linux-kernel - Re: [PATCH] mm: per-thread vma caching

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Date:	Fri, 21 Feb 2014 12:53:39 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: per-thread vma caching

On Fri, 2014-02-21 at 10:13 -0800, Linus Torvalds wrote:
> On Thu, Feb 20, 2014 at 9:28 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> > From: Davidlohr Bueso <davidlohr@...com>
> >
> > This patch is a continuation of efforts trying to optimize find_vma(),
> > avoiding potentially expensive rbtree walks to locate a vma upon faults.
> 
> Ok, so I like this one much better than the previous version.
> 
> However, I do wonder if the per-mm vmacache is actually worth it.
> Couldn't the per-thread one replace it entirely?

I think you are right. I just reran some of the tests and things are
pretty much the same, so we could get rid of it. I originally left it
there because I recall seeing a slightly better hit rate for some java
workloads (map/reduce, specifically), but it wasn't a big deal - some
slots endup being redundant with a per mm cache. It does however
guarantee that we access hot vmas immediately, instead of potentially
slightly more reads when we go into per-thread checking. I'm happy with
the results either way.

> Also, the hash you use for the vmacache index is *particularly* odd.
> 
>         int idx =  (addr >> 10) & 3;
> 
> you're using the top two bits of the address *within* the page.
> There's a lot of places that round addresses down to pages, and in
> general it just looks really odd to use an offset within a page as an
> index, since in some patterns (linear accesses, whatever), the page
> faults will always be to the beginning of the page, so index 0 ends up
> being special.

Ah, this comes from tediously looking at access patterns. I actually
printed pages of them. I agree that it is weird, and I'm by no means
against changing it. However, the results are just too good, specially
for ebizzy, so I decided to keep it, at least for now. I am open to
alternatives.

Thanks,
Davidlohr

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