linux-kernel - Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

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Date:	Thu, 29 Nov 2007 12:50:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	jdike@...toit.com, user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

* Miklos Szeredi <miklos@...redi.hu> wrote:

> > how come UML idled for 30 msecs here, while the workload was 
> > supposed to be CPU-bound? It's not IO bound anywhere, right? No SMP 
> > artifacts either, right?
> 
> Yes.  The UML kernel is UP, and I don't think 'date' or 'bash' want to 
> do any disk I/O.
> 
> Could disk I/O be blocking the tty?  I think UML uses separate threads 
> for these, but I don't know the details.

even if the workload is fully CPU bound externally - internally a 
request to the external (CPU-bound) thread will look like an asynchonous 
request - so small amounts of idle time can be within UML, legitimately. 
(for the amount of time it takes for the external thread to service the 
request)

30 msecs already sounds a bit excessive (this is on your T60 with 
Core2Duo, right?), and 1-2 seconds noticeable latency is definitely 
excessive.

	Ingo
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