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

[Ying Han <yinghan@google.com>]

On Sun, Feb 5, 2012 at 5:46 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 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
>> >
>> >> ---
>> >>  Documentation/cgroups/memory.txt |   25 ++++++++++++-------------
>> >>  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.
>> >>  By using memsw limit, you can avoid system OOM which can be caused by swap
>> >>  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.
>> >> -
>> >>  * What happens when a cgroup hits memory.memsw.limit_in_bytes
>> >>  When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
>> >>  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.


Thank you for the clarification. So the "global LRU" should be
interpreted as global pressure, i guess? I can imagine some extra
complexities on two limit (in memory & swap) vs one limit
(memory+swap).

I will go ahead post the first patch and leave the swap change behind.
Apparently I don't know the initial design much, and feel free to post
the second half.

--Ying

>
> Thanks,
> -Kame
>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>