[PATCH v2] memcg: update documentation to describe usage_in_bytes

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Mon, 28 Mar 2011 19:31:08 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Mon, 28 Mar 2011 11:48:20 +0200
> Michal Hocko <mhocko@suse.cz> wrote:
> 
> > On Mon 28-03-11 18:11:27, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 28 Mar 2011 09:43:42 +0200
> > > Michal Hocko <mhocko@suse.cz> wrote:
> > > 
> > > > On Mon 28-03-11 13:25:50, Daisuke Nishimura wrote:
> > > > > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > [...]
> > > > > +5.5 usage_in_bytes
> > > > > +
> > > > > +As described in 2.1, memory cgroup uses res_counter for tracking and limiting
> > > > > +the memory usage. memory.usage_in_bytes shows the current res_counter usage for
> > > > > +memory, and DOESN'T show a actual usage of RSS and Cache. It is usually bigger
> > > > > +than the actual usage for a performance improvement reason. 
> > > > 
> > > > Isn't an explicit mention about caching charges better?
> > > > 
> > > 
> > > It's difficult to distinguish which is spec. and which is implemnation details...
> > 
> > Sure. At least commit log should contain the implementation details IMO,
> > though.
> > 
> > > 
> > > My one here ;)
> > > ==
> > > 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 usage, it's an fuzz value for efficient
> > > access. (Of course, when necessary, it's synchronized.)
> > > In usual, the value (RSS+CACHE) in memory.stat shows more exact value. IOW,
> > 
> > - In usual, the value (RSS+CACHE) in memory.stat shows more exact value. IOW,
> > + (RSS+CACHE) value from memory.stat shows more exact value and should be used
> > + by userspace. IOW,
> > 
> > ?
> > 
> 
> seems good. Nishimura-san, could you update ?
> 
> Thanks,
> -Kame
> 
Thank you very much for your comments. This is the updated one.

===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

Since 569b846d(memcg: coalesce uncharge during unmap/truncate), we do batched
(delayed) uncharge at truncation/unmap. And since cdec2e42(memcg: coalesce
charging via percpu storage), we have percpu cache for res_counter.

These changes improved performance of memory cgroup very much, but made
res_counter->usage usually have a bigger value than the actual value of memory usage.
So, *.usage_in_bytes, which show res_counter->usage, are not desirable for precise
values of memory(and swap) usage anymore.

Instead of removing these files completely(because we cannot know res_counter->usage
without them), this patch updates the meaning of those files.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 Documentation/cgroups/memory.txt |   15 +++++++++++++--
 1 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 7781857..4f49d91 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -52,8 +52,10 @@ 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 memory(RSS+Cache) usage.
- memory.memsw.usage_in_bytes	 # show current memory+Swap usage
+ memory.usage_in_bytes		 # show current res_counter usage for memory
+				 (See 5.5 for details)
+ memory.memsw.usage_in_bytes	 # show current res_counter 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
@@ -453,6 +455,15 @@ 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 an 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).
+
 6. Hierarchy support
 
 The memory controller supports a deep hierarchy and hierarchical accounting.
-- 
1.7.1

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