Re: [Qemu-block] [PATCH] virtio-blk: notify guest directly

[Sergio Lopez]

On Fri, Mar 02, 2018 at 01:38:52PM +0000, Stefan Hajnoczi wrote:
> On Tue, Dec 19, 2017 at 1:33 PM, sochin.jiang <address@hidden> wrote:
> >  Till now, we've already notify guest as a batch mostly, an
> >  extra BH won't decrease guest interrupts much, but cause a
> >  significant notification loss. Generally, we could have 15%
> >  or so performance lost in single queue IO models, as I tested.
> 
> Recent performance testing has shown that virtio-blk can underperform
> virtio-scsi due to the extra latency added by the BH.
> 
> The virtqueue EVENT_IDX feature mitigates interrupts when the guest
> interrupt handler has not had a chance to run yet.  Therefore, virtio
> already offers one level of interrupt mitigation and the BH adds
> additional latency on top.

FWIW, I'm writing a patch that disables the BH (notifying after each completion
instead) when EVENT_IDX is present. I'll send both the patch and some fio
numbers next week.

Sergio.