[Devel] Re: [PATCH 02/10] memcg: Uncharge all kmem when deleting a cgroup.

[Suleiman Souhlal]

On Tue, Feb 28, 2012 at 11:00 AM, Glauber Costa <glommer at parallels.com> wrote:
> On 02/27/2012 07:58 PM, Suleiman Souhlal wrote:
>>
>> A later patch will also use this to move the accounting to the root
>> cgroup.
>>
>
> Suleiman,
>
> Did you do any measurements to figure out how long does it take, average,
> for dangling caches to go away ? Under memory pressure, let's say

Unfortunately, I don't have any such measurements, other than a very artificial:

# mkdir /dev/cgroup/memory/c
# echo 1073741824 > /dev/cgroup/memory/c/memory.limit_in_bytes
# sync && echo 3 > /proc/sys/vm/drop_caches
# echo $$ > /dev/cgroup/memory/c/tasks
# find / > /dev/null
# grep '(c)' /proc/slabinfo | wc -l
42
# echo $$ > /dev/cgroup/memory/tasks
# rmdir /dev/cgroup/memory/c
# grep '(c)dead' /proc/slabinfo | wc -l
42
# sleep 60 && sync && for i in `seq 1 1000`; do echo 3 >
/proc/sys/vm/drop_caches ; done
# grep '(c)dead' /proc/slabinfo | wc -l
6
# sleep 60 && grep '(c)dead' /proc/slabinfo | wc -l
5
# sleep 60 && grep '(c)dead' /proc/slabinfo | wc -l
5

(Note that this is without any per-memcg shrinking patch applied. With
shrinking, things will be a bit better, because deleting the cgroup
will force the dentries to get shrunk.)

Some of these dead caches may take a long time to go away, but we
haven't found them to be a problem for us, so far.

-- Suleiman