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

[Ying Han <yinghan@google.com>]

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

Then i can see the kworkers start running when the memcg A under
memory pressure. There was no other memcgs configured under root.

$ cat /dev/cgroup/memory/memory.async_control
0

--Ying



>> Some careful description at this place in the code might help clear
>> things up.
>>
> yes, I'll fix and add text, consider better name.
>
>> Perhaps s/USE_AUTO_ASYNC/AUTO_ASYNC_IN_USE/ is what you meant.
>>
> Ah, good name :)
>
>>>
>>> ...
>>>
>>> +static void mem_cgroup_may_async_reclaim(struct mem_cgroup *mem)
>>> +{
>>> +     if (!test_bit(USE_AUTO_ASYNC, &mem->async_flags))
>>> +             return;
>>> +     if (res_counter_margin(&mem->res) <= MEMCG_ASYNC_MARGIN) {
>>> +             /* Fill here */
>>> +     }
>>> +}
>>
>> I'd expect a function called foo_may_bar() to return a bool.
>>
> ok,
>
>> But given the lack of documentation and no-op implementation, I have o
>> idea what's happening here!
>>
> yes. Hmm, maybe adding an empty function here and comments on the
> function will make this better.
>
> Thank you for review.
> -Kame
>

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