linux-kernel - Re: [PATCH] mm: cache largest vma

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Date:	Wed, 13 Nov 2013 18:59:48 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: cache largest vma


* Davidlohr Bueso <davidlohr@...com> wrote:

> On Mon, 2013-11-11 at 12:47 -0800, Davidlohr Bueso wrote:
> > On Mon, 2013-11-11 at 13:04 +0100, Ingo Molnar wrote:
> > > * Michel Lespinasse <walken@...gle.com> wrote:
> > > 
> > > > On Sun, Nov 10, 2013 at 8:12 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> > > > > 2) Oracle Data mining (4K pages)
> > > > > +------------------------+----------+------------------+---------+
> > > > > |    mmap_cache type     | hit-rate | cycles (billion) | stddev  |
> > > > > +------------------------+----------+------------------+---------+
> > > > > | no mmap_cache          | -        | 63.35            | 0.20207 |
> > > > > | current mmap_cache     | 65.66%   | 19.55            | 0.35019 |
> > > > > | mmap_cache+largest VMA | 71.53%   | 15.84            | 0.26764 |
> > > > > | 4 element hash table   | 70.75%   | 15.90            | 0.25586 |
> > > > > | per-thread mmap_cache  | 86.42%   | 11.57            | 0.29462 |
> > > > > +------------------------+----------+------------------+---------+
> > > > >
> > > > > This workload sure makes the point of how much we can benefit of 
> > > > > caching the vma, otherwise find_vma() can cost more than 220% extra 
> > > > > cycles. We clearly win here by having a per-thread cache instead of 
> > > > > per address space. I also tried the same workload with 2Mb hugepages 
> > > > > and the results are much more closer to the kernel build, but with the 
> > > > > per-thread vma still winning over the rest of the alternatives.
> > > > >
> > > > > All in all I think that we should probably have a per-thread vma 
> > > > > cache. Please let me know if there is some other workload you'd like 
> > > > > me to try out. If folks agree then I can cleanup the patch and send it 
> > > > > out.
> > > > 
> > > > Per thread cache sounds interesting - with per-mm caches there is a real 
> > > > risk that some modern threaded apps pay the cost of cache updates 
> > > > without seeing much of the benefit. However, how do you cheaply handle 
> > > > invalidations for the per thread cache ?
> > > 
> > > The cheapest way to handle that would be to have a generation counter for 
> > > the mm and to couple cache validity to a specific value of that. 
> > > 'Invalidation' is then the free side effect of bumping the generation 
> > > counter when a vma is removed/moved.
> 
> Wouldn't this approach make us invalidate all vmas even when we 
> just want to do it for one? [...]

Yes. If it's implemented as some sort of small, vma-size-weighted 
LRU, then all these 'different' caches go away and there's just 
this single LRU cache with a handful of entries cached.

This cache is then invalidated on munmap() et al. Which should be 
fine, mmap()/munmap() is a slowpath relative to find_vma().

Thanks,

	Ingo
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