Re: [PATCH v3 0/5] vhost: optimize enqueue

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

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On Thu, Sep 22, 2016 at 05:01:41PM +0800, Jianbo Liu wrote:
> On 22 September 2016 at 14:58, Wang, Zhihong <zhihong.wang@intel.com> wrote:
> >
> >
> >> -----Original Message-----
> >> From: Jianbo Liu [mailto:jianbo.liu@linaro.org]
> >> Sent: Thursday, September 22, 2016 1:48 PM
> >> To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> >> Cc: Wang, Zhihong <zhihong.wang@intel.com>; Maxime Coquelin
> >> <maxime.coquelin@redhat.com>; dev@dpdk.org
> >> Subject: Re: [dpdk-dev] [PATCH v3 0/5] vhost: optimize enqueue
> >>
> >> On 22 September 2016 at 10:29, Yuanhan Liu <yuanhan.liu@linux.intel.com>
> >> wrote:
> >> > On Wed, Sep 21, 2016 at 08:54:11PM +0800, Jianbo Liu wrote:
> >> >> >> > My setup consists of one host running a guest.
> >> >> >> > The guest generates as much 64bytes packets as possible using
> >> >> >>
> >> >> >> Have you tested with other different packet size?
> >> >> >> My testing shows that performance is dropping when packet size is
> >> more
> >> >> >> than 256.
> >> >> >
> >> >> >
> >> >> > Hi Jianbo,
> >> >> >
> >> >> > Thanks for reporting this.
> >> >> >
> >> >> >  1. Are you running the vector frontend with mrg_rxbuf=off?
> >> >> >
> >> Yes, my testing is mrg_rxbuf=off, but not vector frontend PMD.
> >>
> >> >> >  2. Could you please specify what CPU you're running? Is it Haswell
> >> >> >     or Ivy Bridge?
> >> >> >
> >> It's an ARM server.
> >>
> >> >> >  3. How many percentage of drop are you seeing?
> >> The testing result:
> >> size (bytes)     improvement (%)
> >> 64                   3.92
> >> 128                 11.51
> >> 256                  24.16
> >> 512                  -13.79
> >> 1024                -22.51
> >> 1500                -12.22
> >> A correction is that performance is dropping if byte size is larger than 512.
> >
> >
> > Jianbo,
> >
> > Could you please verify does this patch really cause enqueue perf to drop?
> >
> > You can test the enqueue path only by set guest to do rxonly, and compare
> > the mpps by show port stats all in the guest.
> >
> >
> Tested with testpmd, host: txonly, guest: rxonly
> size (bytes)     improvement (%)
> 64                    4.12
> 128                   6
> 256                   2.65
> 512                   -1.12
> 1024                 -7.02

There is a difference between Zhihong's code and the old I spotted in
the first time: Zhihong removed the avail_idx prefetch. I understand
the prefetch becomes a bit tricky when mrg-rx code path is considered;
thus, I didn't comment on that.

That's one of the difference that, IMO, could drop a regression. I then
finally got a chance to add it back.

A rough test shows it improves the performance of 1400B packet size greatly
in the "txonly in host and rxonly in guest" case: +33% is the number I get
with my test server (Ivybridge).

I guess this might/would help your case as well. Mind to have a test
and tell me the results?

BTW, I made it in rush; I haven't tested the mrg-rx code path yet.

Thanks.

	--yliu

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commit e5852d04bf87c02d6d0d8e6d8ded4c33030b9c9e
Author: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Date:   Tue Sep 27 17:51:15 2016 +0800

    xxxx
    
    Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 381dc27..41bfeba 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -61,6 +61,8 @@ struct buf_vector {
 	uint32_t desc_idx;
 };
 
+#define NR_AVAIL_IDX_PREFETCH	32
+
 /**
  * Structure contains variables relevant to RX/TX virtqueues.
  */
@@ -70,7 +72,7 @@ struct vhost_virtqueue {
 	struct vring_used	*used;
 	uint32_t		size;
 
