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Message-ID: <20160530155521.GA5427@redhat.com> Date: Mon, 30 May 2016 18:55:21 +0300 From: "Michael S. Tsirkin" <mst@...hat.com> To: Jason Wang <jasowang@...hat.com> Cc: kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH V2 2/2] vhost_net: conditionally enable tx polling 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? > > 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@...hat.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
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