded. The memory.stat file gives accounting information. Now, the number ofcaches, 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 parallelpage fault test, multi-process test may be better than multi-threadtest 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 isterminated 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 ofsome of the pages cached in the cgroup (page cache pages). To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) andseeing what happens will be helpful. 4.2 Task migration When a task migrates from one cgroup to another, its charge is notcarried forward by default. The pages allocated from the original cgroup stillremain charged to it, the charge is dropped when the page is freed orreclaimed. 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, acgroup might have some charge associated with it, even though alltasks have migrated away from it. (because we charge against pages, notagainst tasks.) We move the stats to root (if use_hierarchy==0) or parent (ifuse_hierarchy==1), and no change on the charge except unchargingfrom 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 statuscache - # 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 usagewriteback - # 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.Lines 1–500 of 876. Load more
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 - 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 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.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 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_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. 2.7.3 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 (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.