From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark McLoughlin Subject: Re: [PATCH 5/6] kvm: qemu: virtio-net: handle all tx in I/O thread without timer Date: Fri, 31 Oct 2008 09:16:59 +0000 Message-ID: <1225444619.3758.21.camel@blaa> References: <> <1225389113-28332-1-git-send-email-markmc@redhat.com> <1225389113-28332-2-git-send-email-markmc@redhat.com> <1225389113-28332-3-git-send-email-markmc@redhat.com> <1225389113-28332-4-git-send-email-markmc@redhat.com> <1225389113-28332-5-git-send-email-markmc@redhat.com> <1225389113-28332-6-git-send-email-markmc@redhat.com> <490A09EF.2030006@codemonkey.ws> Reply-To: Mark McLoughlin Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Avi Kivity , kvm@vger.kernel.org To: Anthony Liguori Return-path: Received: from mx2.redhat.com ([66.187.237.31]:36268 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752392AbYJaJSL (ORCPT ); Fri, 31 Oct 2008 05:18:11 -0400 In-Reply-To: <490A09EF.2030006@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: 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 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 --- 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