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
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