Re: [PATCH 0/9] kvm tools, qcow: Improve QCOW performance

Ingo Molnar
2011-07-11T10:36:51Z

* Pekka Enberg <penberg@kernel.org> wrote:

> > As Ingo already said, the cache mode is probably the major
> > difference. From what I can see in your code, cache=writeback
> > would be the equivalent for what tools/kvm is doing, however
> > cache=none (i.e. O_DIRECT) is what people usually do with qemu.
>
> Yup, I posted 'cache=writeback' results too which are much closer
> to tools/kvm numbers.

tools/kvm/ seems to be about 20% faster on average:

511229952 bytes (511 MB) copied, 7.68312 s, 66.5 MB/s
511229952 bytes (511 MB) copied, 7.54065 s, 67.8 MB/s
511229952 bytes (511 MB) copied, 9.34749 s, 54.7 MB/s
511229952 bytes (511 MB) copied, 9.2421 s, 55.3 MB/s
511229952 bytes (511 MB) copied, 9.9364 s, 51.5 MB/s
511229952 bytes (511 MB) copied, 10.0337 s, 51.0 MB/s
511229952 bytes (511 MB) copied, 9.39502 s, 54.4 MB/s

versus the qemu numbers:

511229952 bytes (511 MB) copied, 10.0879 s, 50.7 MB/s
511229952 bytes (511 MB) copied, 4.92686 s, 104 MB/s
511229952 bytes (511 MB) copied, 13.1955 s, 38.7 MB/s
511229952 bytes (511 MB) copied, 10.7322 s, 47.6 MB/s
511229952 bytes (511 MB) copied, 9.46115 s, 54.0 MB/s
511229952 bytes (511 MB) copied, 14.9963 s, 34.1 MB/s
511229952 bytes (511 MB) copied, 11.1701 s, 45.8 MB/s

but indeed there's (much) more variability in the Qemu numbers,
suggesting some cache artifact.

Are all of these measurements done via /dev/shm, to stabilize the
numbers and to remove disk IO delay artifacts?

Thanks,

Ingo