On Sat, Oct 26, 2013 at 03:03:09PM +0200, Max Reitz wrote: > Am 20.09.2013 12:32, schrieb Stefan Hajnoczi: > > On Thu, Sep 19, 2013 at 05:07:56PM +0200, Max Reitz wrote: > >> As far as I understand, the I/O speed (the duration of an I/O > >> operation) should be pretty much the same for all scenarios, > >> however, the latency is the value in question (since the overlap > >> checks should affect the latency only). > > The other value to look at is the host CPU consumption per I/O. In > > other words, the CPU overhead added by performing the extra checks: > > > > efficiency = avg throughput / avg cpu utilization > > > > Once CPU consumption reaches 100% the workload is CPU-bound and we have > > a bottleneck. > > > > Hopefully the efficiency doesn't change noticably either, then we know > > there is no big impact from the extra checks. > > > > Stefan > > Okay, after fixing the VM state in qcow2, I was now finally able to > actually perform the CPU benchmark. On second thought, it wasn't really > neccessary, since I performed most of the tests in RAM anyway, so the > CPU was already the bottleneck for these tests. > > I ran bonnie++ (bonnie++ -s 4g -n 0 -x 16) from an arch live CD ISO on a > 5 GB qcow2 image formatted as ext4, both residing in /tmp; I prepared > the VM state to the point where I just had to press Enter to perform the > test and shut down the VM. I then performed a snapshot and used this > image as the basis for two tests, one with no overlap checks enabled and > one with all of them enabled. > > The time output for both qemu instances was respectively: > > echo 'sendkey ret' | time $QEMU_DIR/x86_64-softmmu/qemu-system-x86_64 > -cdrom arch.iso -drive file=base.qcow2,overlap-check=none -enable-kvm > -vga std -m 512 -loadvm 0 -monitor stdio > d 294.42s user 117.72s system 98% cpu 6:58.00 total > > echo 'sendkey ret' | time $QEMU_DIR/x86_64-softmmu/qemu-system-x86_64 > -cdrom arch.iso -drive file=base.qcow2,overlap-check=all -enable-kvm > -vga std -m 512 -loadvm 0 -monitor stdio > d 298.87s user 119.55s system 100% cpu 6:56.37 total > > So, as you can see, the CPU time differs only marginally (using all > overlap checks instead of none took 1.52 % more CPU time).
Good, looks like the impact isn't very noticable. I wonder if that 1.52% is reproducible or just noise, did you run the benchmark multiple times? Stefan
