Re: [PATCH v2] kvm tools: Add QCOW level2 caching support

[Ingo Molnar <mingo@elte.hu>]

* Prasad Joshi <prasadjoshi124@gmail.com> wrote:

> Summary of performance numbers
> ==============================
> There is not much difference with sequential character operations are
> performed, the code with caching performed better by small margin. The caching
> code performance raised by 12% with sequential block output and dropped by
> 0.5% with sequential block input. The caching code also suffered with
> Random seeks and performed badly by 12%. The performance numbers drastically
> improved with sequential creates (62%) and delete operations (30%).

Looking at the numbers i think it's pretty clear that from this point 
on the quality of IO tests should be improved: Bonnie is too noisy 
and does not cut it anymore for finer enhancements.

To make measurements easier you could also do a simple trick: put 
*all* of the disk image into /dev/shm and add a command-line debug 
option that add a fixed-amount udelay(1000) call every time the code 
reads from the disk image.

This introduces a ~1msec delay and thus simulates IO, but the delays 
are *constant* [make sure you use a high-res timers kernel], so they 
do not result in nearly as much measurement noise as real block IO 
does.

The IO delays will still be there, so any caching advantages (and CPU 
overhead reductions) will be measurable very clearly.

This way you are basically 'emulating' a real disk drive but you will 
emulate uniform latencies, which makes measurements a lot more 
reliable - while still relevant to the end result.

So if under such a measurement model you can prove an improvement 
with a patch, that improvement will be there with real disks as well 
- just harder to prove.

Wanna try this? I really think you are hitting the limits of your 
current measurement methodology and you will be wasting time running 
more measurements instead of saving time doing more intelligent 
measurements ;-)

Thanks,

	Ingo