[Qemu-devel] [PATCH] [qemu-img] CPU consuming optimization

[Dmitry Konishchev]

Hi! I was wondering why qemu-img consumes so much CPU when it converts
one partially allocated qcow2 image to another qcow2 image and I've
written a patch which improves the situation a little.

I have an image:
> $ qemu-img info ubuntu.10.04.qcow2
> image: ubuntu.10.04.qcow2
> file format: qcow2
> virtual size: 20G (21474836480 bytes)
> disk size: 2.2G
> cluster_size: 65536

I create a new copy on write image:
> $ qemu-img create -f qcow2 -o backing_file=ubuntu.10.04.qcow2 volume.qcow2 
> 100G
... and use it for a while.

Then I want to create a non-copy on write image from it to send it to someone:
> qemu-img convert -O qcow2 volume.qcow2 snapshot.qcow2


The last operation consumes a lot of CPU, so I run qemu-img under
profiler and realized, that most of CPU time is consumed by
is_not_zero() function. I had made a couple of optimizations on it and
got the following output for `time qemu-img convert -O qcow2
volume.qcow2 snapshot.qcow2`:

x86_64 machine:

Original qemu-img:
real 0m56.159s
user 0m34.670s
sys  0m12.079s

Patched qemu-img:
real 0m34.805s
user 0m18.445s
sys  0m12.552s


x86 machine:

Original qemu-img:
real 1m13.991s
user 0m24.734s
sys  0m6.604s

Patched qemu-img:
real 1m6.898s
user 0m16.021s
sys  0m6.700s

Please, see on the consumed user CPU time. I think that the
optimization worth it, so it will be awesome if you accept this patch
(see the attachment).

Thanks for your attention.

0001-is_not_zero-optimization-in-qemu-img.patch
Description: Text Data