Re: [Libguestfs] Virt-v2v performance benchmarking part 3
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.
Thanks,
Laszlo
The -W flag was only used for -o rhv-upload and not for any other
input or output method. See write_out_of_order in the 1.44 code.
Rich.