Re: [PATCH V2 2/2] vhost_net: conditionally enable tx polling
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2016-05-30 15:55:26
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kvm, lkml
On Mon, May 30, 2016 at 02:47:54AM -0400, Jason Wang wrote:
We always poll tx for socket, this is sub optimal since:
- it will be only used when we exceed the sndbuf of the socket.
- since we use two independent polls for tx and vq, this will slightly
increase the waitqueue traversing time and more important, vhost
could not benefit from commit
9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
tun_do_read for better sleep/wakeup efficiency") even if we've
stopped rx polling during handle_rx since tx poll were still left in
the waitqueue.
Why is this an issue?
sock_def_write_space only wakes up when queue is half empty,
not on each packet.
if ((atomic_read(&sk->sk_wmem_alloc) << 1) <= sk->sk_sndbuf)
I suspect the issue is with your previous patch,
it now pokes at the spinlock on data path
where it used not to.
Is that right?
quoted hunk ↗ jump to hunk
Fix this by conditionally enable tx polling only when -EAGAIN were met. Test shows about 8% improvement on guest rx pps. Before: ~1350000 After: ~1460000 Signed-off-by: Jason Wang <jasowang@redhat.com> --- drivers/vhost/net.c | 3 +++ 1 file changed, 3 insertions(+)diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index e91603b..5a05fa0 100644 --- a/drivers/vhost/net.c +++ b/drivers/vhost/net.c@@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net) goto out; vhost_disable_notify(&net->dev, vq); + vhost_net_disable_vq(net, vq); hdr_size = nvq->vhost_hlen; zcopy = nvq->ubufs;@@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net) % UIO_MAXIOV; } vhost_discard_vq_desc(vq, 1); + if (err == -EAGAIN) + vhost_net_enable_vq(net, vq); break; } if (err != len)-- 1.8.3.1