linux-kernel - Re: -rt doesn't compile for UML

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Message-Id: <E1IxOwS-000752-HF@dorka.pomaz.szeredi.hu>
Date:	Wed, 28 Nov 2007 16:35:12 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jdike@...toit.com
CC:	mingo@...e.hu, user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: -rt doesn't compile for UML

> > When writing a big file to a ubd disk, everything in uml slows down to
> > a crawl, even though CPU usage is minimal.
> 
> What exactly is your test case?

Something like:

  dd if=/host/tmp/bigfile of=/tmp/bigfile bs=1048576 count=100

and run 'top' or 'watch something' in another terminal, which
sometimes completely stops updating during the write.

> I have some patches which theoretically should help I/O performance
> which I've been sitting on because I can't find a case where they
> help.

It's not I/O performance I'm worried about, but scheduling latencies.
CPU use inside and outside the UML is minimal, so I can't see where
this slowdown comes from.

Miklos
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