linux-kernel - Re: [PATCH] mm: vmscan: check mem cgroup over reclaimed

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Date:	Wed, 25 Jan 2012 11:20:53 -0800
From:	Ying Han <yinghan@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	linux-mm@...ck.org, Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: check mem cgroup over reclaimed

On Tue, Jan 24, 2012 at 5:47 PM, Hillf Danton <dhillf@...il.com> wrote:
> On Wed, Jan 25, 2012 at 7:22 AM, Ying Han <yinghan@...gle.com> wrote:
>> On Mon, Jan 23, 2012 at 7:45 PM, Hillf Danton <dhillf@...il.com> wrote:
>>> With soft limit available, what if nr_to_reclaim set to be the number of
>>> pages exceeding soft limit? With over reclaim abused, what are the targets
>>> of soft limit?
>>
>> The nr_to_reclaim is set to SWAP_CLUSTER_MAX (32) for direct reclaim
>> and ULONG_MAX for background reclaim. Not sure we can set it, but it
>> is possible the res_counter_soft_limit_excess equal to that target
>> value. The current soft limit mechanism provides a clue of WHERE to
>> reclaim pages when there is memory pressure, it doesn't change the
>> reclaim target as it was before.
>>
>
> Decrement in sc->nr_to_reclaim was tried in another patch, you already saw it.
>
>> Overreclaim a cgroup under its softlimit is bad, but we should be
>> careful not introducing side effect before providing the guarantee.
>
> Yes 8-)
>
>> Here, the should_continue_reclaim() has logic of freeing a bit more
>> order-0 pages for compaction. The logic got changed after this.
>>
>
> Compaction is to increase the successful rate of THP allocation, and in turn
> to back up higher performance. In soft limit, performance guarantee is not
> extra request but treated with less care.
>
> Which one you prefer, compaction or guarantee?

The compaction is something we already supporting, while the softlimit
implementation is a new design. I would say that we need to guarantee
no regression introduced by any new code.

--Ying

> Thanks
> Hillf
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