Re: [RFC][PATCH 0/7] memcg async reclaim

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 May 2011 19:02:16 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> Hi, thank you for all comments on previous patches for watermarks for memcg.
> 
> This is a new series as 'async reclaim', no watermark.
> This version is a RFC again and I don't ask anyone to test this...but
> comments/review are appreciated. 
> 
> Major changes are
>   - no configurable watermark
>   - hierarchy support
>   - more fix for static scan rate round robin scanning of memcg.
> 
> (assume x86-64 in following.)
> 
> 'async reclaim' works when
>    - usage > limit - 4MB.
> until
>    - usage < limit - 8MB.
> 
> when the limit is larger than 128MB. This value of margin to limit
> has some purpose for helping to reduce page fault latency at using
> Transparent hugepage.
> 
> Considering THP, we need to reclaim HPAGE_SIZE(2MB) of pages when we hit
> limit and consume HPAGE_SIZE(2MB) immediately. Then, the application need to
> scan 2MB per each page fault and get big latency. So, some margin > HPAGE_SIZE
> is required. I set it as 2*HPAGE_SIZE/4*HPAGE_SIZE, here. The kernel
> will do async reclaim and reduce usage to limit - 8MB in background.
> 
> BTW, when an application gets a page, it tend to do some action to fill the
> gotton page. For example, reading data from file/network and fill buffer.
> This implies the application will have a wait or consumes cpu other than
> reclaiming memory. So, if the kernel can help memory freeing in background
> while application does another jobs, application latency can be reduced.
> Then, this kind of asyncronous reclaim of memory will be a help for reduce
> memory reclaim latency by memcg. But the total amount of cpu time consumed
> will not have any difference.
> 
> This patch series implements
>   - a logic for trigger async reclaim
>   - help functions for async reclaim
>   - core logic for async reclaim, considering memcg's hierarchy.
>   - static scan rate memcg reclaim.
>   - workqueue for async reclaim.
> 
> Some concern is that I didn't implement a code for handle the case
> most of pages are mlocked or anon memory in swapless system. I need some
> detection logic to avoid hopless async reclaim.
> 

What (user-visible) problem is this patchset solving?

IOW, what is the current behaviour, what is wrong with that behaviour
and what effects does the patchset have upon that behaviour?

The sole answer from the above is "latency spikes".  Anything else?

Have these spikes been observed and measured?  We should have a
testcase/worload with quantitative results to demonstrate and measure
the problem(s), so the effectiveness of the proposed solution can be
understood.

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