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SubjectRe: [PATCH 0/8] memcg async reclaim v2
On Mon, 23 May 2011 15:38:31 -0700
Ying Han <yinghan@google.com> wrote:

> Hi Kame:
>
> I applied and tested the patchset on top of mmotm-2011-05-12-15-52. I
> admit that I didn't look the patch closely yet, which I plan to do
> next. Now i have few quick questions based on the testing result:
>
> Test:
> 1) create a 2g memcg and enable async_control
> $ mkdir /dev/cgroup/memory/A
> $ echo 2g >/dev/cgroup/memory/A/memory.limit_in_bytes
> $ echo 1 >/dev/cgroup/memory/A/memory.async_control
>
> 2) read a 20g file in the memcg
> $ echo $$ >/dev/cgroup/memory/A/tasks
> $ time cat /export/hdc3/dd_A/tf0 > /dev/zero
>
> real 4m26.677s
> user 0m0.222s
> sys 0m28.481s
>
> Here are the questions:
>
> 1. I monitored the "top" while the test is running. The amount of
> cputime the kworkers take worries me, and the following top output
> stays pretty consistent while the "cat" is running/
>

memcg-async's kworker is kworker/u:x .....because of UNBOUND_WQ.
Then, kworker you see is for other purpose....Hmm, from trace log,
most of them are for "draining" per-cpu memcg cache. I'll prepare a patch.



> Tasks: 152 total, 2 running, 150 sleeping, 0 stopped, 0 zombie
> Cpu(s): 0.1%us, 1.2%sy, 0.0%ni, 87.6%id, 10.6%wa, 0.0%hi, 0.5%si, 0.0%st
> Mem: 32963480k total, 2694728k used, 30268752k free, 3888k buffers
> Swap: 0k total, 0k used, 0k free, 2316500k cached
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 389 root 20 0 0 0 0 R 45 0.0 1:36.24
> kworker/3:1
> 23127 root 20 0 0 0 0 S 44 0.0 0:13.44
> kworker/4:2
> 393 root 20 0 0 0 0 S 43 0.0 2:02.28
> kworker/7:1
> 32 root 20 0 0 0 0 S 42 0.0 1:54.02
> kworker/6:0
> 1230 root 20 0 0 0 0 S 42 0.0 1:22.01
> kworker/2:2
> 23130 root 20 0 0 0 0 S 31 0.0 0:04.04
> kworker/0:2
> 391 root 20 0 0 0 0 S 22 0.0 1:45.79
> kworker/5:1
> 23109 root 20 0 3104 228 180 D 10 0.0 0:08.56 cat
>
> I attached the tracing output of the kworkers while they are running
> by doing the following:
>
> $ mount -t debugfs nodev /sys/kernel/debug/
> $ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
> $ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
>
> 2. I can not justify the cputime on the kworkers. I am looking for the
> patch which we exports the time before and after workitem on memcg
> basis. I recall we have that in previous post, sorry I missed that
> patch somehere.
>
> # cat /cgroup/memory/A/memory.stat
> ....
> direct_elapsed_ns 0
> wmark_elapsed_ns 103566424
> direct_scanned 0
> wmark_scanned 29303
> direct_freed 0
> wmark_freed 29290
>

I didn't include this for this version because you and others working on
memory.stat file. I wanted to avoid to add new mess ;)
I'll include it again in v3.



> 3. Here is the outout of memory.stat after the test, the last one is
> the memory.failcnt. As far as I remember, the failcnt is far higher
> than the result i got on previous testing (per-memcg-per-kswapd
> patch). This is all clean file pages which shouldn't be hard to
> reclaim.
>
> cache 2147151872
> rss 94208
> mapped_file 0
> pgpgin 5242945
> pgpgout 4718715
> pgfault 274
> pgmajfault 0
> 1050041
>
> Please let me know if the current version isn't ready for testing, and
> I will wait :)
>

This version has tweaked to be less cpu hogging than previous one. So,
hit_limit increases. I'll drop some tweakes I added in v2 for starting from
a simple one.

I'll post v3 in this week. But if dirty_ratio is ready, I think it should be
merged 1st. But it's merge window....
Thanks,
-Kame






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