[Libguestfs] Virt-v2v performance benchmarking part 3

[Laszlo Ersek]

On 01/12/22 15:04, Nir Soffer wrote:
> On Wed, Jan 12, 2022 at 1:12 PM Laszlo Ersek <lersek at redhat.com> wrote:
>>
>> On 01/11/22 11:56, Richard W.M. Jones wrote:
>>> On Tue, Jan 11, 2022 at 08:07:39AM +0100, Laszlo Ersek wrote:
>>>> hmmmm, not necessarily; according to the manual, "qemu-img convert" uses
>>>> (by default) 8 co-routines. There's also the -W flag ("out of order
>>>> writes"), which I don't know if the original virt-v2v used.
>>>
>>> I'm never sure how qemu coroutines map to threads.  I assume it's not
>>> 1-1, and it's somehow connected to the iothread setting?
>>
>> (I don't know the answer, but) this difference should not matter here I
>> believe; the QEMU co-routines submit async IO one way or another (IIRC),
>> so once those are in the kernel's hands, it shouldn't matter (from an IO
>> performance perspective) whether the userspace process waits on their
>> completions with heavy-weight threads, or co-routines or whatever. What
>> matters is that the kernel has multiple IO reqs in flight at the same time.
> 
> In practice this matters a lot. Using -W can be 6x times faster with qemu-img
> convert.
> 
> See how I tested this here:
> https://bugzilla.redhat.com/1511891#c57

Sorry, I didn't mean the difference between "-W" and "no -W". I meant
the difference between "NPTL threads submitting async IO requests" and
"coroutines submitting async IO requests".

Thanks
Laszlo