Re: [PATCH] memcg: fix up documentation on global LRU.

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

On Fri, 3 Feb 2012 12:15:59 -0800
Ying Han <yinghan@google.com> wrote:

> On Fri, Feb 3, 2012 at 8:11 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > On Thu 02-02-12 17:37:13, Ying Han wrote:
> >> In v3.3-rc1, the global LRU has been removed with commit
> >> "mm: make per-memcg LRU lists exclusive". The patch fixes up the memcg docs.
> >>
> >> Signed-off-by: Ying Han <yinghan@google.com>
> >
> > For the global LRU removal
> > Acked-by: Michal Hocko <mhocko@suse.cz>
> >
> > see the comment about the swap extension bellow.
> >
> > Thanks
> >
> >> ---
> >> A Documentation/cgroups/memory.txt | A  25 ++++++++++++-------------
> >> A 1 files changed, 12 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> >> index 4c95c00..847a2a4 100644
> >> --- a/Documentation/cgroups/memory.txt
> >> +++ b/Documentation/cgroups/memory.txt
> > [...]
> >> @@ -209,19 +208,19 @@ In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
> >> A By using memsw limit, you can avoid system OOM which can be caused by swap
> >> A shortage.
> >>
> >> -* why 'memory+swap' rather than swap.
> >> -The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
> >> -to move account from memory to swap...there is no change in usage of
> >> -memory+swap. In other words, when we want to limit the usage of swap without
> >> -affecting global LRU, memory+swap limit is better than just limiting swap from
> >> -OS point of view.
> >> -
> >> A * What happens when a cgroup hits memory.memsw.limit_in_bytes
> >> A When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
> >> A in this cgroup. Then, swap-out will not be done by cgroup routine and file
> >> -caches are dropped. But as mentioned above, global LRU can do swapout memory
> >> -from it for sanity of the system's memory management state. You can't forbid
> >> -it by cgroup.
> >> +caches are dropped.
> >> +
> >> +TODO:
> >> +* use 'memory+swap' rather than swap was due to existence of global LRU.
> 
> I wasn't sure about the initial comment while making the patch. Since
> it mentions something about global LRU, which i figured we need to
> revisit it anyway.
> 

The "global LRU" here means 'the health of the whole memory management".
memory+swap guarantees memcg will never be obstacles for routines which
works for system memory management.

soft-limit _is_ a hint for global lru. but memory+swap will never be.

Thanks,
-Kame

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