linux-kernel - Re: [PATCH] mm: MADV_WILLNEED implementation for anonymous memory

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Date:	Thu, 31 Jan 2008 02:18:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	hugh@...itas.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	npiggin@...e.de, riel@...hat.com, mztabzr@...inter.de,
	mpm@...enic.com
Subject: Re: [PATCH] mm: MADV_WILLNEED implementation for anonymous memory

On Thu, 31 Jan 2008 11:10:13 +0100 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> 
> On Thu, 2008-01-31 at 02:05 -0800, Andrew Morton wrote:
> > On Thu, 31 Jan 2008 10:53:26 +0100 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > > 
> > > On Thu, 2008-01-31 at 01:47 -0800, Andrew Morton wrote:
> > > > On Thu, 31 Jan 2008 10:35:18 +0100 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > > > 
> > > > > 
> > > > > On Thu, 2008-01-31 at 01:12 -0800, Andrew Morton wrote:
> > > > > 
> > > > > > Implementation-wise: make_pages_present() _can_ be converted to do this. 
> > > > > > But it's a lot of patching, and the result will be a cleaner, faster and
> > > > > > smaller core MM.  Whereas your approach is easy, but adds more code and
> > > > > > leaves the old stuff slow-and-dirty.
> > > > > > 
> > > > > > Guess which approach is preferred? ;)
> > > > > 
> > > > > Ok, I'll look at using make_pages_present().
> > > > 
> > > > Am still curious to know what inspired this change.  What are the use
> > > > cases?  Performance testing results, etc?
> > > 
> > > Ah, that is Lennarts Pulse Audio thing, he has samples in memory which
> > > might not have been used for a while, and he wants to be able to
> > > pre-fetch those when he suspects they might need to be played. So that
> > > once the audio thread comes along and stuffs them down /dev/dsp its all
> > > nice in memory.
> > > 
> > > Since its all soft real-time at best he feels its better to do a best
> > > effort at not hitting swap than it is to strain the system with mlock
> > > usage.
> > 
> > hrm.  Does he know about pthread_create()?
> 
> I'm very sure he does. So you're suggesting to just create a thread and
> touch that memory and be done with it?
> 
> Lennart?

That would get him out of trouble.  But it certainly makes _sense_ for the
kernel to implement MADV_WILLNEED for anon memory.  From a consistency POV.

But I don't know that the usefulness of the feature is worth actually
expending code on.  Heck, after five-odd years I'm still asking every
second person I meet "why don't you use fadvise()?"  (Reponse: ooooh!)


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