On Wed, Jan 12, 2022 at 1:12 PM Laszlo Ersek <[email protected]> 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 Nir _______________________________________________ Libguestfs mailing list [email protected] https://listman.redhat.com/mailman/listinfo/libguestfs
