Re: QEMU PIC indirection patch for in-kernel APIC work

Dong, Eddie
2007-04-05T14:52:16Z

Avi Kivity wrote:
>> With PIC in Xen, CPU2K gets 6.5% performance gain in old 1000HZ linux
>> kernel, KB gets 14% gain. We also did a shared PIC model which share
>> PIC state among Qemu & VMM with less LOC in VMM, it can get
>> similar performance gain (5.8% in my test).
>> BTW, at that time, PIT is in VMM already.
>>
>
> I expect that the gain in kvm will be smaller. Xen has to schedule
> dom0 to process the event channel (possibly on another cpu), dom0 has
> to schedule qemu-dm (again, possibly on another cpu), qemu does its
> thing, and then Xen has to schedule domU again. With kvm, we are
> always on the same cpu, and the only overhead is the system call,
> which is a few hundred nanoseconds. I expect with current hardware
> that it will be negligible (as a vmexit is measured in microseconds),
> but to become measurable as hardware improves.
Yes very possible.
We can take a quick mesurement to see how many cycles are spent in a
dummy
I/O emulation in KVM/Qemu. In Xen, one of my old P4 3.8GHZ platform
takes
about 50-60K cycles. We can see how many is it in KVM.
BTW, today Linux kernel is no longer 1000HZ :-)
thx,eddie