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Diffstat (limited to 'Documentation/cgroups/memory.txt')
| -rw-r--r-- | Documentation/cgroups/memory.txt | 162 |
1 files changed, 115 insertions, 47 deletions
diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt index 4372e6b8a353..8b8c28b9864c 100644 --- a/Documentation/cgroups/memory.txt +++ b/Documentation/cgroups/memory.txt @@ -18,16 +18,16 @@ 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 + Memory-hungry applications can be isolated and limited to a smaller amount of memory. -b. Create a cgroup with limited amount of memory, this can be used +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 +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) @@ -38,12 +38,12 @@ Features: - optionally, memory+swap usage can be accounted and limited. - hierarchical accounting - soft limit - - moving(recharging) account at moving a task is selectable. + - moving (recharging) account at moving a task is selectable. - usage threshold notifier - oom-killer disable knob and oom-notifier - Root cgroup has no limit controls. - Kernel memory support is work in progress, and the current version provides + Kernel memory support is a work in progress, and the current version provides basically functionality. (See Section 2.7) Brief summary of control files. @@ -71,6 +71,11 @@ Brief summary of control files. 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 @@ -144,9 +149,9 @@ Figure 1 shows the important aspects of the controller 3. Each page has a pointer to the page_cgroup, which in turn knows the cgroup it belongs to -The accounting is done as follows: mem_cgroup_charge() is invoked to setup -the necessary data structures and check if the cgroup that is being charged -is over its limit. If it is then reclaim is invoked on the cgroup. +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. @@ -163,13 +168,13 @@ for earlier. A file page will be accounted for as Page Cache when it's inserted into inode (radix-tree). While it's mapped into the page tables of processes, duplicate accounting is carefully avoided. -A RSS page is unaccounted when it's fully unmapped. A PageCache page is +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 also also accounted. +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 read multiple swaps at once. +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. @@ -209,7 +214,7 @@ memsw.limit_in_bytes. Example: Assume a system with 4G of swap. A task which allocates 6G of memory (by mistake) under 2G memory limitation will use all swap. In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap. -By using memsw limit, you can avoid system OOM which can be caused by swap +By using the memsw limit, you can avoid system OOM which can be caused by swap shortage. * why 'memory+swap' rather than swap. @@ -217,7 +222,7 @@ The global LRU(kswapd) can swap out arbitrary pages. Swap-out means to move account from memory to swap...there is no change in usage of memory+swap. In other words, when we want to limit the usage of swap without affecting global LRU, memory+swap limit is better than just limiting swap from -OS point of view. +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 @@ -236,7 +241,7 @@ 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 +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 @@ -268,20 +273,73 @@ 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. +cgroup may or may not be accounted. The memory used is accumulated into +memory.kmem.usage_in_bytes, or in a separate counter when it makes sense. +(currently only for tcp). +The main "kmem" counter is fed into the main counter, so kmem charges will +also be visible from the user counter. Currently no soft limit is implemented for kernel memory. It is future work to trigger slab reclaim when those limits are reached. 2.7.1 Current Kernel Memory resources accounted +* stack pages: every process consumes some stack pages. By accounting into +kernel memory, we prevent new processes from being created when the kernel +memory usage is too high. + +* slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy +of each kmem_cache is created everytime the cache is touched by the first time +from inside the memcg. The creation is done lazily, so some objects can still be +skipped while the cache is being created. All objects in a slab page should +belong to the same memcg. This only fails to hold when a task is migrated to a +different memcg during the page allocation by the cache. + * sockets memory pressure: some sockets protocols have memory pressure thresholds. The Memory Controller allows them to be controlled individually per cgroup, instead of globally. * tcp memory pressure: sockets memory pressure for the tcp protocol. +2.7.3 Common use cases + +Because the "kmem" counter is fed to the main user counter, kernel memory can +never be limited completely independently of user memory. Say "U" is the user +limit, and "K" the kernel limit. There are three possible ways limits can be +set: + + U != 0, K = unlimited: + This is the standard memcg limitation mechanism already present before kmem + accounting. Kernel memory is completely ignored. + + U != 0, K < U: + Kernel memory is a subset of the user memory. This setup is useful in + deployments where the total amount of memory per-cgroup is overcommited. + Overcommiting kernel memory limits is definitely not recommended, since the + box can still run out of non-reclaimable memory. + In this case, the admin could set up K so that the sum of all groups is + never greater than the total memory, and freely set U at the cost of his + QoS. + + U != 0, K >= U: + Since kmem charges will also be fed to the user counter and reclaim will be + triggered for the cgroup for both kinds of memory. This setup gives the + admin a unified view of memory, and it is also useful for people who just + want to track kernel memory usage. + 3. User Interface 0. Configuration @@ -290,6 +348,7 @@ 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 @@ -316,7 +375,7 @@ We can check the usage: # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes 1216512 -A successful write to this file does not guarantee a successful set of +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 @@ -350,7 +409,7 @@ Trying usual test under memory controller is always helpful. 