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SubjectRe: [PATCH 2/2] virtio_blk: blk-mq support
Hello Jens,

On Tue, Oct 29, 2013 at 03:34:01PM -0600, Jens Axboe wrote:
> On 10/28/2013 02:52 AM, Christoph Hellwig wrote:
> > On Mon, Oct 28, 2013 at 01:17:54PM +1030, Rusty Russell wrote:
> >> Let's pretend I'm stupid.
> >>
> >> We don't actually have multiple queues through to the host, but we're
> >> pretending to, because it makes the block layer go faster?
> >>
> >> Do I want to know *why* it's faster? Or should I look the other way?
> >
> > You shouldn't. To how multiple queues benefit here I'd like to defer to
> > Jens, given the single workqueue I don't really know where to look here.
> The 4 was chosen to "have some number of multiple queues" and to be able
> to exercise that part, no real performance testing was done by me after
> the implementation to verify whether it was faster at 1, 2, 4, or
> others. But it was useful for that! For merging, we can easily just make
> it 1 since that's the most logical transformation. I can set some time
> aside to play with multiple queues and see if we gain anything, but that
> can be done post merge.
> > The real benefit that unfortunately wasn't obvious from the description
> > is that even with just a single queue the blk-multiqueue infrastructure
> > will be a lot faster, because it is designed in a much more streaminline
> > fashion and avoids lots of lock roundtrips both during submission itself
> > and for submission vs complettion. Back when I tried to get virtio-blk
> > to perform well on high-end flash (the work that Asias took over later)
> > the queue_lock contention was the major issue in virtio-blk and this
> > patch gets rid of that even with a single queue.
> >
> > A good example are the patches from Nick to move scsi drivers over to
> > the infrastructure that only support a single queue. Even that gave
> > over a 10 fold improvement over the old code.
> >
> > Unfortunately I do not have access to this kind of hardware at the
> > moment, but I'd love to see if Asias or anyone at Red Hat could redo
> > those old numbers.
> I've got a variety of fast devices, so should be able to run that.
> Asias, let me know what your position is, it'd be great to have
> independent results.

I'd love to run the test but I do not have fast devices around. It would
be nice if Nick can give a try ;-)

1) Set .nr_hw_queues to 1 instead 4 for now.
2) Drop some more unused code in the other mail I sent

Otherwise, feel free to add:

Acked-by: Asias He <>

> --
> Jens Axboe


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