Re: VM performance issue in KVM guests.

[Avi Kivity]

On 04/12/2010 05:04 AM, Zhang, Xiantao wrote:
> > What was the performance hit?  What was your I/O setup (image format,
> > using aio?)
> >      
> The issue only happens when vcpu number is over-committed(e.g. vcpu/pcpu>2) and physical cpus are saturated. For example,  when run webbench in windows OS in this case, its performance drops by 80%.  In our experiment, we are using image file through virtio, and I think aio should be used by default also.
>    

Is this on a machine that does pause-loop exits?  The current handing of 
PLE is very suboptimal.  With proper directed yield we should be much 
better there.

Without PLE, we need paravirtualized spinlocks, no way around it.

> > > After analysis about Linux scheduler, we found it is indeed caused
> > > by the known features of Linux schduler, such as AFFINE_WAKEUPS,
> > > SYNC_WAKEUPS etc. With these features on, linux schduler often tries
> > > to schedule the vcpu threads of one guests to one same logical
> > > processor when vcpus are over-committed and logical processors are
> > > saturated. Once the vcpu threads of one VM are scheduled to the same
> > > LP, system performance drops dramatically with some workloads(like
> > > webbench running in windows OS).
> > >
> > >        
> > Were the affine wakeups due to the kernel (emulated guest IPIs) or
> > qemu?
> >      
> We have basic guesses about the reasone, one is wakeup affinity between vcpu threads due to IPI, and the other is wakeup affinity between io theads and vcpu threads.
>    

It would be good to find out.

> > Most likely it also hits non-virtualized loads as well.  If the
> > scheduler pulls two long-running threads to the same cpu, performance
> > will take a hit.
> >      
> Since the hit only happens when physical cpus are saturated, and sheduling non-virtualized multiple threads of one process to same processor can benefit the performance due to cache share or other affinities, but you know it hurts performance a lot once schedule two vcpu theads to a same processor due to mutual spin-lock in guests.
>    

Spin loops need to be addressed first, they are known to kill 
performance in overcommit situations.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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