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Re: [xen-4.12-testing test] 169199: regressions - FAIL
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 8 Apr 2022 13:56:35 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, osstest service owner <osstest-admin@xxxxxxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>
- Delivery-date: Fri, 08 Apr 2022 11:56:52 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08.04.2022 13:26, Andrew Cooper wrote:
> On 08/04/2022 12:08, Julien Grall wrote:
>> Hi,
>>
>> On 08/04/2022 12:01, Roger Pau Monné wrote:
>>>>> I could add a suitable dom0_max_vcpus parameter to osstest. XenServer
>>>>> uses 16 for example.
>>>>
>>>> I'm afraid a fixed number won't do, the more that iirc there are
>>>> systems with just a few cores in the pool (and you don't want to
>>>> over-commit by default).
>>>
>>> But this won't over commit, it would just assign dom0 16 vCPUs at
>>> most, if the system has less than 16 vCPUs that's what would be
>>> assigned to dom0.
>>
>> AFAICT, this is not the case on Arm. If you ask 16 vCPUs, then you
>> will get that number even if there are 8 pCPUs.
Same on x86, afaict.
>> In fact, the documentation of dom0_max_vcpus suggests that the numbers
>> of vCPUs can be more than the number of pCPUs.
>
> XenServer uses dom0_max_vcpus=1-16 so we dont oversubscribe (even if
> CPUs get turned off in firmware), but top out at 16.
>
> It is possible to use this option to create more vcpus, but whether dom0
> decides to do anything with them is up to dom0. Linux won't go any
> further than it can see CPUs listed in the ACPI tables (and yes, this is
> a host/guest laying violation for PV dom0 where dom0 sees the system
> ACPI tables.)
That has changed not so long ago, Linux will now use all vCPU-s
supplied by Xen. Since I was able to over-size Dom0 with XenoLinux,
I wanted to have the ability also with the upstream version.
Jan
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