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|
Memory Resource Controller
NOTE: This document is hopelessly outdated and it asks for a complete
rewrite. It still contains a useful information so we are keeping it
here but make sure to check the current code if you need a deeper
understanding.
NOTE: The Memory Resource Controller has generically been referred to as the
memory controller in this document. Do not confuse memory controller
used here with the memory controller that is used in hardware.
(For editors)
In this document:
When we mention a cgroup (cgroupfs's directory) with memory controller,
we call it "memory cgroup". When you see git-log and source code, you'll
see patch's title and function names tend to use "memcg".
In this document, we avoid using it.
Benefits and Purpose of the memory controller
The memory controller isolates the memory behaviour of a group of tasks
from the rest of the system. The article on LWN [12] mentions some probable
uses of the memory controller. The memory controller can be used to
a. Isolate an application or a group of applications
Memory-hungry applications can be isolated and limited to a smaller
amount of memory.
b. Create a cgroup with a limited amount of memory; this can be used
as a good alternative to booting with mem=XXXX.
c. Virtualization solutions can control the amount of memory they want
to assign to a virtual machine instance.
d. A CD/DVD burner could control the amount of memory used by the
rest of the system to ensure that burning does not fail due to lack
of available memory.
e. There are several other use cases; find one or use the controller just
for fun (to learn and hack on the VM subsystem).
Current Status: linux-2.6.34-mmotm(development version of 2010/April)
Features:
- accounting anonymous pages, file caches, swap caches usage and limiting them.
- pages are linked to per-memcg LRU exclusively, and there is no global LRU.
- optionally, memory+swap usage can be accounted and limited.
- hierarchical accounting
- soft limit
- moving (recharging) account at moving a task is selectable.
- usage threshold notifier
- memory pressure notifier
- oom-killer disable knob and oom-notifier
- Root cgroup has no limit controls.
Kernel memory support is a work in progress, and the current version provides
basically functionality. (See Section 2.7)
Brief summary of control files.
tasks # attach a task(thread) and show list of threads
cgroup.procs # show list of processes
cgroup.event_control # an interface for event_fd()
memory.usage_in_bytes # show current usage for memory
(See 5.5 for details)
memory.memsw.usage_in_bytes # show current usage for memory+Swap
(See 5.5 for details)
memory.limit_in_bytes # set/show limit of memory usage
memory.memsw.limit_in_bytes # set/show limit of memory+Swap usage
memory.failcnt # show the number of memory usage hits limits
memory.memsw.failcnt # show the number of memory+Swap hits limits
memory.max_usage_in_bytes # show max memory usage recorded
memory.memsw.max_usage_in_bytes # show max memory+Swap usage recorded
memory.soft_limit_in_bytes # set/show soft limit of memory usage
memory.stat # show various statistics
memory.use_hierarchy # set/show hierarchical account enabled
memory.force_empty # trigger forced move charge to parent
memory.pressure_level # set memory pressure notifications
memory.swappiness # set/show swappiness parameter of vmscan
(See sysctl's vm.swappiness)
memory.move_charge_at_immigrate # set/show controls of moving charges
memory.oom_control # set/show oom controls.
memory.numa_stat # show the number of memory usage per numa node
memory.kmem.limit_in_bytes # set/show hard limit for kernel memory
memory.kmem.usage_in_bytes # show current kernel memory allocation
memory.kmem.failcnt # show the number of kernel memory usage hits limits
memory.kmem.max_usage_in_bytes # show max kernel memory usage recorded
memory.kmem.tcp.limit_in_bytes # set/show hard limit for tcp buf memory
memory.kmem.tcp.usage_in_bytes # show current tcp buf memory allocation
memory.kmem.tcp.failcnt # show the number of tcp buf memory usage hits limits
memory.kmem.tcp.max_usage_in_bytes # show max tcp buf memory usage recorded
1. History
The memory controller has a long history. A request for comments for the memory
controller was posted by Balbir Singh [1]. At the time the RFC was posted
there were several implementations for memory control. The goal of the
RFC was to build consensus and agreement for the minimal features required
for memory control. The first RSS controller was posted by Balbir Singh[2]
in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
RSS controller. At OLS, at the resource management BoF, everyone suggested
that we handle both page cache and RSS together. Another request was raised
to allow user space handling of OOM. The current memory controller is
at version 6; it combines both mapped (RSS) and unmapped Page
Cache Control [11].
