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Re: [Xen-devel] What's the different for "dom0_max_vcpus=4 dom0_vcpus_pi

To: lei yang <yanglei.fage@xxxxxxxxx>, Xen Mailing List <xen-users@xxxxxxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] What's the different for "dom0_max_vcpus=4 dom0_vcpus_pin" and "dom0_max_vcpus=4" ?
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 21 May 2010 07:57:50 +0100
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Thread-topic: [Xen-devel] What's the different for "dom0_max_vcpus=4 dom0_vcpus_pin" and "dom0_max_vcpus=4" ?
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On 21/05/2010 04:09, "lei yang" <yanglei.fage@xxxxxxxxx> wrote:

> Hi experts,
> Q1:What's the different for "dom0_max_vcpus=4 dom0_vcpus_pin" and 
> "dom0_max_vcpus=4" ? which will get better performance

The former restricts Xen to only run Dom0 VCPUs 0-3 on physical CPUs 0-3.
It's not really a performance thing, but instead is to mean that some things
like cpufreq management can work directly from dom0. We don't really use the
dom0_vcpus_pin option much.

> Q2: dom0_max_vcpus=4 means "core0-3 will be just used by dom0" or means "4
> cores(not dedicate cores) will be used by dom0, eg: core2-5 or core3-6?

Dom0's VCPUs can be scheduled on any physical CPU at any time, unles
dom0_vcpus_pin is specified.

> Q3.what does mean "nosmp" , xen, dom0,domU, will just use one core????if so,
> can we specify the use different core. eg: dom0=core2 domU=core3? xen will use
> just one core?

It brings Xen up on the boot CPU only. Nothing in the system will use the
other CPUs. You don't want to use this option really.

 -- Keir

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