454 lines
17 KiB
Plaintext
454 lines
17 KiB
Plaintext
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 Anonymous, Page Cache (mapped and unmapped) and
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Swap Cache memory 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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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 anon pages (RSS) and cache pages (Page Cache) are accounted.
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(some pages which never be reclaimable and will not be on global LRU
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are not accounted. we just accounts pages under usual vm management.)
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RSS pages are accounted at page_fault unless they've already been accounted
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for earlier. A file page will be accounted for as Page Cache when it's
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inserted into inode (radix-tree). While it's mapped into the page tables of
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processes, duplicate accounting is carefully avoided.
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A RSS page is unaccounted when it's fully unmapped. A PageCache page is
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unaccounted when it's removed from radix-tree.
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At page migration, accounting information is kept.
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Note: we just account pages-on-lru because our purpose is to control amount
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of used pages. not-on-lru pages are tend to be out-of-control from vm view.
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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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Exception: If CONFIG_CGROUP_CGROUP_MEM_RES_CTLR_SWAP is not used..
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When you do swapoff and make swapped-out pages of shmem(tmpfs) to
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be backed into memory in force, charges for pages are accounted against the
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caller of swapoff rather than the users of shmem.
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2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)
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Swap Extension allows you to record charge for swap. A swapped-in page is
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charged back to original page allocator if possible.
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When swap is accounted, following files are added.
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- memory.memsw.usage_in_bytes.
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- memory.memsw.limit_in_bytes.
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usage of mem+swap is limited by memsw.limit_in_bytes.
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* why 'mem+swap' rather than swap.
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The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
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to move account from memory to swap...there is no change in usage of
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mem+swap. In other words, when we want to limit the usage of swap without
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affecting global LRU, mem+swap limit is better than just limiting swap from
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OS point of view.
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* What happens when a cgroup hits memory.memsw.limit_in_bytes
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When a cgroup his memory.memsw.limit_in_bytes, it's useless to do swap-out
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in this cgroup. Then, swap-out will not be done by cgroup routine and file
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caches are dropped. But as mentioned above, global LRU can do swapout memory
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from it for sanity of the system's memory management state. You can't forbid
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it by cgroup.
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2.5 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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NOTE: Reclaim does not work for the root cgroup, since we cannot set any
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limits on the root cgroup.
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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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NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
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NOTE: We cannot set limits on the root cgroup any more.
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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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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.
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Such charges are freed(at default) or moved to its parent. When moved,
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both of RSS and CACHES are moved to parent.
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If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also.
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Charges recorded in swap information is not updated at removal of cgroup.
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Recorded information is discarded and a cgroup which uses swap (swapcache)
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will be charged as a new owner of it.
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5. Misc. interfaces.
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5.1 force_empty
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memory.force_empty interface is provided to make cgroup's memory usage empty.
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You can use this interface only when the cgroup has no tasks.
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When writing anything to this
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# echo 0 > memory.force_empty
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Almost all pages tracked by this memcg will be unmapped and freed. Some of
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pages cannot be freed because it's locked or in-use. Such pages are moved
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to parent and this cgroup will be empty. But this may return -EBUSY in
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some too busy case.
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Typical use case of this interface is that calling this before rmdir().
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Because rmdir() moves all pages to parent, some out-of-use page caches can be
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moved to the parent. If you want to avoid that, force_empty will be useful.
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5.2 stat file
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memory.stat file includes following statistics
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cache - # of bytes of page cache memory.
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rss - # of bytes of anonymous and swap cache memory.
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pgpgin - # of pages paged in (equivalent to # of charging events).
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pgpgout - # of pages paged out (equivalent to # of uncharging events).
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active_anon - # of bytes of anonymous and swap cache memory on active
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lru list.
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inactive_anon - # of bytes of anonymous memory and swap cache memory on
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inactive lru list.
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active_file - # of bytes of file-backed memory on active lru list.
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inactive_file - # of bytes of file-backed memory on inactive lru list.
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unevictable - # of bytes of memory that cannot be reclaimed (mlocked etc).
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The following additional stats are dependent on CONFIG_DEBUG_VM.
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inactive_ratio - VM internal parameter. (see mm/page_alloc.c)
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recent_rotated_anon - VM internal parameter. (see mm/vmscan.c)
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recent_rotated_file - VM internal parameter. (see mm/vmscan.c)
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recent_scanned_anon - VM internal parameter. (see mm/vmscan.c)
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recent_scanned_file - VM internal parameter. (see mm/vmscan.c)
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Memo:
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recent_rotated means recent frequency of lru rotation.
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recent_scanned means recent # of scans to lru.
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showing for better debug please see the code for meanings.
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Note:
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Only anonymous and swap cache memory is listed as part of 'rss' stat.
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This should not be confused with the true 'resident set size' or the
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amount of physical memory used by the cgroup. Per-cgroup rss
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accounting is not done yet.
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5.3 swappiness
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Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
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Following cgroups' swapiness can't be changed.
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- root cgroup (uses /proc/sys/vm/swappiness).
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- a cgroup which uses hierarchy and it has child cgroup.
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- a cgroup which uses hierarchy and not the root of hierarchy.
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6. Hierarchy support
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The memory controller supports a deep hierarchy and hierarchical accounting.
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The hierarchy is created by creating the appropriate cgroups in the
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cgroup filesystem. Consider for example, the following cgroup filesystem
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hierarchy
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root
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/ | \
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/ | \
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a b c
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d e
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In the diagram above, with hierarchical accounting enabled, all memory
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usage of e, is accounted to its ancestors up until the root (i.e, c and root),
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that has memory.use_hierarchy enabled. If one of the ancestors goes over its
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limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
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children of the ancestor.
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6.1 Enabling hierarchical accounting and reclaim
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The memory controller by default disables the hierarchy feature. Support
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can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
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# echo 1 > memory.use_hierarchy
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The feature can be disabled by
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# echo 0 > memory.use_hierarchy
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NOTE1: Enabling/disabling will fail if the cgroup already has other
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cgroups created below it.
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NOTE2: This feature can be enabled/disabled per subtree.
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7. Soft limits
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Soft limits allow for greater sharing of memory. The idea behind soft limits
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is to allow control groups to use as much of the memory as needed, provided
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a. There is no memory contention
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b. They do not exceed their hard limit
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When the system detects memory contention or low memory control groups
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are pushed back to their soft limits. If the soft limit of each control
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group is very high, they are pushed back as much as possible to make
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sure that one control group does not starve the others of memory.
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Please note that soft limits is a best effort feature, it comes with
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no guarantees, but it does its best to make sure that when memory is
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heavily contended for, memory is allocated based on the soft limit
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hints/setup. Currently soft limit based reclaim is setup such that
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it gets invoked from balance_pgdat (kswapd).
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7.1 Interface
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Soft limits can be setup by using the following commands (in this example we
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assume a soft limit of 256 megabytes)
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# echo 256M > memory.soft_limit_in_bytes
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If we want to change this to 1G, we can at any time use
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# echo 1G > memory.soft_limit_in_bytes
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NOTE1: Soft limits take effect over a long period of time, since they involve
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reclaiming memory for balancing between memory cgroups
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NOTE2: It is recommended to set the soft limit always below the hard limit,
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otherwise the hard limit will take precedence.
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8. 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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