Patchwork [V2,6/6] vhost_net: correctly limit the max pending buffers

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Submitter Jason Wang
Date Aug. 30, 2013, 4:29 a.m.
Message ID <1377836962-49780-7-git-send-email-jasowang@redhat.com>
Download mbox | patch
Permalink /patch/271098/
State Superseded
Delegated to: David Miller
Headers show

Comments

Jason Wang - Aug. 30, 2013, 4:29 a.m.
As Michael point out, We used to limit the max pending DMAs to get better cache
utilization. But it was not done correctly since it was one done when there's no
new buffers submitted from guest. Guest can easily exceeds the limitation by
keeping sending packets.

So this patch moves the check into main loop. Tests shows about 5%-10%
improvement on per cpu throughput for guest tx. But a 5% drop on per cpu
transaction rate for a single session TCP_RR.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/vhost/net.c |   15 ++++-----------
 1 files changed, 4 insertions(+), 11 deletions(-)
Michael S. Tsirkin - Sept. 2, 2013, 5:56 a.m.
On Fri, Aug 30, 2013 at 12:29:22PM +0800, Jason Wang wrote:
> As Michael point out, We used to limit the max pending DMAs to get better cache
> utilization. But it was not done correctly since it was one done when there's no
> new buffers submitted from guest. Guest can easily exceeds the limitation by
> keeping sending packets.
> 
> So this patch moves the check into main loop. Tests shows about 5%-10%
> improvement on per cpu throughput for guest tx. But a 5% drop on per cpu
> transaction rate for a single session TCP_RR.

Any explanation for the drop? single session TCP_RR is unlikely to
exceed VHOST_MAX_PEND, correct?

> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/vhost/net.c |   15 ++++-----------
>  1 files changed, 4 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index d09c17c..592e1f2 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -363,6 +363,10 @@ static void handle_tx(struct vhost_net *net)
>  		if (zcopy)
>  			vhost_zerocopy_signal_used(net, vq);
>  
> +		if ((nvq->upend_idx + vq->num - VHOST_MAX_PEND) % UIO_MAXIOV ==
> +		    nvq->done_idx)
> +			break;
> +
>  		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
>  					 ARRAY_SIZE(vq->iov),
>  					 &out, &in,
> @@ -372,17 +376,6 @@ static void handle_tx(struct vhost_net *net)
>  			break;
>  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>  		if (head == vq->num) {
> -			int num_pends;
> -
> -			/* If more outstanding DMAs, queue the work.
> -			 * Handle upend_idx wrap around
> -			 */
> -			num_pends = likely(nvq->upend_idx >= nvq->done_idx) ?
> -				    (nvq->upend_idx - nvq->done_idx) :
> -				    (nvq->upend_idx + UIO_MAXIOV -
> -				     nvq->done_idx);
> -			if (unlikely(num_pends > VHOST_MAX_PEND))
> -				break;
>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>  				vhost_disable_notify(&net->dev, vq);
>  				continue;
> -- 
> 1.7.1
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Jason Wang - Sept. 2, 2013, 6:30 a.m.
On 09/02/2013 01:56 PM, Michael S. Tsirkin wrote:
> On Fri, Aug 30, 2013 at 12:29:22PM +0800, Jason Wang wrote:
>> As Michael point out, We used to limit the max pending DMAs to get better cache
>> utilization. But it was not done correctly since it was one done when there's no
>> new buffers submitted from guest. Guest can easily exceeds the limitation by
>> keeping sending packets.
>>
>> So this patch moves the check into main loop. Tests shows about 5%-10%
>> improvement on per cpu throughput for guest tx. But a 5% drop on per cpu
>> transaction rate for a single session TCP_RR.
> Any explanation for the drop? single session TCP_RR is unlikely to
> exceed VHOST_MAX_PEND, correct?