4.1 Troubleshooting Sometimes a user might find that the application under a cgroup is -terminated by OOM killer. There are several causes for this: +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 @@ -358,7 +417,7 @@ terminated by OOM killer. There are several causes for this: A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of some of the pages cached in the cgroup (page cache pages). -To know what happens, disable OOM_Kill by 10. OOM Control(see below) and +To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and seeing what happens will be helpful. 4.2 Task migration @@ -399,13 +458,18 @@ About use_hierarchy, see Section 6. 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 + moved to parent (if use_hierarchy==1) or root (if use_hierarchy==0) and this cgroup will be empty. - Typical use case of this interface is that calling this before rmdir(). + 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 @@ -466,6 +530,10 @@ Note: 5.3 swappiness Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only. +Please note that unlike the global swappiness, memcg knob set to 0 +really prevents from any swapping even if there is a swap storage +available. This might lead to memcg OOM killer if there are no file +pages to reclaim. Following cgroups' swappiness can't be changed. - root cgroup (uses /proc/sys/vm/swappiness). @@ -486,7 +554,7 @@ You can reset failcnt by writing 0 to failcnt file. For efficiency, as other kernel components, memory cgroup uses some optimization to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the -method and doesn't show 'exact' value of memory(and swap) usage, it's an fuzz +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). @@ -496,8 +564,8 @@ value in memory.stat(see 5.2). This is similar to numa_maps but operates on a per-memcg basis. This is useful for providing visibility into the numa locality information within an memcg since the pages are allowed to be allocated from any physical -node. One of the usecases is evaluating application performance by -combining this information with the application's cpu allocation. +node. One of the use cases is evaluating application performance by +combining this information with the application's CPU allocation. We export "total", "file", "anon" and "unevictable" pages per-node for each memcg. The ouput format of memory.numa_stat is: @@ -561,10 +629,10 @@ 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 +Please note that soft limits is a best-effort feature; it comes with no guarantees, but it does its best to make sure that when memory is heavily contended for, memory is allocated based on the soft limit -hints/setup. Currently soft limit based reclaim is setup such that +hints/setup. Currently soft limit based reclaim is set up such that it gets invoked from balance_pgdat (kswapd). 7.1 Interface @@ -592,7 +660,7 @@ page tables. 8.1 Interface -This feature is disabled by default. It can be enabled(and disabled again) by +This feature is disabled by default. It can be enabledi (and disabled again) by writing to memory.move_charge_at_immigrate of the destination cgroup. If you want to enable it: @@ -601,8 +669,8 @@ If you want to enable it: Note: Each bits of move_charge_at_immigrate has its own meaning about what type of charges should be moved. See 8.2 for details. -Note: Charges are moved only when you move mm->owner, IOW, a leader of a thread - group. +Note: 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. @@ -612,25 +680,25 @@ And if you want disable it again: # echo 0 > memory.move_charge_at_immigrate -8.2 Type of charges which can be move +8.2 Type of charges which can be moved -Each bits of move_charge_at_immigrate has its own meaning about what type of -charges should be moved. But in any cases, it must be noted that an account of -a page or a swap can be moved only when it is charged to the task's current(old) -memory cgroup. +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. + 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) + 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 + | 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 + | 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 @@ -640,11 +708,11 @@ memory cgroup. 9. Memory thresholds -Memory cgroup implements memory thresholds using cgroups notification +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 application need: +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 @@ -659,24 +727,24 @@ It's applicable for root and non-root cgroup. memory.oom_control file is for OOM notification and other controls. -Memory cgroup implements OOM notifier using cgroup notification +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, application need: +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 -Application will be notified through eventfd when OOM happens. -OOM notification doesn't work for root cgroup. +The application will be notified through eventfd when OOM happens. +OOM notification doesn't work for the root cgroup. -You can disable OOM-killer by writing "1" to memory.oom_control file, as: +You can disable the OOM-killer by writing "1" to memory.oom_control file, as: #echo 1 > memory.oom_control -This operation is only allowed to the top cgroup of sub-hierarchy. +This operation is only allowed to the top cgroup of a sub-hierarchy. If OOM-killer is disabled, tasks under cgroup will hang/sleep in memory cgroup's OOM-waitqueue when they request accountable memory. |
