Re: [PATCH 5/6] kvm: qemu: virtio-net: handle all tx in I/O thread without timer

[Mark McLoughlin <markmc@redhat.com>]

On Thu, 2008-10-30 at 14:24 -0500, Anthony Liguori wrote:

> Instead of using an event fd, perhaps you could just schedule a bottom 
> half?  I think that would be a whole lot cleaner.

Nice, I hadn't noticed the bottom halves. Much cleaner, indeed.

Results are a little better too:

                                |       guest->host tput     |      host->guest tput
  netperf, 10x20s runs (Gb/s)   |    min/ mean/   max/stddev |   min/  mean/   max/stddev
  ------------------------------+----------------------------+---------------------------
  eventfd, 1k                   | 4.260/ 6.739/ 8.020/ 1.075 | 3.640/ 3.777/ 4.020/ 0.110
  eventfd, 16k                  | 8.530/ 8.867/ 9.110/ 0.146 | 7.220/ 7.276/ 7.360/ 0.040
  eventfd, 65k                  | 9.200/ 9.282/ 9.870/ 0.198 | 7.850/ 7.924/ 8.000/ 0.045

  bottom half, bh, 1k           | 4.890/ 6.712/ 7.800/ 0.800 | 4.900/ 5.002/ 5.130/ 0.067
  bottom half, bh, 16k          | 8.640/ 8.827/ 9.290/ 0.186 | 7.920/ 7.963/ 8.020/ 0.034
  bottom half, bh, 65k          | 9.310/ 9.323/ 9.350/ 0.010 | 8.740/ 8.769/ 8.800/ 0.016

                                |       guest->host %idle       |      host->guest %idle
  netperf, 10x20s runs (Gb/s)   |   avg/cpu#0/cpu#1/cpu#2/cpu#3 | avg/cpu#0/cpu#1/cpu#2/cpu#3
  ------------------------------+--------------------------------+-----------------------------
  eventfd, 1k                   | 57.68/99.81/99.96/18.76/21.91 | 65.49/99.84/99.97/15.20/52.46
  eventfd, 16k                  | 61.66/99.85/99.98/13.01/40.83 | 61.43/99.79/99.93/0.97/47.61
  eventfd, 65k                  | 62.57/99.99/99.95/13.13/39.28 | 60.15/99.81/99.97/1.27/43.10

  bottom half, 1k               | 57.29/99.83/99.81/22.30/15.93 | 63.36/99.98/99.95/2.92/51.26
  bottom half, 16k              | 62.21/99.81/99.96/15.74/42.40 | 60.21/99.81/99.97/0.92/43.78
  bottom half, 65k              | 62.25/99.99/99.96/12.67/37.45 | 58.63/99.83/99.96/0.92/36.71

(With this approach, patches 2-4 aren't needed anymore, but they're
still worthwhile cleanups IMHO)

Cheers,
Mark.

From: Mark McLoughlin <markmc@redhat.com>
Subject: kvm: qemu: virtio-net: handle all tx in I/O thread without timer

By removing the tx timer altogether and doing all the copies in the
I/O thread, we can keep the I/O churning away in parallel with the
guest generating more I/O.

In my tests, this significantly increases guest->host throughput,
causes a minor increase in host->guest throughput, reduces CPU
utilization somewhat and greatly reduces roundtrip times.

Even aside from the benchmark results, removing the arbitrary 150us
timer is a nicer option than coming up with a heuristic to make it
vary according to load. Finally, on kernels which don't have a
suitably low posix timer latency, we won't be scuppered by effectively
having e.g. a 1ms timer.

Note, this highlights that the I/O thread may become a scalability
concern and we might want to consider e.g. an I/O thread per device.

