linux-kernel - Re: [PATCH] x86_64: Make NR_IRQS configurable in Kconfig

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Date:	Tue, 08 Aug 2006 10:17:53 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Paul Mackerras <paulus@...ba.org>, Andrew Morton <akpm@...l.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Randy.Dunlap" <rdunlap@...otime.net>,
	"Protasevich, Natalie" <Natalie.Protasevich@...sys.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH] x86_64: Make NR_IRQS configurable in Kconfig

On Tue, 2006-08-08 at 07:14 +0200, Andi Kleen wrote:
> > > > Drawback would be some more TLB misses.
> > > 
> > > yup.  On some (important) architectures - I'm not sure which architectures
> > > do the bigpage-for-kernel trick.
> > 
> > I looked at optimizing the per-cpu data accessors on PowerPC and only
> > ever saw fractions of a percent change in overall performance, which
> > says to me that we don't actually use per-cpu data all that much.  So
> > unless you make per-cpu data really really slow, I doubt that we'll
> > see any significant performance difference.
> 
> The main problem is that we would need a "vmalloc reserve first; allocate pages
> later" interface. On x86 it would be easy by just splitting up vmalloc/vmap a bit
> again. Does anybody else see problems with implementing that on any
> other architecture? 

"vmalloc reserve first; allocate pages later" would be a really nice
feature. We could use this on s390 to implement the virtual mem_map
array spanning the whole 64 bit address range (with holes in it). To
make it perfect a "deallocate pages; keep vmalloc reserve" should be
added, then we could free parts of the mem_map array again on hot memory
remove. 
I don't see a problem for s390.

-- 
blue skies,
  Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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