Re: [Qemu-devel] [PATCH] virtio: limit avail bytes lookahead

[Edivaldo de Araujo Pereira]

Dear friends, 

Please excuse-me for not reporting earlier... I confirm that the patch by 
Michael really fixes the problem I've reported. The regression has gone away 
when I used it, so I think it is good to be applied.

Thanks,

Edivaldo de Araújo Pereira


--- Em ter, 27/11/12, Michael S. Tsirkin <address@hidden> escreveu:

> De: Michael S. Tsirkin <address@hidden>
> Assunto: Re: [PATCH] virtio: limit avail bytes lookahead
> Para: "Amit Shah" <address@hidden>
> Cc: "Stefan Hajnoczi" <address@hidden>, "Edivaldo de Araujo Pereira" 
> <address@hidden>, address@hidden, "Anthony Liguori" <address@hidden>, "Bug 
> 1066055" <address@hidden>
> Data: Terça-feira, 27 de Novembro de 2012, 8:25
> On Thu, Nov 01, 2012 at 06:07:21PM
> +0200, Michael S. Tsirkin wrote:
> > Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f
> introduced
> > a regression in virtio-net performance because it
> looks
> > into the ring aggressively while we really only care
> > about a single packet worth of buffers.
> > To fix, add parameters limiting lookahead, and
> > use in virtqueue_avail_bytes.
> > 
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > Reported-by: Edivaldo de Araujo Pereira <address@hidden>
> 
> Ping.
> Anthony - going to apply this?
> 
> 
> > ---
> > 
> > Edivaldo could you please confirm this fixes
> regression?
> > 
> > diff --git a/hw/virtio-serial-bus.c
> b/hw/virtio-serial-bus.c
> > index d20bd8b..a761cdc 100644
> > --- a/hw/virtio-serial-bus.c
> > +++ b/hw/virtio-serial-bus.c
> > @@ -297,7 +297,7 @@ size_t
> virtio_serial_guest_ready(VirtIOSerialPort *port)
> >      if (use_multiport(port->vser)
> && !port->guest_connected) {
> >          return 0;
> >      }
> > -    virtqueue_get_avail_bytes(vq,
> &bytes, NULL);
> > +    virtqueue_get_avail_bytes(vq,
> &bytes, NULL, UINT_MAX, 0);
> >      return bytes;
> >  }
> >  
> > diff --git a/hw/virtio.c b/hw/virtio.c
> > index ec8b7d8..f40a8c5 100644
> > --- a/hw/virtio.c
> > +++ b/hw/virtio.c
> > @@ -336,7 +336,8 @@ static unsigned
> virtqueue_next_desc(hwaddr desc_pa,
> >  }
> >  
> >  void virtqueue_get_avail_bytes(VirtQueue *vq,
> unsigned int *in_bytes,
> > -             
>              
>    unsigned int *out_bytes)
> > +             
>              
>    unsigned int *out_bytes,
> > +             
>              
>    unsigned max_in_bytes, unsigned
> max_out_bytes)
> >  {
> >      unsigned int idx;
> >      unsigned int total_bufs, in_total,
> out_total;
> > @@ -385,6 +386,9 @@ void
> virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int
> *in_bytes,
> >              } else
> {
> >               
>   out_total += vring_desc_len(desc_pa, i);
> >              }
> > +            if (in_total
> >= max_in_bytes && out_total >= max_out_bytes)
> {
> > +             
>   goto done;
> > +            }
> >          } while ((i =
> virtqueue_next_desc(desc_pa, i, max)) != max);
> >  
> >          if (!indirect)
> > @@ -392,6 +396,7 @@ void
> virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int
> *in_bytes,
> >          else
> >             
> total_bufs++;
> >      }
> > +done:
> >      if (in_bytes) {
> >          *in_bytes =
> in_total;
> >      }
> > @@ -405,12 +410,8 @@ int
> virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
> >  {
> >      unsigned int in_total, out_total;
> >  
> > -    virtqueue_get_avail_bytes(vq,
> &in_total, &out_total);
> > -    if ((in_bytes && in_bytes <
> in_total)
> > -        || (out_bytes &&
> out_bytes < out_total)) {
> > -        return 1;
> > -    }
> > -    return 0;
> > +    virtqueue_get_avail_bytes(vq,
> &in_total, &out_total, in_bytes, out_bytes);
> > +    return in_bytes <= in_total
> && out_bytes <= out_total;
> >  }
> >  
> >  void virtqueue_map_sg(struct iovec *sg, hwaddr
> *addr,
> > diff --git a/hw/virtio.h b/hw/virtio.h
> > index ac482be..1278328 100644
> > --- a/hw/virtio.h
> > +++ b/hw/virtio.h
> > @@ -150,7 +150,8 @@ int virtqueue_pop(VirtQueue *vq,
> VirtQueueElement *elem);
> >  int virtqueue_avail_bytes(VirtQueue *vq, unsigned
> int in_bytes,
> >               
>             unsigned int
> out_bytes);
> >  void virtqueue_get_avail_bytes(VirtQueue *vq,
> unsigned int *in_bytes,
> > -             
>              
>    unsigned int *out_bytes);
> > +             
>              
>    unsigned int *out_bytes,
> > +             
>              
>    unsigned max_in_bytes, unsigned
> max_out_bytes);
> >  
> >  void virtio_notify(VirtIODevice *vdev, VirtQueue
> *vq);
> >  
>