On Thu, 2011-12-01 at 09:58 +0200, Michael S. Tsirkin wrote:
> On Thu, Dec 01, 2011 at 01:12:25PM +1030, Rusty Russell wrote:
> > On Wed, 30 Nov 2011 18:11:51 +0200, Sasha Levin <levinsasha928@xxxxxxxxx> wrote:
> > > On Tue, 2011-11-29 at 16:58 +0200, Avi Kivity wrote:
> > > > On 11/29/2011 04:54 PM, Michael S. Tsirkin wrote:
> > > > > >
> > > > > > Which is actually strange, weren't indirect buffers introduced to make
> > > > > > the performance *better*? From what I see it's pretty much the
> > > > > > same/worse for virtio-blk.
> > > > >
> > > > > I know they were introduced to allow adding very large bufs.
> > > > > See 9fa29b9df32ba4db055f3977933cd0c1b8fe67cd
> > > > > Mark, you wrote the patch, could you tell us which workloads
> > > > > benefit the most from indirect bufs?
> > > > >
> > > >
> > > > Indirects are really for block devices with many spindles, since there
> > > > the limiting factor is the number of requests in flight. Network
> > > > interfaces are limited by bandwidth, it's better to increase the ring
> > > > size and use direct buffers there (so the ring size more or less
> > > > corresponds to the buffer size).
> > > >
> > >
> > > I did some testing of indirect descriptors under different workloads.
> >
> > MST and I discussed getting clever with dynamic limits ages ago, but it
> > was down low on the TODO list. Thanks for diving into this...
> >
> > AFAICT, if the ring never fills, direct is optimal. When the ring
> > fills, indirect is optimal (we're better to queue now than later).
> >
> > Why not something simple, like a threshold which drops every time we
> > fill the ring?
> >
> > struct vring_virtqueue
> > {
> > ...
> > int indirect_thresh;
> > ...
> > }
> >
> > virtqueue_add_buf_gfp()
> > {
> > ...
> >
> > if (vq->indirect &&
> > (vq->vring.num - vq->num_free) + out + in > vq->indirect_thresh)
> > return indirect()
> > ...
> >
> > if (vq->num_free < out + in) {
> > if (vq->indirect && vq->indirect_thresh > 0)
> > vq->indirect_thresh--;
> >
> > ...
> > }
> >
> > Too dumb?
> >
> > Cheers,
> > Rusty.
>
> We'll presumably need some logic to increment is back,
> to account for random workload changes.
> Something like slow start?
We can increment it each time the queue was less than 10% full, it
should act like slow start, no?
--
Sasha.
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