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

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Date:	Wed, 25 Jan 2012 09:47:13 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Ying Han <yinghan@...gle.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 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?

Thanks
Hillf
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