memcg: update documentation to describe usage_in_bytes
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>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index b6ed61c9..7c16347 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -52,8 +52,10 @@
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 @@
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.