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Re: [Xen-devel] [PATCH RFC 44/49] xen: round up max vcpus to scheduling granularity


  • To: Juergen Gross <jgross@xxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 1 Apr 2019 09:50:32 +0100
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Mon, 01 Apr 2019 08:50:42 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 29/03/2019 15:09, Juergen Gross wrote:
> Make sure the number of vcpus is always a multiple of the scheduling
> granularity. Note that we don't support a scheduling granularity above
> one on ARM.

I'm afraid that I don't think this is a clever move.  In turn, this
brings into question the approach to idle handling.

Firstly, with a proposed socket granularity, this would be 128 on some
systems which exist today.  Furthermore, consider the case where
cpupool0 has a granularity of 1, and a second pool has a granularity of
2.  A domain can be created with an odd number of vcpus and operate in
pool0 fine, but can't now be moved to pool1.

If at all possible, I think it would be better to try and reuse the idle
cpus for holes like this.  Seeing as you've been playing with this code
a lot, what is your assessment?

~Andrew

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