On Thu, Nov 14, 2013 at 07:37:54PM +0100, Max Reitz wrote: > On 05.11.2013 09:51, Stefan Hajnoczi wrote: > > 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 > > I just ran three tests for each (alternating between the modes), the > results are as following (comparing the total time): > > overlap-check=none: 421.29 s, 412.37 s, 414.60 s > Average: 416.09 s > Standard deviation: 3.79 s > > overlap-check=all: 420.02 s, 415.11 s, 423.37 s > Average: 419.50 s > Standard deviation: 3.39 s > > So, using all overlap checks is nearly consistently slower – however, > the difference is exactly within the single standard deviation. There is > a difference, but first of all, it is pretty much unremarkable, and > second, remember all tests are run in tmpfs. This is the absolute > maximum slowdown we'll ever experience.
Thanks! Stefan
