linux-kernel - Re: [PATCH] Physical Memory Management [0/1]

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Message-ID: <20090515112656.GD16682@one.firstfloor.org>
Date:	Fri, 15 May 2009 13:26:56 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Michał Nazarewicz <m.nazarewicz@...sung.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, m.szyprowski@...sung.com,
	kyungmin.park@...sung.com, linux-mm@...ck.org
Subject: Re: [PATCH] Physical Memory Management [0/1]

On Fri, May 15, 2009 at 12:47:23PM +0200, Michał Nazarewicz wrote:
> On Fri, 15 May 2009 12:18:11 +0200, Andi Kleen wrote:
> > That's not correct, support for multiple huge page sizes was recently
> > added. The interface is a bit clumpsy admittedly, but it's there.
> 
> I'll have to look into that further then.  Having said that, I cannot
> create a huge page SysV shared memory segment with pages of specified
> size, can I?

sysv shared memory supports huge pages, but there is currently
no interface to specify the intended page size, you always
get the default.

> 
> > However for non fragmentation purposes you probably don't
> > want too many different sizes anyways, the more sizes, the worse
> > the fragmentation. Ideal is only a single size.
> 
> Unfortunately, sizes may very from several KiBs to a few MiBs.

Then your approach will likely not be reliable.

> On the other hand, only a handful of apps will use PMM in our system
> and at most two or three will be run at the same time so hopefully
> fragmentation won't be so bad.  But yes, I admit it is a concern.

Such tight restrictions might work for you, but for mainline Linux the quality 
standards are higher.
 
> > As Peter et.al. explained earlier varying buffer sizes don't work
> > anyways.
> 
> Either I missed something or Peter and Adrew only pointed the problem
> we all seem to agree exists: a problem of fragmentation.

Multiple buffer sizes lead to fragmentation.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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