Note also that when tuning for a specific workload, which CPU
the I/O thread is pinned to is important.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
---
 qemu/hw/virtio-net.c |   60 +++++++++++++++++++++----------------------------
 1 files changed, 26 insertions(+), 34 deletions(-)

diff --git a/qemu/hw/virtio-net.c b/qemu/hw/virtio-net.c
index bc2ede6..bc198ef 100644
--- a/qemu/hw/virtio-net.c
+++ b/qemu/hw/virtio-net.c
@@ -35,8 +35,6 @@
 #define VIRTIO_NET_F_HOST_ECN	13	/* Host can handle TSO[6] w/ ECN in. */
 #define VIRTIO_NET_F_HOST_UFO	14	/* Host can handle UFO in. */
 
-#define TX_TIMER_INTERVAL 150000 /* 150 us */
-
 /* The config defining mac address (6 bytes) */
 struct virtio_net_config
 {
@@ -68,8 +66,7 @@ typedef struct VirtIONet
     VirtQueue *rx_vq;
     VirtQueue *tx_vq;
     VLANClientState *vc;
-    QEMUTimer *tx_timer;
-    int tx_timer_active;
+    QEMUBH *tx_bh;
 } VirtIONet;
 
 /* TODO
@@ -227,13 +224,14 @@ static void virtio_net_receive(void *opaque, const uint8_t *buf, int size)
 }
 
 /* TX */
-static void virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
+static int virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
 {
     VirtQueueElement elem;
     int has_vnet_hdr = tap_has_vnet_hdr(n->vc->vlan->first_client);
+    int num_packets = 0;
 
     if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
-        return;
+        return num_packets;
 
     while (virtqueue_pop(vq, &elem)) {
 	ssize_t len = 0;
@@ -256,38 +254,29 @@ static void virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
 
 	virtqueue_push(vq, &elem, len);
 	virtio_notify(&n->vdev, vq);
+
+	num_packets++;
     }
+
+    return num_packets;
 }
 
 static void virtio_net_handle_tx(VirtIODevice *vdev, VirtQueue *vq)
 {
     VirtIONet *n = to_virtio_net(vdev);
 
-    if (n->tx_timer_active) {
-	vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
-	qemu_del_timer(n->tx_timer);
-	n->tx_timer_active = 0;
-	virtio_net_flush_tx(n, vq);
-    } else {
-	qemu_mod_timer(n->tx_timer,
-		       qemu_get_clock(vm_clock) + TX_TIMER_INTERVAL);
-	n->tx_timer_active = 1;
-	vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
-    }
+    vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+    qemu_bh_schedule(n->tx_bh);
 }
 
-static void virtio_net_tx_timer(void *opaque)
+static void virtio_net_tx_bh(void *opaque)
 {
     VirtIONet *n = opaque;
 
-    n->tx_timer_active = 0;
-
-    /* Just in case the driver is not ready on more */
-    if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
-        return;
-
-    n->tx_vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
-    virtio_net_flush_tx(n, n->tx_vq);
+    if (!virtio_net_flush_tx(n, n->tx_vq))
+        n->tx_vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
+    else
+        qemu_bh_schedule(n->tx_bh);
 }
 
 static void virtio_net_save(QEMUFile *f, void *opaque)
@@ -297,7 +286,6 @@ static void virtio_net_save(QEMUFile *f, void *opaque)
     virtio_save(&n->vdev, f);
 
     qemu_put_buffer(f, n->mac, 6);
-    qemu_put_be32(f, n->tx_timer_active);
 }
 
 static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
@@ -310,12 +298,15 @@ static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
     virtio_load(&n->vdev, f);
 
     qemu_get_buffer(f, n->mac, 6);
-    n->tx_timer_active = qemu_get_be32(f);
 
-    if (n->tx_timer_active) {
-	qemu_mod_timer(n->tx_timer,
-		       qemu_get_clock(vm_clock) + TX_TIMER_INTERVAL);
-    }
+    return 0;
+}
+
+static int virtio_net_uninit(PCIDevice *dev)
+{
+    VirtIONet *n = (VirtIONet *)dev;
+
+    qemu_bh_delete(n->tx_bh);
 
     return 0;
 }
@@ -341,8 +332,9 @@ PCIDevice *virtio_net_init(PCIBus *bus, NICInfo *nd, int devfn)
     n->vc = qemu_new_vlan_client(nd->vlan, virtio_net_receive,
                                  virtio_net_can_receive, n);
 
-    n->tx_timer = qemu_new_timer(vm_clock, virtio_net_tx_timer, n);
-    n->tx_timer_active = 0;
+    n->tx_bh = qemu_bh_new(virtio_net_tx_bh, n);
+
+    n->vdev.pci_dev.unregister = virtio_net_uninit;
 
     register_savevm("virtio-net", virtio_net_id++, 1,
 		    virtio_net_save, virtio_net_load, n);
-- 
1.5.4.3