linux-kernel - Re: [PATCH -mm v2 00/11] mm: scalable and unified arch_get_unmapped

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Date:	Sat, 23 Jun 2012 12:03:48 -0400
From:	"John Stoffel" <john@...ffel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"John Stoffel" <john@...ffel.org>, Rik van Riel <riel@...riel.com>,
	linux-mm@...ck.org, aarcange@...hat.com, peterz@...radead.org,
	minchan@...il.com, kosaki.motohiro@...il.com, andi@...stfloor.org,
	hannes@...xchg.org, mel@....ul.ie, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 00/11] mm: scalable and unified
 arch_get_unmapped_area

>>>>> "Andrew" == Andrew Morton <akpm@...ux-foundation.org> writes:

Andrew> On Fri, 22 Jun 2012 10:24:58 -0400
Andrew> "John Stoffel" <john@...ffel.org> wrote:

>> >>>>> "Rik" == Rik van Riel <riel@...riel.com> writes:
>> 
Rik> A long time ago, we decided to limit the number of VMAs per
Rik> process to 64k. As it turns out, there actually are programs
Rik> using tens of thousands of VMAs.
>> 
>> 
Rik> Performance
>> 
Rik> Testing performance with a benchmark that allocates tens
Rik> of thousands of VMAs, unmaps them and mmaps them some more
Rik> in a loop, shows promising results.
>> 
>> How are the numbers for applications which only map a few VMAs?  Is
>> there any impact there?
>> 

Andrew> Johannes did a test for that: https://lkml.org/lkml/2012/6/22/219

I don't see that in his results.  But maybe (probably) I don't
understand what types of applications this change is supposed to
help.  I guess I worry that this will just keep slowing down other
apps. 

His tests seemed to be for just one VMA remapped with thousands in
use.  Or am I missing the fact that all VMAs are in the same pool?  

Andrew> Some regression with such a workload is unavoidable, I expect.
Andrew> We have to work out whether the pros outweigh the cons.  This
Andrew> involves handwaving.

Yup, it does.  Proof by vigorous handwaving is a time honored
tradition.  

And I do see that the numbers aren't that much poorer, I just keep
thinking that if we can speed up the corner case, can't we also speed
up the normal case with a better algorithm or data structure?

John
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