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

[Max Reitz <mreitz@redhat.com>]

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.

Max