diff mbox series

[v3] vhost: batch used descs chains write-back with packed ring

Message ID 20181220100022.3531-1-maxime.coquelin@redhat.com (mailing list archive)
State Superseded, archived
Delegated to: Maxime Coquelin
Headers show
Series [v3] vhost: batch used descs chains write-back with packed ring | expand

Checks

Context Check Description
ci/Intel-compilation success Compilation OK
ci/mellanox-Performance-Testing success Performance Testing PASS
ci/intel-Performance-Testing success Performance Testing PASS

Commit Message

Maxime Coquelin Dec. 20, 2018, 10 a.m. UTC
Instead of writing back descriptors chains in order, let's
write the first chain flags last in order to improve batching.

With Kernel's pktgen benchmark, ~3% performance gain is measured.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/virtio_net.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

Comments

Tiwei Bie Dec. 20, 2018, 10:07 a.m. UTC | #1
On Thu, Dec 20, 2018 at 11:00:22AM +0100, Maxime Coquelin wrote:
> Instead of writing back descriptors chains in order, let's
> write the first chain flags last in order to improve batching.
> 
> With Kernel's pktgen benchmark, ~3% performance gain is measured.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/virtio_net.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)

Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Michael S. Tsirkin Dec. 20, 2018, 2:30 p.m. UTC | #2
On Thu, Dec 20, 2018 at 11:00:22AM +0100, Maxime Coquelin wrote:
> Instead of writing back descriptors chains in order, let's
> write the first chain flags last in order to improve batching.
> 
> With Kernel's pktgen benchmark, ~3% performance gain is measured.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/virtio_net.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 8c657a101..66ccd3c35 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>  {
>  	int i;
>  	uint16_t used_idx = vq->last_used_idx;
> +	uint16_t head_idx = vq->last_used_idx;
> +	uint16_t head_flags = 0;
>  
>  	/* Split loop in two to save memory barriers */
>  	for (i = 0; i < vq->shadow_used_idx; i++) {
> @@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>  			flags &= ~VRING_DESC_F_AVAIL;
>  		}
>  
> -		vq->desc_packed[vq->last_used_idx].flags = flags;
> +		if (i > 0) {
> +			vq->desc_packed[vq->last_used_idx].flags = flags;
>  
> -		vhost_log_cache_used_vring(dev, vq,
> +			vhost_log_cache_used_vring(dev, vq,
>  					vq->last_used_idx *
>  					sizeof(struct vring_packed_desc),
>  					sizeof(struct vring_packed_desc));
> +		} else {
> +			head_idx = vq->last_used_idx;
> +			head_flags = flags;
> +		}
>  
>  		vq->last_used_idx += vq->shadow_used_packed[i].count;
>  		if (vq->last_used_idx >= vq->size) {
> @@ -140,7 +147,15 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>  		}
>  	}
>  
> +	vq->desc_packed[head_idx].flags = head_flags;
> +
>  	rte_smp_wmb();
> +
> +	vhost_log_cache_used_vring(dev, vq,
> +				head_idx *
> +				sizeof(struct vring_packed_desc),
> +				sizeof(struct vring_packed_desc));
> +
>  	vq->shadow_used_idx = 0;
>  	vhost_log_cache_sync(dev, vq);

How about moving rte_smp_wmb into logging functions?
This way it's free with log disabled even on arm...

