[vfio-users] CPU Question

[ALG Bass]

So if you have an 8-core processor like the FX8350, you can assign all 8
cores to the guest and Linux will manage everything fine? Nothing will
hang?
Is the performance comparable to having a certain amount of cores pinned to
the guest?

On Fri, Sep 4, 2015 at 2:17 PM, Bradley Davis <bradleydavisjr at gmail.com>
wrote:

> It appears my last message wasn't delivered by google.
>
> What is the feature called that Xeon E5s have that others do not? I'm not
> seeing references to ACS on Intel ARC. I have an Intel Xeon X5660 and i'm
> trying to see if my processor has the feature. Maybe I just got lucky with
> my motherboard having each PCIe slot in its IOMMU group.
>
> On Thu, Sep 3, 2015 at 2:55 PM, Blank Field <ihatethisfield at gmail.com>
> wrote:
>
>> There are two ways of assigning CPU cores to the VM:
>> 1. The basic one. Just specifies the number of threads that QEMU creates,
>> then the OS handles CPU affinity by itself.
>> 2. The CPU pinning. Here you specify that QEMU can use logical cores #1,
>> 3, 5 and 7 for example.
>> Pinning the sibling cores will give you a small performance boost due to
>> caching and common(or not) elements, for example, we've pinned logical core
>> 1 and logical core 0, where core #1 is a hyper-threaded core from core
>> #0(these are called siblings, afair), and we need to transfer data from L1
>> cache of core #0 to L1 cache of core #1. Since that cache is in the same
>> place(that's actually a deep question), we do it instantly and gain
>> performance.
>>
>> So, there is a thread on the mailing list where aw submitted his tool for
>> measuring CPU to CPU latency using netperf.
>> It would be profitable for you to assign cores with minimal latency
>> between them.
>>
>> There's the bright side of this - you can assign every core.
>> The guest will do less work than expected when the host is loaded enough,
>> but the host will work at almost the same speed as usual when the guest is
>> loaded, pushing out the QEMU out of CPU.
>> In usual gaming proccess, even on my 4 core CPU that situations happen
>> very rarely, and i play some CPU-intensive games.
>>
>> There's a lot of technical detail that i've missed, correct me or ask
>> questions, maybe someone smarter than me will answer them precisely right.
>>
>> 2015-09-03 22:11 GMT+03:00 Daniel <flac at posteo.de>:
>>
>>> I'm planning to use a i7 6700k (4x 4 Ghz, supports Vt-d) which has
>>> hyperthreading. How many cores and how many threads would you assign to the
>>> VM? Is it possible to let the host use 4 threads (each connected to a
>>> different physical core) and to let the VM use 4 threads (each connected to
>>> a different physical core)? So that the VM has access to all 4 physical
>>> cores of the CPU to get highest gaming perfomance. I searched around but I
>>> didn't find a clear answer.
>>>
>>> Regards,
>>> Daniel
>>>
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>>> vfio-users at redhat.com
>>> https://www.redhat.com/mailman/listinfo/vfio-users
>>>
>>
>>
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>
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