Unlikely to exceed. Recheck the result, looks like it was not stable
enough. Will re-test and report.
>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>>  drivers/vhost/net.c |   15 ++++-----------
>>  1 files changed, 4 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index d09c17c..592e1f2 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -363,6 +363,10 @@ static void handle_tx(struct vhost_net *net)
>>  		if (zcopy)
>>  			vhost_zerocopy_signal_used(net, vq);
>>  
>> +		if ((nvq->upend_idx + vq->num - VHOST_MAX_PEND) % UIO_MAXIOV ==
>> +		    nvq->done_idx)
>> +			break;
>> +
>>  		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
>>  					 ARRAY_SIZE(vq->iov),
>>  					 &out, &in,
>> @@ -372,17 +376,6 @@ static void handle_tx(struct vhost_net *net)
>>  			break;
>>  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>>  		if (head == vq->num) {
>> -			int num_pends;
>> -
>> -			/* If more outstanding DMAs, queue the work.
>> -			 * Handle upend_idx wrap around
>> -			 */
>> -			num_pends = likely(nvq->upend_idx >= nvq->done_idx) ?
>> -				    (nvq->upend_idx - nvq->done_idx) :
>> -				    (nvq->upend_idx + UIO_MAXIOV -
>> -				     nvq->done_idx);
>> -			if (unlikely(num_pends > VHOST_MAX_PEND))
>> -				break;
>>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>>  				vhost_disable_notify(&net->dev, vq);
>>  				continue;
>> -- 
>> 1.7.1
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Jason Wang - Sept. 2, 2013, 8:37 a.m.
On 09/02/2013 02:30 PM, Jason Wang wrote:
> On 09/02/2013 01:56 PM, Michael S. Tsirkin wrote:
>> > On Fri, Aug 30, 2013 at 12:29:22PM +0800, Jason Wang wrote:
>>> >> As Michael point out, We used to limit the max pending DMAs to get better cache
>>> >> utilization. But it was not done correctly since it was one done when there's no
>>> >> new buffers submitted from guest. Guest can easily exceeds the limitation by
>>> >> keeping sending packets.
>>> >>
>>> >> So this patch moves the check into main loop. Tests shows about 5%-10%
>>> >> improvement on per cpu throughput for guest tx. But a 5% drop on per cpu
>>> >> transaction rate for a single session TCP_RR.
>> > Any explanation for the drop? single session TCP_RR is unlikely to
>> > exceed VHOST_MAX_PEND, correct?
> Unlikely to exceed. Recheck the result, looks like it was not stable
> enough. Will re-test and report.

Re-tested with more iterations, result shows no difference on TCP_RR
test and 5%-10% improvement on per cpu throughput for guest tx.

Will post V3 soon.
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Patch

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index d09c17c..592e1f2 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -363,6 +363,10 @@  static void handle_tx(struct vhost_net *net)
 		if (zcopy)
 			vhost_zerocopy_signal_used(net, vq);
 
+		if ((nvq->upend_idx + vq->num - VHOST_MAX_PEND) % UIO_MAXIOV ==
+		    nvq->done_idx)
+			break;
+
 		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
 					 ARRAY_SIZE(vq->iov),
 					 &out, &in,
@@ -372,17 +376,6 @@  static void handle_tx(struct vhost_net *net)
 			break;
 		/* Nothing new?  Wait for eventfd to tell us they refilled. */
 		if (head == vq->num) {
-			int num_pends;
-
-			/* If more outstanding DMAs, queue the work.
-			 * Handle upend_idx wrap around
-			 */
-			num_pends = likely(nvq->upend_idx >= nvq->done_idx) ?
-				    (nvq->upend_idx - nvq->done_idx) :
-				    (nvq->upend_idx + UIO_MAXIOV -
-				     nvq->done_idx);
-			if (unlikely(num_pends > VHOST_MAX_PEND))
-				break;
 			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
 				vhost_disable_notify(&net->dev, vq);
 				continue;