628f423553
Shrinking memory usage at limit change. [akpm@linux-foundation.org: coding-style fixes] Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com> Acked-by: Pavel Emelyanov <xemul@openvz.org> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Paul Menage <menage@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
276 lines
10 KiB
Text
276 lines
10 KiB
Text
Memory Resource Controller
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NOTE: The Memory Resource Controller has been generically been referred
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to as the memory controller in this document. Do not confuse memory controller
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used here with the memory controller that is used in hardware.
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Salient features
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a. Enable control of both RSS (mapped) and Page Cache (unmapped) pages
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b. The infrastructure allows easy addition of other types of memory to control
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c. Provides *zero overhead* for non memory controller users
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d. Provides a double LRU: global memory pressure causes reclaim from the
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global LRU; a cgroup on hitting a limit, reclaims from the per
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cgroup LRU
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NOTE: Swap Cache (unmapped) is not accounted now.
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Benefits and Purpose of the memory controller
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The memory controller isolates the memory behaviour of a group of tasks
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from the rest of the system. The article on LWN [12] mentions some probable
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uses of the memory controller. The memory controller can be used to
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a. Isolate an application or a group of applications
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Memory hungry applications can be isolated and limited to a smaller
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amount of memory.
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b. Create a cgroup with limited amount of memory, this can be used
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as a good alternative to booting with mem=XXXX.
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c. Virtualization solutions can control the amount of memory they want
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to assign to a virtual machine instance.
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d. A CD/DVD burner could control the amount of memory used by the
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rest of the system to ensure that burning does not fail due to lack
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of available memory.
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e. There are several other use cases, find one or use the controller just
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for fun (to learn and hack on the VM subsystem).
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1. History
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The memory controller has a long history. A request for comments for the memory
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controller was posted by Balbir Singh [1]. At the time the RFC was posted
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there were several implementations for memory control. The goal of the
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RFC was to build consensus and agreement for the minimal features required
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for memory control. The first RSS controller was posted by Balbir Singh[2]
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in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
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RSS controller. At OLS, at the resource management BoF, everyone suggested
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that we handle both page cache and RSS together. Another request was raised
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to allow user space handling of OOM. The current memory controller is
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at version 6; it combines both mapped (RSS) and unmapped Page
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Cache Control [11].
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2. Memory Control
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Memory is a unique resource in the sense that it is present in a limited
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amount. If a task requires a lot of CPU processing, the task can spread
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its processing over a period of hours, days, months or years, but with
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memory, the same physical memory needs to be reused to accomplish the task.
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The memory controller implementation has been divided into phases. These
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are:
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1. Memory controller
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2. mlock(2) controller
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3. Kernel user memory accounting and slab control
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4. user mappings length controller
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The memory controller is the first controller developed.
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2.1. Design
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The core of the design is a counter called the res_counter. The res_counter
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tracks the current memory usage and limit of the group of processes associated
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with the controller. Each cgroup has a memory controller specific data
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structure (mem_cgroup) associated with it.
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2.2. Accounting
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+--------------------+
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| mem_cgroup |
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| (res_counter) |
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+--------------------+
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/ ^ \
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/ | \
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+---------------+ | +---------------+
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| mm_struct | |.... | mm_struct |
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| | | | |
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+---------------+ | +---------------+
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+ --------------+
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+---------------+ +------+--------+
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| page +----------> page_cgroup|
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| | | |
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+---------------+ +---------------+
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(Figure 1: Hierarchy of Accounting)
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Figure 1 shows the important aspects of the controller
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1. Accounting happens per cgroup
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2. Each mm_struct knows about which cgroup it belongs to
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3. Each page has a pointer to the page_cgroup, which in turn knows the
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cgroup it belongs to
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The accounting is done as follows: mem_cgroup_charge() is invoked to setup
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the necessary data structures and check if the cgroup that is being charged
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is over its limit. If it is then reclaim is invoked on the cgroup.
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More details can be found in the reclaim section of this document.
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If everything goes well, a page meta-data-structure called page_cgroup is
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allocated and associated with the page. This routine also adds the page to
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the per cgroup LRU.
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2.2.1 Accounting details
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All mapped pages (RSS) and unmapped user pages (Page Cache) are accounted.
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RSS pages are accounted at the time of page_add_*_rmap() unless they've already
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been accounted for earlier. A file page will be accounted for as Page Cache;
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it's mapped into the page tables of a process, duplicate accounting is carefully
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avoided. Page Cache pages are accounted at the time of add_to_page_cache().
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The corresponding routines that remove a page from the page tables or removes
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a page from Page Cache is used to decrement the accounting counters of the
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cgroup.
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2.3 Shared Page Accounting
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Shared pages are accounted on the basis of the first touch approach. The
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cgroup that first touches a page is accounted for the page. The principle
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behind this approach is that a cgroup that aggressively uses a shared
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page will eventually get charged for it (once it is uncharged from
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the cgroup that brought it in -- this will happen on memory pressure).
