linux-kernel - RE: [PATCH -v2 -mm] add extra free kbytes tunable

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Date:	Tue, 11 Oct 2011 16:23:22 -0400
From:	Satoru Moriya <satoru.moriya@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Satoru Moriya <smoriya@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"lwoodman@...hat.com" <lwoodman@...hat.com>,
	Seiji Aguchi <saguchi@...hat.com>,
	"hughd@...gle.com" <hughd@...gle.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>
Subject: RE: [PATCH -v2 -mm] add extra free kbytes tunable

On 10/11/2011 03:55 PM, Andrew Morton wrote:
> On Tue, 11 Oct 2011 15:32:11 -0400
> Satoru Moriya <satoru.moriya@....com> wrote:
> 
>> On 10/10/2011 06:37 PM, Andrew Morton wrote:
>>> On Fri, 7 Oct 2011 20:08:19 -0700 (PDT) David Rientjes 
>>> <rientjes@...gle.com> wrote:
>>>
>>>> On Thu, 1 Sep 2011, Rik van Riel wrote:
>>
>> Actually page allocator decreases min watermark to 3/4 * min 
>> watermark for rt-task. But in our case some applications create a lot 
>> of processes and if all of them are rt-task, the amount of watermark
>> bonus(1/4 * min watermark) is not enough.
>>
>> If we can tune the amount of bonus, it may be fine. But that is 
>> almost all same as extra free kbytes.
> 
> This situation is detectable at runtime.  If realtime tasks are being 
> stalled in the page allocator then start to increase the free-page 
> reserves.  A little control system.

Detecting at runtime is too late for some latency critical systems.
At that system, we must avoid a stall before it happens.

Also, if we increase the free-page reserves a.k.a min_free_kbytes,
the possibility of direct reclaim on other workloads increases.
I think it's a bad side effect.

Thanks,
Satoru
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