Re: [Qemu-devel] [PATCH v5 0/8] Add metadata overlap checks

[Stefan Hajnoczi <stefanha@redhat.com>]

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