[vfio-users] cpu usage in guest != cpu usage in host, even with exclusive pinning

Jonathan Scruggs j.scruggs at gmail.com
Tue Apr 5 08:53:25 UTC 2016


I don't know what halt_poll_ns does, but wouldn't setting it to 0 disable
it and is disabling it bad?

So, what does halt_poll_ns do anyways?

On 4 April 2016 at 19:17, Okky Hendriansyah <okky.htf at gmail.com> wrote:

> On Mon, Apr 4, 2016 at 8:20 AM, Jayme Howard <g.prime at gmail.com> wrote:
>
>> Okay wow, Alex.  That made a huge difference immediately.  That dropped
>> me from 90% to 33% on the main thread, and all the children are below 10%.
>>
>
>> On Sun, Apr 3, 2016 at 8:07 PM, Alex Williamson <
>> alex.l.williamson at gmail.com> wrote:
>>
>>> If this is the issue I think it is, the unraid folks bisected this and
>>> found the culprit as:
>>>
>>> aca6ff2 KVM: dynamic halt-polling
>>>
>>> (
>>> https://github.com/torvalds/linux/commit/aca6ff29c4063a8d467cdee241e6b3bf7dc4a171
>>> )
>>>
>>> It seems to be a poor interaction of the halt polling interval vs the
>>> timer ticks on win10.  You can pick a different polling interval with the
>>> kvm module option halt_poll_ns.  The default is 500000.  It seemed that
>>> setting this to 400000 or lower resolves the issue.  You can do this via a
>>> modprobe entry, 'options kvm halt_poll_ns=400000' or on the kernel command
>>> line with kvm.halt_poll_ns=400000.  You can also change it dynamically via
>>> 'echo 400000 > /sys/module/kvm/parameters/halt_poll_ns'
>>>
>>
> I can also confirm that tuning the halt_poll_ns kvm module parameter on
> kernel 4.4.5 drops the host CPU usage! Haven't done any game benchmark but
> from listening to music the CPU load is as low as it is on 4.1.20. Although
> reducing the value to what Alex (400000) had suggested is probably
> sufficient, I'm currently trying out the value of 0 for it just like what
> Unraid 6.20 beta 20 changelog had it set on default [1]. Thanks for the
> info, Alex.
>
> [1]
> http://dnld.lime-technology.com/beta/unRAIDServer-6.2.0-beta20-x86_64.txt
>
> Best regards,
> Okky Hendriansyah
>
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