2. Memory Control
Memory is a unique resource in the sense that it is present in a limited
amount. If a task requires a lot of CPU processing, the task can spread
its processing over a period of hours, days, months or years, but with
memory, the same physical memory needs to be reused to accomplish the task.
The memory controller implementation has been divided into phases. These
are:
1. Memory controller
2. mlock(2) controller
3. Kernel user memory accounting and slab control
4. user mappings length controller
The memory controller is the first controller developed.
2.1. Design
The core of the design is a counter called the page_counter. The
page_counter tracks the current memory usage and limit of the group of
processes associated with the controller. Each cgroup has a memory controller
specific data structure (mem_cgroup) associated with it.
2.2. Accounting
+--------------------+
| mem_cgroup |
| (page_counter) |
+--------------------+
/ ^ \
/ | \
+---------------+ | +---------------+
| mm_struct | |.... | mm_struct |
| | | | |
+---------------+ | +---------------+
|
+ --------------+
|
+---------------+ +------+--------+
| page +----------> page_cgroup|
| | | |
+---------------+ +---------------+
(Figure 1: Hierarchy of Accounting)
Figure 1 shows the important aspects of the controller
1. Accounting happens per cgroup
2. Each mm_struct knows about which cgroup it belongs to
3. Each page has a pointer to the page_cgroup, which in turn knows the
cgroup it belongs to
The accounting is done as follows: mem_cgroup_charge_common() is invoked to
set up the necessary data structures and check if the cgroup that is being
charged is over its limit. If it is, then reclaim is invoked on the cgroup.
More details can be found in the reclaim section of this document.
If everything goes well, a page meta-data-structure called page_cgroup is
updated. page_cgroup has its own LRU on cgroup.
(*) page_cgroup structure is allocated at boot/memory-hotplug time.
2.2.1 Accounting details
All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
Some pages which are never reclaimable and will not be on the LRU
are not accounted. We just account pages under usual VM management.
RSS pages are accounted at page_fault unless they've already been accounted
for earlier. A file page will be accounted for as Page Cache when it's
inserted into inode (radix-tree). While it's mapped into the page tables of
processes, duplicate accounting is carefully avoided.
An RSS page is unaccounted when it's fully unmapped. A PageCache page is
unaccounted when it's removed from radix-tree. Even if RSS pages are fully
unmapped (by kswapd), they may exist as SwapCache in the system until they
are really freed. Such SwapCaches are also accounted.
A swapped-in page is not accounted until it's mapped.
Note: The kernel does swapin-readahead and reads multiple swaps at once.
This means swapped-in pages may contain pages for other tasks than a task
causing page fault. So, we avoid accounting at swap-in I/O.
At page migration, accounting information is kept.
Note: we just account pages-on-LRU because our purpose is to control amount
of used pages; not-on-LRU pages tend to be out-of-control from VM view.
2.3 Shared Page Accounting
Shared pages are accounted on the basis of the first touch approach. The
cgroup that first touches a page is accounted for the page. The principle
behind this approach is that a cgroup that aggressively uses a shared
page will eventually get charged for it (once it is uncharged from
the cgroup that brought it in -- this will happen on memory pressure).
But see section 8.2: when moving a task to another cgroup, its pages may
be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
Exception: If CONFIG_MEMCG_SWAP is not used.
