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Re: [Qemu-devel] [patch] non-blocking disk IO

From: Jens Axboe
Subject: Re: [Qemu-devel] [patch] non-blocking disk IO
Date: Tue, 4 Oct 2005 14:22:55 +0200

On Tue, Oct 04 2005, Christian MICHON wrote:
> hmmm....
> the static binary crashes, the one with ptreadGC2.dll is ok.
> What I tried to measure, on windows host, kqemu-0.7.2
> loaded, was:
> (1) tar -jxf linux-2.6.10.tar.bz2
> (2) make allnoconfig; make
> I confirm ~18% speed improvement in case (1), but no change
> more or less for case (2). I believe maybe someone with a
> windows host SMP could get different figures. If so, please
> update these accordingly and let the list know.
> Indeed, threading on a busy single CPU doesn't make it faster.

That's not quite true. The io thread basically takes no real CPU, but it
often has to wait for io to become available. So even (especially!) for
a single cpu system, you definitely want to make sure that blocking io
doesn't eat away cpu cycles from a cpu bound case like the compile.

If you did the kernel compile with a hot disk cache, I'm not surprised
you're not seeing a performance benefit of the non-blocking io patch.
Even for a cold cache compile it will generally be cpu bound.

Jens Axboe

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