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

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Date:	Wed, 12 Oct 2011 15:58:31 -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 10/12/2011 03:20 PM, Andrew Morton wrote:
> On Wed, 12 Oct 2011 09:09:17 -0400
> Rik van Riel<riel@...hat.com>  wrote:

>> 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.
>
> That problem remains with this patch - it just takes a larger burst.
>
> Unless the admin somehow manages to configure the tunable large enough
> to cover the largest burst, and there aren't other applications
> allocating memory during that burst, and the time between bursts is
> sufficient for kswapd to be able to sufficiently replenish free-page
> reserves.  All of which sounds rather unlikely.

It depends on the system. For a setup which is packed to
the brim with workloads, this patch is not likely to help.
On the other hand, on a system that is packed to the brim
with workloads, you are unlikely to get low latencies anyway.

For situations where people really care about low latencies,
I imagine having dedicated hardware for a workload is not at
all unusual, and the patch works for that.

>>> 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 :(
>
> Do we actually have a real-world application which is hurting from
> this?

Satoru-san?
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