linux-kernel - Re: unixbench context switch perfomance & cpu topology

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Date:   Mon, 22 Jan 2018 13:08:59 +0100
From:   Mike Galbraith <efault@....de>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: unixbench context switch perfomance & cpu topology

On Mon, 2018-01-22 at 19:47 +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?

Probably because with that topology, there is no shared llc, thus no
cross-core scheduling, micro-benchmark waker/wakee are stacked.  If
your benchmark does nothing but schedule, stacking makes beautiful (but
utterly meaningless) numbers.

	-Mike