Messages in this thread |  | | Subject | Re: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi | From | zhenwei pi <> | Date | Thu, 21 Oct 2021 11:02:15 +0800 |
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On 10/21/21 4:12 AM, Sean Christopherson wrote: > On Wed, Oct 20, 2021, Wanpeng Li wrote: >> On Wed, 20 Oct 2021 at 20:08, zhenwei pi <pizhenwei@bytedance.com> wrote: >>> >>> Although host side exposes KVM PV SEND IPI feature to guest side, >>> guest should still have a chance to disable it. >>> >>> A typicall case of this parameter: >>> If the host AMD server enables AVIC feature, the flat mode of APIC >>> get better performance in the guest. >> >> Hmm, I didn't find enough valuable information in your posting. We >> observe AMD a lot before. >> https://lore.kernel.org/all/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@mail.gmail.com/T/#u > > I too would like to see numbers. I suspect the answer is going to be that > AVIC performs poorly in CPU overcommit scenarios because of the cost of managing > the tables and handling "failed delivery" exits, but that AVIC does quite well > when vCPUs are pinned 1:1 and IPIs rarely require an exit to the host. >
Test env: CPU: AMD EPYC 7642 48-Core Processor
Kmod args(enable avic and disable nested): modprobe kvm-amd nested=0 avic=1 npt=1
QEMU args(disable x2apic): ... -cpu host,x2apic=off ...
Benchmark tool: https://github.com/bytedance/kvm-utils/tree/master/microbenchmark/apic-ipi
~# insmod apic_ipi.ko options=5 && dmesg -c
apic_ipi: 1 NUMA node(s) apic_ipi: apic [flat] apic_ipi: apic->send_IPI[default_send_IPI_single+0x0/0x40] apic_ipi: apic->send_IPI_mask[kvm_send_ipi_mask+0x0/0x10] apic_ipi: IPI[kvm_send_ipi_mask] from CPU[0] to CPU[1] apic_ipi: total cycles 375671259, avg 3756 apic_ipi: IPI[flat_send_IPI_mask] from CPU[0] to CPU[1] apic_ipi: total cycles 221961822, avg 2219
apic->send_IPI_mask[kvm_send_ipi_mask+0x0/0x10] -> This line show current send_IPI_mask is kvm_send_ipi_mask(because of PV SEND IPI FEATURE)
apic_ipi: IPI[kvm_send_ipi_mask] from CPU[0] to CPU[1] apic_ipi: total cycles 375671259, avg 3756 -->These lines show the average cycles of each kvm_send_ipi_mask: 3756
apic_ipi: IPI[flat_send_IPI_mask] from CPU[0] to CPU[1] apic_ipi: total cycles 221961822, avg 2219 -->These lines show the average cycles of each flat_send_IPI_mask: 2219
-- zhenwei pi
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