[PATCH] virtio_net tx performance fix

[Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

[-- Attachment #1: Type: text/plain, Size: 1733 bytes --]

>From f582caf612b446e42f1e80d5ef12c5b7322efd03 Mon Sep 17 00:00:00 2001
From: Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
Date: Mon, 28 Jan 2008 02:09:48 +0200
Subject: [PATCH] virtio_net tx performance fix

There was a problem with the location of the notify call in
add_buff function:
When VRING_USED_F_NO_NOTIFY is set, the host does not kick the
guest when packets were transmitted, as a result the guest runs
out of tx buffers sometimes. This is fine but the problem lies
when add_buf fails, it called notify and the host sends all the
pending tx pkts. When enable_cb was called, more_used(vq) returned
false so eventually the skb was dropped.

Moving notify from add_buf to enable_cb fixes this flow problem.
The tx performance boosted from 220Mbps to 850Mbps.

Signed-off-by: Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
---
 drivers/virtio/virtio_ring.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 3a28c13..592bbc9 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -87,8 +87,6 @@ static int vring_add_buf(struct virtqueue *_vq,
 	if (vq->num_free < out + in) {
 		pr_debug("Can't add buf len %i - avail = %i\n",
 			 out + in, vq->num_free);
-		/* We notify *even if* VRING_USED_F_NO_NOTIFY is set here. */
-		vq->notify(&vq->vq);
 		END_USE(vq);
 		return -ENOSPC;
 	}
@@ -232,6 +230,7 @@ static bool vring_enable_cb(struct virtqueue *_vq)
 	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
 	mb();
 	if (unlikely(more_used(vq))) {
+		vq->notify(&vq->vq);
 		vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
 		END_USE(vq);
 		return false;
-- 
1.5.3.7



[-- Attachment #2: 0001-virtio_net-tx-performance-fix.patch --]
[-- Type: application/mbox, Size: 1730 bytes --]

[-- Attachment #3: Type: text/plain, Size: 228 bytes --]

-------------------------------------------------------------------------
This SF.net email is sponsored by: Microsoft
Defy all challenges. Microsoft(R) Visual Studio 2008.
http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/

[-- Attachment #4: Type: text/plain, Size: 186 bytes --]

_______________________________________________
kvm-devel mailing list
kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org
https://lists.sourceforge.net/lists/listinfo/kvm-devel