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

[Stefan Hajnoczi <stefanha@gmail.com>]

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