linux-kernel - Re: [PATCH 2.6.28 1/2] memory: improve find

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Message-ID: <20090122231358.GA27033@elte.hu>
Date:	Fri, 23 Jan 2009 00:13:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Daniel Lowengrub <lowdanie@...il.com>
Cc:	Hugh Dickins <hugh@...itas.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.28 1/2] memory: improve find_vma


* Daniel Lowengrub <lowdanie@...il.com> wrote:

> On Thu, Jan 22, 2009 at 7:22 PM, Hugh Dickins <hugh@...itas.com> wrote:
> > Do you have some performance figures to support this patch?
> > Some of the lmbench tests may be appropriate.
> >
> > The thing is, expanding vm_area_struct to include another pointer
> > will have its own cost, which may well outweigh the efficiency
> > (in one particular case) which you're adding.  Expanding mm_struct
> > for this would be much more palatable, but I don't think that flies.
> >
> > And it seems a little greedy to require both an rbtree and a doubly
> > linked list for working our way around the vmas.
> >
> > I suspect that originally your enhancement would only have hit when
> > extending the stack: which I guess isn't enough to justify the cost.
> > But it could well be that unmapped area handling has grown more
> > complex down the years, and you get some hits now from that.
> >
> Thanks for the reply.
> I ran an lmbench test on the 2.6.28 kernel and on the same kernel
> after applying the patch.  Here's a portion of the results with the
> format of
> test : standard kernel / kernel after patch
> 
> Simple syscall: 0.7419 / 0.4244 microseconds
> Simple read: 1.2071 / 0.7270 microseconds

there must be a significant measurement mistake here: none of your patches 
affect the 'simple syscall' path, nor the sys_read() path.

	Ingo
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