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

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Date:	Tue, 8 Aug 2006 07:14:21 +0200
From:	Andi Kleen <ak@...e.de>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	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 Tuesday 08 August 2006 07:09, Paul Mackerras wrote:
> Andrew Morton writes:

[adding linux-arch; talking about doing extensible per cpu areas
by prereserving virtual space and then later fill it up as needed]

> > > 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? 

This wouldn't be truly demand paged, just pages initialized on allocation.

-Andi

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