Re: [Libguestfs] Virt-v2v performance benchmarking part 3

On Wed, Jan 12, 2022 at 1:12 PM Laszlo Ersek <lersek(a)redhat.com&gt; 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