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

[Anthony Liguori <anthony@codemonkey.ws>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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 <mst@redhat.com>
>> Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
>
> Ping.
> Anthony - going to apply this?

Yes, I've got it queued now.  In the future, please top post patches.

Regards,

Anthony Liguori

>
>
>> ---
>> 
>> 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);
>>