2018-01-22 20:08 GMT+08:00 Mike Galbraith <efault@xxxxxx>: > 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. The waker and wakee are just sporadic on the same logical cpu in the guest(-smp 8, sockets=8, cores=1, threads=1) during the testing, in addition, binding the waker/wakee to one logical cpu in the guest(-smp 8, sockets=1, cores=4, threads=2) also can get the performance as better as 8 sockets setup. Regards, Wanpeng Li
