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

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Message-Id: <20111011135445.f580749b.akpm@linux-foundation.org>
Date:	Tue, 11 Oct 2011 13:54:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Satoru Moriya <satoru.moriya@....com>
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 Tue, 11 Oct 2011 16:23:22 -0400
Satoru Moriya <satoru.moriya@....com> wrote:

> 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.

It's pretty darn obvious that the kernel can easily see the situation
developing before it happens.  By comparing a few integers.

Look, please don't go bending over backwards like this to defend a bad
patch.  It's a bad patch!  It would be better not to have to merge it. 
Let's do something better.

> 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.

extra_free_kbytes has the same side-effect.
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