On Mon, 3 Jul 2017, Yang Zhang wrote: > The background is that we(Alibaba Cloud) do get more and more complaints from > our customers in both KVM and Xen compare to bare-mental.After investigations, > the root cause is known to us: big cost in message passing workload(David show > it in KVM forum 2015) > > A typical message workload like below: > vcpu 0 vcpu 1 > 1. send ipi 2. doing hlt > 3. go into idle 4. receive ipi and wake up from hlt > 5. write APIC time twice 6. write APIC time twice to > to stop sched timer reprogram sched timer > 7. doing hlt 8. handle task and send ipi to > vcpu 0 > 9. same to 4. 10. same to 3 > > One transaction will introduce about 12 vmexits(2 hlt and 10 msr write). The > cost of such vmexits will degrades performance severely. Linux kernel already > provide idle=poll to mitigate the trend. But it only eliminates the IPI and > hlt vmexit. It has nothing to do with start/stop sched timer. A compromise > would be to turn off NOHZ kernel, but it is not the default config for new > distributions. You still can turn if off on the kernel command line via nohz=off Thanks, tglx
