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

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Message-ID: <4E95917D.3080507@redhat.com>
Date:	Wed, 12 Oct 2011 09:09:17 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Satoru Moriya <satoru.moriya@....com>,
	David Rientjes <rientjes@...gle.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 04:54 PM, Andrew Morton wrote:
> 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.

The problem is that we may be dealing with bursts, not steady
states of allocations.  Without knowing the size of a burst,
we have no idea when we should wake up kswapd to get enough
memory freed ahead of the application's allocations.

> 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.

I would love it if we could come up with something better,
and have thought about it a lot.

However, so far we do not seem to have an alternative yet :(

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