|Subject:||PCI device perf limitation|
|Date:||Fri, 27 Mar 2020 15:17:59 -0700|
I noticed a perf issue in my own pci device, and managed to reproduce it with the pci-testdev with a small patch. Here:
Before my patch, I could start a VM with 32 of these devices happily. I can ssh to the VM within a minute. I also see that once the VM is up, I see small amount of kvm activity, i.e. less than 100 kvm exits a second.
After the patch, I can start no more than 3 devices. If I do, I seems like QEMU gets to a state of thrashing. The VM never comes up, and I see the following:
kvm statistics - summary
Event Total %Total CurAvg/s
kvm_fpu 52166411 46.8 2773738
kvm_userspace_exit 26083186 23.4 1386869
kvm_vcpu_wakeup 25415616 22.8 1386869
I tried to trace the kvm exit reason, but I get:
qemu-system-x86 83801  10892345.982869: kvm:kvm_userspace_exit: reason KVM_EXIT_UNKNOWN (0)
I am wondering if this is known performance limitation on io memory region? Is there a way around it?
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