On Thu, Apr 04, 2013 at 04:32:01PM -0700, Christoffer Dall wrote: > [...] > > >> >> to give us some idea how much performance we would gain from each approach? Thoughput should be completely unaffected anyway, since virtio just coalesces kicks internally. > >> > > >> > Latency is dominated by the scheduling latency. > >> > This means virtio-net is not the best benchmark. > >> > >> So what is a good benchmark? > > > > E.g. ping pong stress will do but need to look at CPU utilization, > > that's what is affected, not latency. > > > >> Is there any difference in speed at all? I strongly doubt it. One of virtio's main points is to reduce the number of kicks. > > > > For this stage of the project I think microbenchmarks are more appropriate. > > Doubling the price of exit is likely to be measureable. 30 cycles likely > > not ... > > > I don't quite understand this point here. If we don't have anything > real-world where we can measure a decent difference, then why are we > doing this? I would agree with Alex that the three test scenarios > proposed by him should be tried out before adding this complexity, > measured in CPU utilization or latency as you wish. Sure, plan to do real world benchmarks for PV MMIO versus PIO as well. I don't see why I should bother implementing hypercalls given that the kvm maintainer says they won't be merged. > FWIW, ARM always uses MMIO and provides hardware decoding of all sane > (not user register-writeback) instruction, but the hypercall vs. mmio > looks like this: > > hvc: 4,917 > mmio_kernel: 6,248 So 20% difference? That's not far from what happens on my intel laptop: vmcall 1519 outl_to_kernel 1745 10% difference here. > > But I doubt that an hvc wrapper around mmio decoding would take care > of all this difference, because the mmio operation needs to do other > work not realated to emulating the instruction in software, which > you'd have to do for an hvc anyway (populate kvm_mmio structure etc.) > > -Christoffer Instead of speculating, someone with relevant hardware could just try this, but kvm unittest doesn't seem to have arm support at the moment. Anyone working on this? -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
