Re: [PATCH 6/8] memcg asynchronous memory reclaim interface

[Ying Han <yinghan@google.com>]

On Mon, May 23, 2011 at 5:11 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Mon, 23 May 2011 16:36:20 -0700
> Ying Han <yinghan@google.com> wrote:
>
>> On Fri, May 20, 2011 at 4:56 PM, Hiroyuki Kamezawa
>> <kamezawa.hiroyuki@gmail.com> wrote:
>> > 2011/5/21 Andrew Morton <akpm@linux-foundation.org>:
>> >> On Fri, 20 May 2011 12:46:36 +0900
>> >> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> >>
>> >>> This patch adds a logic to keep usage margin to the limit in asynchronous way.
>> >>> When the usage over some threshould (determined automatically), asynchronous
>> >>> memory reclaim runs and shrink memory to limit - MEMCG_ASYNC_STOP_MARGIN.
>> >>>
>> >>> By this, there will be no difference in total amount of usage of cpu to
>> >>> scan the LRU
>> >>
>> >> This is not true if "don't writepage at all (revisit this when
>> >> dirty_ratio comes.)" is true.  Skipping over dirty pages can cause
>> >> larger amounts of CPU consumption.
>> >>
>> >>> but we'll have a chance to make use of wait time of applications
>> >>> for freeing memory. For example, when an application read a file or socket,
>> >>> to fill the newly alloated memory, it needs wait. Async reclaim can make use
>> >>> of that time and give a chance to reduce latency by background works.
>> >>>
>> >>> This patch only includes required hooks to trigger async reclaim and user interfaces.
>> >>> Core logics will be in the following patches.
>> >>>
>> >>>
>> >>> ...
>> >>>
>> >>>  /*
>> >>> + * For example, with transparent hugepages, memory reclaim scan at hitting
>> >>> + * limit can very long as to reclaim HPAGE_SIZE of memory. This increases
>> >>> + * latency of page fault and may cause fallback. At usual page allocation,
>> >>> + * we'll see some (shorter) latency, too. To reduce latency, it's appreciated
>> >>> + * to free memory in background to make margin to the limit. This consumes
>> >>> + * cpu but we'll have a chance to make use of wait time of applications
>> >>> + * (read disk etc..) by asynchronous reclaim.
>> >>> + *
>> >>> + * This async reclaim tries to reclaim HPAGE_SIZE * 2 of pages when margin
>> >>> + * to the limit is smaller than HPAGE_SIZE * 2. This will be enabled
>> >>> + * automatically when the limit is set and it's greater than the threshold.
>> >>> + */
>> >>> +#if HPAGE_SIZE != PAGE_SIZE
>> >>> +#define MEMCG_ASYNC_LIMIT_THRESH      (HPAGE_SIZE * 64)
>> >>> +#define MEMCG_ASYNC_MARGIN         (HPAGE_SIZE * 4)
>> >>> +#else /* make the margin as 4M bytes */
>> >>> +#define MEMCG_ASYNC_LIMIT_THRESH      (128 * 1024 * 1024)
>> >>> +#define MEMCG_ASYNC_MARGIN            (8 * 1024 * 1024)
>> >>> +#endif
>> >>
>> >> Document them, please.  How are they used, what are their units.
>> >>
>> >
>> > will do.
>> >
>> >
>> >>> +static void mem_cgroup_may_async_reclaim(struct mem_cgroup *mem);
>> >>> +
>> >>> +/*
>> >>>   * The memory controller data structure. The memory controller controls both
>> >>>   * page cache and RSS per cgroup. We would eventually like to provide
>> >>>   * statistics based on the statistics developed by Rik Van Riel for clock-pro,
>> >>> @@ -278,6 +303,12 @@ struct mem_cgroup {
>> >>>        */
>> >>>       unsigned long   move_charge_at_immigrate;
>> >>>       /*
>> >>> +      * Checks for async reclaim.
>> >>> +      */
>> >>> +     unsigned long   async_flags;
>> >>> +#define AUTO_ASYNC_ENABLED   (0)
>> >>> +#define USE_AUTO_ASYNC               (1)
>> >>
>> >> These are really confusing.  I looked at the implementation and at the
>> >> documentation file and I'm still scratching my head.  I can't work out
>> >> why they exist.  With the amount of effort I put into it ;)
>> >>
>> >> Also, AUTO_ASYNC_ENABLED and USE_AUTO_ASYNC have practically the same
>> >> meaning, which doesn't help things.
>> >>
>> > Ah, yes it's confusing.
>>
>> Sorry I was confused by the memory.async_control interface. I assume
>> that is the knob to turn on/off the bg reclaim on per-memcg basis. But
>> when I tried to turn it off, it seems not working well:
>>
>> $ cat /proc/7248/cgroup
>> 3:memory:/A
>>
>> $ cat /dev/cgroup/memory/A/memory.async_control
>> 0
>>
>
> If enabled and kworker runs, this shows "3", for now.
> I'll make this simpler in the next post.
>
>> Then i can see the kworkers start running when the memcg A under
>> memory pressure. There was no other memcgs configured under root.
>
>
> What kworkers ? For example, many kworkers runs on ext4? on my host.
> If kworker/u:x works, it may be for memcg (for my host)

I am kind of sure they are kworkers from memcg. They start running
right after my test and then stop when i kill that test.

$ cat /dev/cgroup/memory/A/memory.limit_in_bytes
2147483648
$ cat /dev/cgroup/memory/A/memory.async_control
0


  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  393 root      20   0     0    0    0 S   54  0.0   1:30.36
kworker/7:1
  391 root      20   0     0    0    0 S   51  0.0   1:42.35
kworker/5:1
  390 root      20   0     0    0    0 S   43  0.0   1:45.55
kworker/4:1
   11 root      20   0     0    0    0 S   40  0.0   1:36.98
kworker/1:0
   14 root      20   0     0    0    0 S   36  0.0   1:47.04
kworker/0:1
  389 root      20   0     0    0    0 S   24  0.0   0:47.35
kworker/3:1
20071 root      20   0 20.0g 497m 497m D   12  1.5   0:04.99 memtoy
  392 root      20   0     0    0    0 S   10  0.0   1:26.43
kworker/6:1

--Ying

>
> Ok, I'll add statistics in v3.
>
> Thanks,
> -Kame
>
>

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