>  }
> -- 
> 2.17.2
Maxime Coquelin Dec. 20, 2018, 3:32 p.m. UTC | #3
On 12/20/18 3:30 PM, Michael S. Tsirkin wrote:
> On Thu, Dec 20, 2018 at 11:00:22AM +0100, Maxime Coquelin wrote:
>> Instead of writing back descriptors chains in order, let's
>> write the first chain flags last in order to improve batching.
>>
>> With Kernel's pktgen benchmark, ~3% performance gain is measured.
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> ---
>>   lib/librte_vhost/virtio_net.c | 19 +++++++++++++++++--
>>   1 file changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
>> index 8c657a101..66ccd3c35 100644
>> --- a/lib/librte_vhost/virtio_net.c
>> +++ b/lib/librte_vhost/virtio_net.c
>> @@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>   {
>>   	int i;
>>   	uint16_t used_idx = vq->last_used_idx;
>> +	uint16_t head_idx = vq->last_used_idx;
>> +	uint16_t head_flags = 0;
>>   
>>   	/* Split loop in two to save memory barriers */
>>   	for (i = 0; i < vq->shadow_used_idx; i++) {
>> @@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>   			flags &= ~VRING_DESC_F_AVAIL;
>>   		}
>>   
>> -		vq->desc_packed[vq->last_used_idx].flags = flags;
>> +		if (i > 0) {
>> +			vq->desc_packed[vq->last_used_idx].flags = flags;
>>   
>> -		vhost_log_cache_used_vring(dev, vq,
>> +			vhost_log_cache_used_vring(dev, vq,
>>   					vq->last_used_idx *
>>   					sizeof(struct vring_packed_desc),
>>   					sizeof(struct vring_packed_desc));
>> +		} else {
>> +			head_idx = vq->last_used_idx;
>> +			head_flags = flags;
>> +		}
>>   
>>   		vq->last_used_idx += vq->shadow_used_packed[i].count;
>>   		if (vq->last_used_idx >= vq->size) {
>> @@ -140,7 +147,15 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>   		}
>>   	}
>>   
>> +	vq->desc_packed[head_idx].flags = head_flags;
>> +
>>   	rte_smp_wmb();
>> +
>> +	vhost_log_cache_used_vring(dev, vq,
>> +				head_idx *
>> +				sizeof(struct vring_packed_desc),
>> +				sizeof(struct vring_packed_desc));
>> +
>>   	vq->shadow_used_idx = 0;
>>   	vhost_log_cache_sync(dev, vq);
> 
> How about moving rte_smp_wmb into logging functions?
> This way it's free with log disabled even on arm...

That's what I initially suggested in my reply to v2.
Problem is that in split ring case, we already have a barrier before
cache sync, and we need it even if logging is disabled.

But I think you are right, it might be better to have the barrier twice
in split ring case when logging is enabled and none for packed ring when
logging is disabled.

I'll post a v4.

Thanks,
Maxime

>>   }
>> -- 
>> 2.17.2
diff mbox series

Patch

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 8c657a101..66ccd3c35 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -97,6 +97,8 @@  flush_shadow_used_ring_packed(struct virtio_net *dev,
 {
 	int i;
 	uint16_t used_idx = vq->last_used_idx;
+	uint16_t head_idx = vq->last_used_idx;
+	uint16_t head_flags = 0;
 
 	/* Split loop in two to save memory barriers */
 	for (i = 0; i < vq->shadow_used_idx; i++) {
@@ -126,12 +128,17 @@  flush_shadow_used_ring_packed(struct virtio_net *dev,
 			flags &= ~VRING_DESC_F_AVAIL;
 		}
 
-		vq->desc_packed[vq->last_used_idx].flags = flags;
+		if (i > 0) {
+			vq->desc_packed[vq->last_used_idx].flags = flags;
 
-		vhost_log_cache_used_vring(dev, vq,
+			vhost_log_cache_used_vring(dev, vq,
 					vq->last_used_idx *
 					sizeof(struct vring_packed_desc),
 					sizeof(struct vring_packed_desc));
+		} else {
+			head_idx = vq->last_used_idx;
+			head_flags = flags;
+		}
 
 		vq->last_used_idx += vq->shadow_used_packed[i].count;
 		if (vq->last_used_idx >= vq->size) {
@@ -140,7 +147,15 @@  flush_shadow_used_ring_packed(struct virtio_net *dev,
 		}
 	}
 
+	vq->desc_packed[head_idx].flags = head_flags;
+
 	rte_smp_wmb();
+
+	vhost_log_cache_used_vring(dev, vq,
+				head_idx *
+				sizeof(struct vring_packed_desc),
+				sizeof(struct vring_packed_desc));
+
 	vq->shadow_used_idx = 0;
 	vhost_log_cache_sync(dev, vq);
 }