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

[Jon Panozzo]

Also experiencing this with Linux kernel 4.4.5 and QEMU 2.5.0.  Odd behavior indeed.

> On Feb 14, 2016, at 9:34 PM, globalgorrilla at fastmail.fm wrote:
> 
> Still the same with 4.5-rc4 and AUR qemu-git.
> 
> On 23 Jan 2016, at 18:55, Dan Ziemba wrote:
> 
>> On Tue, 2016-01-19 at 16:52 +0100, Friedrich Oslage wrote:
>>> I did some more testing and it turns out to be a kernel error,
>>> introduced somewhere around linux 4.3.
>>> 
>>> Short description of the error: 100% cpu usage while using 3d in a vm
>>> with vfio
>>> 
>>> My last known good kernel is 4.2.8, which gives me about 20% cpu
>>> usage
>>> while playing Diablo III. My first known bad kernel is 4.3.3, which
>>> gives me 100% cpu usage with every game, even while idle-ling in the
>>> menu.
>>> 
>>> On 01/17/2016 10:29 AM, Friedrich Oslage wrote:
>>>> my host system is linux-4.4.0 with qemu-2.5.0 and a 4-core i7.
>>>> Linux is
>>>> booted with isolcpus=1-3,5-7 to reserve 3 cores + threads for the
>>>> Windows 10 VM.
>>>> The VM's 3 cores(2 threads each) are pinned to the respective
>>>> physical
>>>> core/thread. The iothread is pinned to 1-3,5-7.
>> 
>> Interesting, I've also noticed similar behavior since upgrading to
>> kernel 4.4 and qemu 2.5.0.  I am running 4 cores / 8 threads for the VM
>> and pinned to individual host threads by libvirt, but I didn't boot
>> with the isolcpus arg. Emulator threads are pinned to the remaining 4
>> host threads.
>> 
>> Host is a i7-5930K, so 6 cores.  Guest is running Win 8.1.
>> 
>> Just sitting at the pause screen in Fallout: New Vegas, I have a single
>> CPU running at 100% according to both the windows task manager and the
>> host.  The host cpu thread that is at 100% is one of the ones with a
>> vcpu pinned to it.  In the guest, it does appear that the game is the
>> thing using all the CPU cycles, and the GPU is basically idle at this
>> time.
>> 
>> When I was previously running kernel 4.1.15 and qemu 2.4, New Vegas was
>> never demanding enough to max out any one cpu no matter what I was
>> doing with the game.  Same goes for more demanding games such as GTA V.
>>  
>> 
>> I upgraded both kernel and qemu at the same time, so I can't say which
>> caused it.  I haven't really noticed much difference in performance. If
>> anything, it might be slightly better.  I seemed to notice less frame
>> rate drops and stuttering while playing GTA V after the upgrade.
>> 
>> Dan
>> 
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> 
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