[vfio-users] CPU Question

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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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