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Re: [Xen-devel] Performance difference between Xen versions

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Performance difference between Xen versions
From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
Date: Mon, 02 May 2011 10:00:38 +0200
Cc: Keir Fraser <keir.xen@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 05/02/11 09:23, Jan Beulich wrote:
On 02.05.11 at 08:41, Keir Fraser<keir.xen@xxxxxxxxx>  wrote:
On 02/05/2011 06:31, "Juergen Gross"<juergen.gross@xxxxxxxxxxxxxx>  wrote:

Is there any easy explanation for this? Both Xen versions are from SLES
(SLES11 or SLES11 SP1).
I think cpufreq handling was off by default in 3.3, and is on by
default on 4.0. Try turning this off, or using the performance
Jan, you got it! With cpufreq=none Xen 4.0 has more or less the same numbers
as 3.3. Now I wonder why the default is so much slower. I looks as if the
hypervisor would run at a lower speed. I can't believe it should behave like
It runs at lower frequency unless your test offers sufficient load over a
long enough time period. Short microbenchmarks are probably finished before
the frequency governor can react.
Correct. I generally found the default threshold of the ondemand
governor nor very suitable for optimal performance of short lived
jobs, and boot all of my systems with "cpufreq=xen:ondemand,threshold=20".

Thanks, Keir and Jan! You both helped me a lot!

I think the short term solution for our problem is to disable the cpufreq
governor on our BS2000 machines.

On the long run I'd like to make the cpufreq governor a feature of the
cpupool. This would enable an administrator of a large Xen machine
with a heterogeneous load to specify which domains should run at
full speed and which are allowed to save energy at the cost of latency.

What do you think?


Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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