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

[Max Reitz <mreitz@redhat.com>]

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).

Max