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2.4 Reclaim
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Each cgroup maintains a per cgroup LRU that consists of an active
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and inactive list. When a cgroup goes over its limit, we first try
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to reclaim memory from the cgroup so as to make space for the new
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pages that the cgroup has touched. If the reclaim is unsuccessful,
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an OOM routine is invoked to select and kill the bulkiest task in the
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cgroup.
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The reclaim algorithm has not been modified for cgroups, except that
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pages that are selected for reclaiming come from the per cgroup LRU
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list.
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2. Locking
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The memory controller uses the following hierarchy
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1. zone->lru_lock is used for selecting pages to be isolated
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2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone)
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3. lock_page_cgroup() is used to protect page->page_cgroup
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3. User Interface
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0. Configuration
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a. Enable CONFIG_CGROUPS
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b. Enable CONFIG_RESOURCE_COUNTERS
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c. Enable CONFIG_CGROUP_MEM_RES_CTLR
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1. Prepare the cgroups
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# mkdir -p /cgroups
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# mount -t cgroup none /cgroups -o memory
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2. Make the new group and move bash into it
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# mkdir /cgroups/0
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# echo $$ > /cgroups/0/tasks
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Since now we're in the 0 cgroup,
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We can alter the memory limit:
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# echo 4M > /cgroups/0/memory.limit_in_bytes
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NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
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mega or gigabytes.
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# cat /cgroups/0/memory.limit_in_bytes
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4194304
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NOTE: The interface has now changed to display the usage in bytes
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instead of pages
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We can check the usage:
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# cat /cgroups/0/memory.usage_in_bytes
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1216512
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A successful write to this file does not guarantee a successful set of
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this limit to the value written into the file. This can be due to a
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number of factors, such as rounding up to page boundaries or the total
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availability of memory on the system. The user is required to re-read
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this file after a write to guarantee the value committed by the kernel.
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# echo 1 > memory.limit_in_bytes
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# cat memory.limit_in_bytes
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4096
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The memory.failcnt field gives the number of times that the cgroup limit was
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exceeded.
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The memory.stat file gives accounting information. Now, the number of
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caches, RSS and Active pages/Inactive pages are shown.
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The memory.force_empty gives an interface to drop *all* charges by force.
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# echo 1 > memory.force_empty
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will drop all charges in cgroup. Currently, this is maintained for test.
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4. Testing
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Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].
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Apart from that v6 has been tested with several applications and regular
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daily use. The controller has also been tested on the PPC64, x86_64 and
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UML platforms.
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4.1 Troubleshooting
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Sometimes a user might find that the application under a cgroup is
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terminated. There are several causes for this:
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1. The cgroup limit is too low (just too low to do anything useful)
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2. The user is using anonymous memory and swap is turned off or too low
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A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
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some of the pages cached in the cgroup (page cache pages).
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4.2 Task migration
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When a task migrates from one cgroup to another, it's charge is not
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carried forward. The pages allocated from the original cgroup still
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remain charged to it, the charge is dropped when the page is freed or
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reclaimed.
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4.3 Removing a cgroup
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A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
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cgroup might have some charge associated with it, even though all
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tasks have migrated away from it. Such charges are automatically dropped at
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rmdir() if there are no tasks.
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5. TODO
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1. Add support for accounting huge pages (as a separate controller)
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2. Make per-cgroup scanner reclaim not-shared pages first
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3. Teach controller to account for shared-pages
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4. Start reclamation in the background when the limit is
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not yet hit but the usage is getting closer
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Summary
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Overall, the memory controller has been a stable controller and has been
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commented and discussed quite extensively in the community.
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References
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1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
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2. Singh, Balbir. Memory Controller (RSS Control),
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http://lwn.net/Articles/222762/
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3. Emelianov, Pavel. Resource controllers based on process cgroups
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http://lkml.org/lkml/2007/3/6/198
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4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
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http://lkml.org/lkml/2007/4/9/78
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5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
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http://lkml.org/lkml/2007/5/30/244
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6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
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7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
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subsystem (v3), http://lwn.net/Articles/235534/
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8. Singh, Balbir. RSS controller v2 test results (lmbench),
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http://lkml.org/lkml/2007/5/17/232
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9. Singh, Balbir. RSS controller v2 AIM9 results
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http://lkml.org/lkml/2007/5/18/1
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10. Singh, Balbir. Memory controller v6 test results,
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http://lkml.org/lkml/2007/8/19/36
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11. Singh, Balbir. Memory controller introduction (v6),
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http://lkml.org/lkml/2007/8/17/69
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12. Corbet, Jonathan, Controlling memory use in cgroups,
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http://lwn.net/Articles/243795/
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