-	/* Last index used on the available ring */
+	uint16_t		last_avail_idx;
 	uint16_t		last_used_idx;
 #define VIRTIO_INVALID_EVENTFD		(-1)
 #define VIRTIO_UNINITIALIZED_EVENTFD	(-2)
@@ -89,6 +91,9 @@ struct vhost_virtqueue {
 	/* Shadow used ring for performance */
 	struct vring_used_elem	*shadow_used_ring;
 	uint32_t		shadow_used_idx;
+
+	uint16_t		next_avail_idx;
+	uint16_t		avail_idx_buf[NR_AVAIL_IDX_PREFETCH];
 } __rte_cache_aligned;
 
 /* Old kernels have no such macro defined */
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 11a2c1a..1cc22fc 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -170,6 +170,41 @@ flush_used_ring(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	}
 }
 
+/* Fetch NR_AVAIL_IDX_PREFETCH avail entries at once */
+static void
+prefetch_avail_idx(struct vhost_virtqueue *vq)
+{
+	int i;
+
+	for (i = 0; i < NR_AVAIL_IDX_PREFETCH; i++) {
+		vq->avail_idx_buf[i] = vq->avail->ring[(vq->last_avail_idx + i) &
+					(vq->size - 1)];
+	}
+}
+
+static uint16_t
+next_avail_idx(struct vhost_virtqueue *vq)
+{
+	if (vq->next_avail_idx >= NR_AVAIL_IDX_PREFETCH) {
+		prefetch_avail_idx(vq);
+		vq->next_avail_idx = 0;
+		vq->last_avail_idx += NR_AVAIL_IDX_PREFETCH;
+	}
+
+	return vq->avail_idx_buf[vq->next_avail_idx++];
+}
+
+/*
+ * Just peek, but don't move forward the "next_avail_idx" pointer
+ * The caller also has to make sure the point doesn't go beyond
+ * the array.
+ */
+static uint16_t
+peek_next_avail_idx(struct vhost_virtqueue *vq)
+{
+	return vq->avail_idx_buf[vq->next_avail_idx];
+}
+
 static inline int __attribute__((always_inline))
 enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		uint16_t avail_idx, struct rte_mbuf *mbuf,
@@ -193,7 +228,7 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	mbuf_avail = mbuf_len;
 
 	/* get the current desc */
-	desc_current = vq->avail->ring[(vq->last_used_idx) & (vq->size - 1)];
+	desc_current = next_avail_idx(vq);
 	desc_chain_head = desc_current;
 	desc = &vq->desc[desc_current];
 	desc_addr = gpa_to_vva(dev, desc->addr);
@@ -235,9 +270,7 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 				if (avail_idx == vq->last_used_idx)
 					goto error;
 
-				desc_current =
-					vq->avail->ring[(vq->last_used_idx) &
-					(vq->size - 1)];
+				desc_current = next_avail_idx(vq);
 				desc_chain_head = desc_current;
 				desc_chain_len = 0;
 			} else
@@ -298,6 +331,7 @@ notify_guest(struct virtio_net *dev, struct vhost_virtqueue *vq)
 		eventfd_write(vq->callfd, (eventfd_t)1);
 }
 
+
 uint16_t
 rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 	struct rte_mbuf **pkts, uint16_t count)
@@ -331,14 +365,15 @@ rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 
 	/* start enqueuing packets 1 by 1 */
 	vq->shadow_used_idx = 0;
+	vq->next_avail_idx  = 0;
 	used_idx = vq->last_used_idx & (vq->size - 1);
 	avail_idx = *((volatile uint16_t *)&vq->avail->idx);
 	while (pkt_left && avail_idx != vq->last_used_idx) {
 		/* prefetch the next desc */
-		if (pkt_left > 1 && avail_idx != vq->last_used_idx + 1)
-			rte_prefetch0(&vq->desc[vq->avail->ring[
-					(vq->last_used_idx + 1) &
-					(vq->size - 1)]]);
+		if (pkt_left > 1 &&
+		    vq->next_avail_idx + 1 < NR_AVAIL_IDX_PREFETCH) {
+			rte_prefetch0(&vq->desc[peek_next_avail_idx(vq)]);
+		}
 
 		if (enqueue_packet(dev, vq, avail_idx, pkts[pkt_idx],
 					is_mrg_rxbuf))
@@ -347,6 +382,7 @@ rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 		pkt_idx++;
 		pkt_left--;
 	}
+	vq->last_avail_idx += vq->next_avail_idx;
 
 	/* batch update used ring for better performance */
 	if (likely(vq->shadow_used_idx > 0))