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

Mark McLoughlin
2008-10-30T17:53:26Z

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 | 79 ++++++++++++++++++++++++++++---------------------
1 files changed, 45 insertions(+), 34 deletions(-)

diff --git a/qemu/hw/virtio-net.c b/qemu/hw/virtio-net.c
index bc2ede6..0612f5f 100644
--- a/qemu/hw/virtio-net.c
+++ b/qemu/hw/virtio-net.c
@@ -15,6 +15,8 @@
#include "net.h"
#include "qemu-timer.h"
#include "qemu-kvm.h"
+#include "qemu-char.h"
+#include "compatfd.h"

/* from Linux's virtio_net.h */

@@ -35,8 +37,6 @@
#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 +68,7 @@ typedef struct VirtIONet
VirtQueue *rx_vq;
VirtQueue *tx_vq;
VLANClientState *vc;
- QEMUTimer *tx_timer;
- int tx_timer_active;
+ int tx_eventfds[2];
} VirtIONet;

/* TODO
@@ -227,13 +226,14 @@ 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 +256,31 @@ 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_eventfd_write(n->tx_eventfds[1], 1);
}

-static void virtio_net_tx_timer(void *opaque)
+static void virtio_net_tx_event(void *opaque)
{
VirtIONet *n = opaque;

- n->tx_timer_active = 0;
+ qemu_eventfd_read(n->tx_eventfds[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_eventfd_write(n->tx_eventfds[1], 1);
}

static void virtio_net_save(QEMUFile *f, void *opaque)
@@ -297,7 +290,6 @@ 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 +302,16 @@ 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;
+
+ close(n->tx_eventfds[0]);
+ close(n->tx_eventfds[1]);

return 0;
}
@@ -324,13 +320,23 @@ PCIDevice *virtio_net_init(PCIBus *bus, NICInfo *nd, int devfn)
{
VirtIONet *n;
static int virtio_net_id;
+ int eventfds[2];
+
+ if (qemu_eventfd(eventfds) == -1) {
+ fprintf(stderr, "Failed to create eventfds : %s\n",
+ strerror(errno));
+ return NULL;
+ }

n = (VirtIONet *)virtio_init_pci(bus, "virtio-net", 6900, 0x1000,
0, VIRTIO_ID_NET,
0x02, 0x00, 0x00,
6, sizeof(VirtIONet));
- if (!n)
+ if (!n) {
+ close(eventfds[0]);
+ close(eventfds[1]);
return NULL;
+ }

n->vdev.get_config = virtio_net_update_config;
n->vdev.get_features = virtio_net_get_features;
@@ -341,8 +347,13 @@ 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;
+ fcntl(eventfds[0], F_SETFL, O_NONBLOCK);
+ n->tx_eventfds[0] = eventfds[0];
+ fcntl(eventfds[1], F_SETFL, O_NONBLOCK);
+ n->tx_eventfds[1] = eventfds[1];
+
+ n->vdev.pci_dev.unregister = virtio_net_uninit;
+ qemu_set_fd_handler2(n->tx_eventfds[0], NULL, virtio_net_tx_event, NULL, n);

register_savevm("virtio-net", virtio_net_id++, 1,
virtio_net_save, virtio_net_load, n);
--
1.5.4.3