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Re: [Xen-users] Dedicating a physical CPU or Hyperthread to dom0? Strang

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Subject: Re: [Xen-users] Dedicating a physical CPU or Hyperthread to dom0? Strange test results
From: Dominik Klein <dk@xxxxxxxxxxxxxxxx>
Date: Wed, 22 Aug 2007 12:18:31 +0200
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Javier Guerra Giraldez schrieb:
Dominik Klein wrote:
CPU1 = HT1 + HT2
CPU2 = HT3 + HT4

Are you positively sure on this?

Hum. Actually not.

When booting the machine with a non-xen Kernel, I see in /proc/cpuinfo:
processor       : 0
...
physical id     : 0
...

processor       : 1
...
physical id     : 0
...

processor       : 2
...
physical id     : 3
...

processor       : 3
...
physical id     : 3
...

This would strengthen my assumption.
In xen though, both dom0 and domU, I cannot see "physical id" in /proc/cpuinfo.

i think Linux enumerates differently to make it easier to spread load, so it might be

CPU1 = HT1 + HT3
CPU2 = HT2 + HT4

which would mean a totally different interpretation to your experiments.

Correct.

and, of course two processes running on different HTs on the same CPU will affect each other's performance. a multithreaded CPU doesn't have any more ALUs, cache, schedulers, etc. than a singletreaded one; it just have another set of state registers, making it easy to switch from one instruction stream to another. the advantage comes from giving the CPU something else to do while one thread stalls because of a cache miss, or an inter-instruction dependency.

Good point. This said, it is unlikely that the enumeration in Xen is the same as in a non-xen kernel.
Maybe a developer can say something about this.

Regards
Dominik

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