When you do swapoff and make swapped-out pages of shmem(tmpfs) to
be backed into memory in force, charges for pages are accounted against the
caller of swapoff rather than the users of shmem.
2.4 Swap Extension (CONFIG_MEMCG_SWAP)
Swap Extension allows you to record charge for swap. A swapped-in page is
charged back to original page allocator if possible.
When swap is accounted, following files are added.
- memory.memsw.usage_in_bytes.
- memory.memsw.limit_in_bytes.
memsw means memory+swap. Usage of memory+swap is limited by
memsw.limit_in_bytes.
Example: Assume a system with 4G of swap. A task which allocates 6G of memory
(by mistake) under 2G memory limitation will use all swap.
In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
By using the memsw limit, you can avoid system OOM which can be caused by swap
shortage.
* why 'memory+swap' rather than swap.
The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
to move account from memory to swap...there is no change in usage of
memory+swap. In other words, when we want to limit the usage of swap without
affecting global LRU, memory+swap limit is better than just limiting swap from
an OS point of view.
* What happens when a cgroup hits memory.memsw.limit_in_bytes
When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
in this cgroup. Then, swap-out will not be done by cgroup routine and file
caches are dropped. But as mentioned above, global LRU can do swapout memory
from it for sanity of the system's memory management state. You can't forbid
it by cgroup.
2.5 Reclaim
Each cgroup maintains a per cgroup LRU which has the same structure as
global VM. When a cgroup goes over its limit, we first try
to reclaim memory from the cgroup so as to make space for the new
pages that the cgroup has touched. If the reclaim is unsuccessful,
an OOM routine is invoked to select and kill the bulkiest task in the
cgroup. (See 10. OOM Control below.)
The reclaim algorithm has not been modified for cgroups, except that
pages that are selected for reclaiming come from the per-cgroup LRU
list.
NOTE: Reclaim does not work for the root cgroup, since we cannot set any
limits on the root cgroup.
Note2: When panic_on_oom is set to "2", the whole system will panic.
When oom event notifier is registered, event will be delivered.
(See oom_control section)
2.6 Locking
lock_page_cgroup()/unlock_page_cgroup() should not be called under
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.
2.7.2 Common use cases
Because the "kmem" counter is fed to the main user counter, kernel memory can
never be limited completely independently of user memory. Say "U" is the user
limit, and "K" the kernel limit. There are three possible ways limits can be
set:
U != 0, K = unlimited:
This is the standard memcg limitation mechanism already present before kmem
accounting. Kernel memory is completely ignored.
U != 0, K < U:
Kernel memory is a subset of the user memory. This setup is useful in
deployments where the total amount of memory per-cgroup is overcommited.
Overcommiting kernel memory limits is definitely not recommended, since the
box can still run out of non-reclaimable memory.
In this case, the admin could set up K so that the sum of all groups is
never greater than the total memory, and freely set U at the cost of his
QoS.
WARNING: In the current implementation, memory reclaim will NOT be
triggered for a cgroup when it hits K while staying below U, which makes
this setup impractical.
U != 0, K >= U:
Since kmem charges will also be fed to the user counter and reclaim will be
triggered for the cgroup for both kinds of memory. This setup gives the
admin a unified view of memory, and it is also useful for people who just
want to track kernel memory usage.
3. User Interface
3.0. Configuration
a. Enable CONFIG_CGROUPS
b. Enable CONFIG_MEMCG
c. Enable CONFIG_MEMCG_SWAP (to use swap extension)
d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)
3.1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
# mount -t tmpfs none /sys/fs/cgroup
# mkdir /sys/fs/cgroup/memory
# mount -t cgroup none /sys/fs/cgroup/memory -o memory
3.2. Make the new group and move bash into it
# mkdir /sys/fs/cgroup/memory/0
# echo $$ > /sys/fs/cgroup/memory/0/tasks
Since now we're in the 0 cgroup, we can alter the memory limit:
# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
NOTE: We cannot set limits on the root cgroup any more.
# cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
4194304
We can check the usage:
# cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
1216512
A successful write to this file does not guarantee a successful setting of
this limit to the value written into the file. This can be due to a
number of factors, such as rounding up to page boundaries or the total
availability of memory on the system. The user is required to re-read
this file after a write to guarantee the value committed by the kernel.
# echo 1 > memory.limit_in_bytes
# cat memory.limit_in_bytes
4096
The memory.failcnt field gives the number of times that the cgroup limit was
exceeded.
The memory.stat file gives accounting information. Now, the number of
caches, RSS and Active pages/Inactive pages are shown.
4. Testing
For testing features and implementation, see memcg_test.txt.
Performance test is also important. To see pure memory controller's overhead,
testing on tmpfs will give you good numbers of small overheads.
Example: do kernel make on tmpfs.
Page-fault scalability is also important. At measuring parallel
page fault test, multi-process test may be better than multi-thread
test because it has noise of shared objects/status.
But the above two are testing extreme situations.
Trying usual test under memory controller is always helpful.
4.1 Troubleshooting
Sometimes a user might find that the application under a cgroup is
terminated by the OOM killer. There are several causes for this:
1. The cgroup limit is too low (just too low to do anything useful)
2. The user is using anonymous memory and swap is turned off or too low
A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
some of the pages cached in the cgroup (page cache pages).
To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
seeing what happens will be helpful.
4.2 Task migration
When a task migrates from one cgroup to another, its charge is not
carried forward by default. The pages allocated from the original cgroup still
remain charged to it, the charge is dropped when the page is freed or
reclaimed.
You can move charges of a task along with task migration.
See 8. "Move charges at task migration"
4.3 Removing a cgroup
A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
cgroup might have some charge associated with it, even though all
tasks have migrated away from it. (because we charge against pages, not
against tasks.)
We move the stats to root (if use_hierarchy==0) or parent (if
use_hierarchy==1), and no change on the charge except uncharging
from the child.
Charges recorded in swap information is not updated at removal of cgroup.
Recorded information is discarded and a cgroup which uses swap (swapcache)
will be charged as a new owner of it.
About use_hierarchy, see Section 6.
5. Misc. interfaces.
5.1 force_empty
memory.force_empty interface is provided to make cgroup's memory usage empty.
When writing anything to this
# echo 0 > memory.force_empty
the cgroup will be reclaimed and as many pages reclaimed as possible.
The typical use case for this interface is before calling rmdir().
Because rmdir() moves all pages to parent, some out-of-use page caches can be
moved to the parent. If you want to avoid that, force_empty will be useful.
Also, note that when memory.kmem.limit_in_bytes is set the charges due to
kernel pages will still be seen. This is not considered a failure and the
write will still return success. In this case, it is expected that
memory.kmem.usage_in_bytes == memory.usage_in_bytes.
About use_hierarchy, see Section 6.
5.2 stat file
memory.stat file includes following statistics
# per-memory cgroup local status
cache - # of bytes of page cache memory.
rss - # of bytes of anonymous and swap cache memory (includes
transparent hugepages).
rss_huge - # of bytes of anonymous transparent hugepages.
mapped_file - # of bytes of mapped file (includes tmpfs/shmem)
pgpgin - # of charging events to the memory cgroup. The charging
event happens each time a page is accounted as either mapped
anon page(RSS) or cache page(Page Cache) to the cgroup.
pgpgout - # of uncharging events to the memory cgroup. The uncharging
event happens each time a page is unaccounted from the cgroup.
swap - # of bytes of swap usage
dirty - # of bytes that are waiting to get written back to the disk.
writeback - # of bytes of file/anon cache that are queued for syncing to
disk.
inactive_anon - # of bytes of anonymous and swap cache memory on inactive
LRU list.
active_anon - # of bytes of anonymous and swap cache memory on active
LRU list.
inactive_file - # of bytes of file-backed memory on inactive LRU list.
active_file - # of bytes of file-backed memory on active LRU list.
unevictable - # of bytes of memory that cannot be reclaimed (mlocked etc).
