[RFC][PATCH 4/4] memcg: softlimit documenation

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Documentation for softlimit

Changelog: (v1)->(v2)
 - fixed typos.
 - added more precise text.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 Documentation/cgroups/memory.txt |   29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

Index: develop/Documentation/cgroups/memory.txt
===================================================================
--- develop.orig/Documentation/cgroups/memory.txt
+++ develop/Documentation/cgroups/memory.txt
@@ -322,6 +322,35 @@ will be charged as a new owner of it.
   - a cgroup which uses hierarchy and it has child cgroup.
   - a cgroup which uses hierarchy and not the root of hierarchy.
 
+5.4 softlimit
+  Memory cgroup supports softlimit and has 2 parameters for control.
+
+    - memory.softlimit_in_bytes
+	softlimit for this cgroup
+
+    - memory.softlimit_priority
+	priority of this cgroup at softlimit reclaim
+	Allowed priority level is 0-8 and 0 is the lowest, 8 is the highest.
+        If 8 is specified, this will not be target of softlimit.
+
+  At memory shortage of the system (or local node/zone), softlimit helps
+  kswapd(), a global memory reclaim kernel thread, and informs victim cgroup
+  to be shrinked to kswapd.
+
+  Victim selection logic:
+  *Now*, static priority round-robin queue is used.
+
+  The kernel searches from the lowest priroty(0) up to the highest(7).
+  (priority (8) will never be scanned.)
+  If it finds a cgroup which has memory larger than softlimit, steal memory
+  from it. If multiple cgroups are on the same priority, each cgroup will be a
+  victim in turn.
+
+  Note: the kernel splits memory into zones and the system's memory shortage
+  is usually shorgage of zone's memory. So, even if a memcg'spriority is low,
+  it may not be selected because target zone's memory is not included in it.
+
+  Todo: some kind of dynamic-priority scheduler is fine.
 
 6. Hierarchy support
 

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