[vfio-users] Anything more I can do to improve CPU performance?

[Alex Williamson]

On Sun, Sep 6, 2015 at 12:39 PM, Garrett Powell <
garretttracypowell at gmail.com> wrote:

> I've been noticing lately that a particularly CPU intensive game has been
> keeping my CPU usage at >95%, and the game's been unplayable as a result.
> With an i7-4820k clocked at 4.7GHz, I should be getting much better
> performance. I've tried using some of the performance enhancements from the
> Arch forum thread, including enabling hyper-v enlightenments, but that
> caused my graphics driver to exit with error 43.
>
> Here's my launch script:
>
> #!/bin/bash
>
> export QEMU_AUDIO_DRV=pa
> export QEMU_PA_SERVER=localhost
> export PULSE_SERVER=localhost
>
> vfio-bind 0000:06:00.0
>
> synergys --daemon --config /etc/synergy.conf
>
> for i in {4..7}; do
>     echo performance >
> /sys/devices/system/cpu/cpu${i}/cpufreq/scaling_governor
> done
>
> echo 5120 > /proc/sys/vm/nr_hugepages
>
> taskset -ac 4-7 qemu-system-x86_64 \
>
...

> Is there anything more I can do? Am I doing anything wrong?
>

Your taskset is probably killing you, that's not the way to pin vCPUs.
Effectively what you're doing is pinning all the qemu processes to pCPU
4-7, without pinning anything to any one specific processor.  So not only
are your vCPUs floating around among those pCPUs, sometimes running on one,
sometimes on another, but you're forcing them to share those pCPUs with the
emulator itself.  Unfortunately doing vCPU pinning correctly via the
commandline is a pain in the butt.  It's trivially easy via libvirt
though.  Libvirt also allows pinning the emulator to different cpusets and
I offered some interesting experiments using that in a latency post to this
list last week.

Your choice of pCPUs is also poor as you're giving the host one half of
each core (thread0) and the VM the other half (thread1).  So if the host
does anything at all, it's preempting a vCPU.  Remember, threads share a
core and it's going to be a real miracle if the host scheduler and guest
scheduler can coordinate to cooperatively share a core.  Thanks,

Alex
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