# status considering hierarchy (see memory.use_hierarchy settings)
hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
under which the memory cgroup is
hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
hierarchy under which memory cgroup is.
total_<counter> - # hierarchical version of <counter>, which in
addition to the cgroup's own value includes the
sum of all hierarchical children's values of
<counter>, i.e. total_cache
# The following additional stats are dependent on CONFIG_DEBUG_VM.
recent_rotated_anon - VM internal parameter. (see mm/vmscan.c)
recent_rotated_file - VM internal parameter. (see mm/vmscan.c)
recent_scanned_anon - VM internal parameter. (see mm/vmscan.c)
recent_scanned_file - VM internal parameter. (see mm/vmscan.c)
Memo:
recent_rotated means recent frequency of LRU rotation.
recent_scanned means recent # of scans to LRU.
showing for better debug please see the code for meanings.
Note:
Only anonymous and swap cache memory is listed as part of 'rss' stat.
This should not be confused with the true 'resident set size' or the
amount of physical memory used by the cgroup.
'rss + file_mapped" will give you resident set size of cgroup.
(Note: file and shmem may be shared among other cgroups. In that case,
file_mapped is accounted only when the memory cgroup is owner of page
cache.)
5.3 swappiness
Overrides /proc/sys/vm/swappiness for the particular group. The tunable
in the root cgroup corresponds to the global swappiness setting.
Please note that unlike during the global reclaim, limit reclaim
enforces that 0 swappiness really prevents from any swapping even if
there is a swap storage available. This might lead to memcg OOM killer
if there are no file pages to reclaim.
5.4 failcnt
A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
This failcnt(== failure count) shows the number of times that a usage counter
hit its limit. When a memory cgroup hits a limit, failcnt increases and
memory under it will be reclaimed.
You can reset failcnt by writing 0 to failcnt file.
# echo 0 > .../memory.failcnt
5.5 usage_in_bytes
For efficiency, as other kernel components, memory cgroup uses some optimization
to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
value for efficient access. (Of course, when necessary, it's synchronized.)
If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
value in memory.stat(see 5.2).
5.6 numa_stat
This is similar to numa_maps but operates on a per-memcg basis. This is
useful for providing visibility into the numa locality information within
an memcg since the pages are allowed to be allocated from any physical
node. One of the use cases is evaluating application performance by
combining this information with the application's CPU allocation.
Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
per-node page counts including "hierarchical_<counter>" which sums up all
hierarchical children's values in addition to the memcg's own value.
The output format of memory.numa_stat is:
total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...
The "total" count is sum of file + anon + unevictable.
6. Hierarchy support
The memory controller supports a deep hierarchy and hierarchical accounting.
The hierarchy is created by creating the appropriate cgroups in the
cgroup filesystem. Consider for example, the following cgroup filesystem
hierarchy
root
/ | \
/ | \
a b c
| \
| \
d e
In the diagram above, with hierarchical accounting enabled, all memory
usage of e, is accounted to its ancestors up until the root (i.e, c and root),
that has memory.use_hierarchy enabled. If one of the ancestors goes over its
limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
children of the ancestor.
6.1 Enabling hierarchical accounting and reclaim
A memory cgroup by default disables the hierarchy feature. Support
can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
# echo 1 > memory.use_hierarchy
The feature can be disabled by
# echo 0 > memory.use_hierarchy
NOTE1: Enabling/disabling will fail if either the cgroup already has other
cgroups created below it, or if the parent cgroup has use_hierarchy
enabled.
NOTE2: When panic_on_oom is set to "2", the whole system will panic in
case of an OOM event in any cgroup.
7. Soft limits
Soft limits allow for greater sharing of
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