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memcg: add documentation about the kmem controller

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Signed-off-by: Glauber Costa <glommer@parallels.com>
Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Frederic Weisbecker <fweisbec@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: JoonSoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Rik van Riel <riel@redhat.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Glauber Costa 2012-12-18 14:22:22 -08:00 committed by Linus Torvalds
parent 2ad306b17c
commit d5bdae7d59
1 changed files with 58 additions and 1 deletions
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59
Documentation/cgroups/memory.txt
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@ -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
@ -268,20 +273,66 @@ 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.
* 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 +341,7 @@ 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
@ -406,6 +458,11 @@ About use_hierarchy, see Section 6.
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