Re: unixbench context switch perfomance & cpu topology

[Peter Zijlstra]

On Mon, Jan 22, 2018 at 07:47:45PM +0800, Wanpeng Li wrote:
> Hi all,
> 
> We can observe unixbench context switch performance is heavily
> influenced by cpu topology which is exposed to the guest. the score is
> posted below, bigger is better, both the guest and the host kernel are
> 3.15-rc3(we can also reproduce against centos 7.4 693 guest/host), LLC
> is exposed to the guest, kvm adaptive halt-polling is default enabled,
> then start a guest w/ 8 logical cpus.
> 
> 
> 
> unixbench context switch
> -smp 8, sockets=8, cores=1, threads=1    382036
> -smp 8, sockets=4, cores=2, threads=1    132480
> -smp 8, sockets=2, cores=4, threads=1    128032
> -smp 8, sockets=2, cores=2, threads=2    131767
> -smp 8, sockets=1, cores=4, threads=2    132742
> -smp 8, sockets=1, cores=4, threads=2 (guest w/ nohz=off idle=poll)    331471
> 
> I can observe there are a lot of reschedule IPIs sent from one vCPU to
> another vCPU, the context switch workload switches between running and
> idle frequently which results in HLT instruction in the idle path, I
> use idle=poll to avoid vmexit due to HLT and to avoid reschedule IPIs
> since idle task checks TIF_NEED_RESCHED flags in a loop, nohz=off can
> stop to program lapic timer/other nohz stuffs. Any idea why sockets=8
> can get best performance?

I suspect because we load-balance less agressively across nodes than we
do within a cache domain.

Fix you benchmark to pin itself to a single CPU, that's the only
sensible way to obtain this number in any case.