On Fri, 2011-07-29 at 16:58 -0700, Mike Waychison wrote:
> On Fri, Jul 29, 2011 at 3:55 PM, Shirley Ma <mashirle@xxxxxxxxxx>
> wrote:
> > Resubmit it with a typo fix.
> >
> > Signed-off-by: Shirley Ma <xma@xxxxxxxxxx>
> > ---
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 0c7321c..c8201d4 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -429,6 +429,22 @@ static int add_recvbuf_mergeable(struct
> virtnet_info *vi, gfp_t gfp)
> > return err;
> > }
> >
> > +static int fill_one(struct virtnet_info *vi, gfp_t gfp)
> > +{
> > + int err;
> > +
> > + if (vi->mergeable_rx_bufs)
> > + err = add_recvbuf_mergeable(vi, gfp);
> > + else if (vi->big_packets)
> > + err = add_recvbuf_big(vi, gfp);
> > + else
> > + err = add_recvbuf_small(vi, gfp);
> > +
> > + if (err >= 0)
> > + ++vi->num;
> > + return err;
> > +}
> > +
> > /* Returns false if we couldn't fill entirely (OOM). */
> > static bool try_fill_recv(struct virtnet_info *vi, gfp_t gfp)
> > {
> > @@ -436,17 +452,10 @@ static bool try_fill_recv(struct virtnet_info
> *vi, gfp_t gfp)
> > bool oom;
> >
> > do {
> > - if (vi->mergeable_rx_bufs)
> > - err = add_recvbuf_mergeable(vi, gfp);
> > - else if (vi->big_packets)
> > - err = add_recvbuf_big(vi, gfp);
> > - else
> > - err = add_recvbuf_small(vi, gfp);
> > -
> > + err = fill_one(vi, gfp);
> > oom = err == -ENOMEM;
> > if (err < 0)
> > break;
> > - ++vi->num;
> > } while (err > 0);
> > if (unlikely(vi->num > vi->max))
> > vi->max = vi->num;
> > @@ -506,13 +515,13 @@ again:
> > receive_buf(vi->dev, buf, len);
> > --vi->num;
> > received++;
> > - }
> > -
> > - if (vi->num < vi->max / 2) {
> > - if (!try_fill_recv(vi, GFP_ATOMIC))
> > + if (fill_one(vi, GFP_ATOMIC) < 0)
> > schedule_delayed_work(&vi->refill, 0);
> > }
> >
> > + /* notify buffers are refilled */
> > + virtqueue_kick(vi->rvq);
> > +
>
> How does this reduce latency? We are doing the same amount of work
> in both cases, and in both cases the newly available buffers are not
> visible to the device until the virtqueue_kick..
It averages the latency between each receive by filling only one set of
buffers vs. either none buffers or 1/2 ring size buffers fill between
receives.
>
> > /* Out of packets? */
> > if (received < budget) {
> > napi_complete(napi);
> >
> >
> > --
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