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memory controller: soft limit documentation

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Soft limits is a new feature for the memory resource controller, something
similar has existed in the group scheduler in the form of shares.  The CPU
controllers interpretation of shares is very different though.

Soft limits are the most useful feature to have for environments where the
administrator wants to overcommit the system, such that only on memory
contention do the limits become active.  The current soft limits
implementation provides a soft_limit_in_bytes interface for the memory
controller and not for memory+swap controller.  The implementation
maintains an RB-Tree of groups that exceed their soft limit and starts
reclaiming from the group that exceeds this limit by the maximum amount.

This patch:

Add documentation for soft limits

Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Balbir Singh 2009-09-23 15:56:34 -07:00 committed by Linus Torvalds
parent 261fb61a8b
commit a6df63615b
1 changed files with 36 additions and 1 deletions
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37
Documentation/cgroups/memory.txt
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@ -379,7 +379,42 @@ cgroups created below it.
NOTE2: This feature can be enabled/disabled per subtree.
7. TODO
7. Soft limits
Soft limits allow for greater sharing of memory. The idea behind soft limits
is to allow control groups to use as much of the memory as needed, provided
a. There is no memory contention
b. They do not exceed their hard limit
When the system detects memory contention or low memory control groups
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
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
it gets invoked from balance_pgdat (kswapd).
7.1 Interface
Soft limits can be setup by using the following commands (in this example we
assume a soft limit of 256 megabytes)
# echo 256M > memory.soft_limit_in_bytes
If we want to change this to 1G, we can at any time use
# echo 1G > memory.soft_limit_in_bytes
NOTE1: Soft limits take effect over a long period of time, since they involve
reclaiming memory for balancing between memory cgroups
NOTE2: It is recommended to set the soft limit always below the hard limit,
otherwise the hard limit will take precedence.
8. TODO
1. Add support for accounting huge pages (as a separate controller)
2. Make per-cgroup scanner reclaim not-shared pages first