On 19/12/2017 14:33, sochin.jiang wrote:
> From: "sochin.jiang" <sochin.ji...@huawei.com>
>
> Till now, we've already notify guest as a batch mostly, an
> extra BH won't decrease guest interrupts much, but cause a
> significant notification loss. Generally, we could have 15%
> or so performance lost in single queue IO models, as I tested.
Interesting, this was indeed done to decrease interrupt overhead:
commit 5b2ffbe4d99843fd8305c573a100047a8c962327
Author: Ming Lei <tom.leim...@gmail.com>
Date: Sat Jul 12 12:08:53 2014 +0800
virtio-blk: dataplane: notify guest as a batch
Now requests are submitted as a batch, so it is natural
to notify guest as a batch too.
This may suppress interrupt notification to VM a lot:
- in my test, decreased by ~13K/sec
Signed-off-by: Ming Lei <ming....@canonical.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
Can you explain your benchmark setup?
Paolo
> Signed-off-by: sochin.jiang <sochin.ji...@huawei.com>
> ---
> hw/block/dataplane/virtio-blk.c | 19 +++++++------------
> 1 file changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 5556f0e..a261a1d 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -32,7 +32,6 @@ struct VirtIOBlockDataPlane {
>
> VirtIOBlkConf *conf;
> VirtIODevice *vdev;
> - QEMUBH *bh; /* bh for guest notification */
> unsigned long *batch_notify_vqs;
>
> /* Note that these EventNotifiers are assigned by value. This is
> @@ -44,14 +43,7 @@ struct VirtIOBlockDataPlane {
> AioContext *ctx;
> };
>
> -/* Raise an interrupt to signal guest, if necessary */
> -void virtio_blk_data_plane_notify(VirtIOBlockDataPlane *s, VirtQueue *vq)
> -{
> - set_bit(virtio_get_queue_index(vq), s->batch_notify_vqs);
> - qemu_bh_schedule(s->bh);
> -}
> -
> -static void notify_guest_bh(void *opaque)
> +static void notify_guest(void *opaque)
> {
> VirtIOBlockDataPlane *s = opaque;
> unsigned nvqs = s->conf->num_queues;
> @@ -75,7 +67,12 @@ static void notify_guest_bh(void *opaque)
> }
> }
>
> -/* Context: QEMU global mutex held */
> +/* Raise an interrupt to signal guest, if necessary */
> +void virtio_blk_data_plane_notify(VirtIOBlockDataPlane *s, VirtQueue *vq)
> +{
> + set_bit(virtio_get_queue_index(vq), s->batch_notify_vqs);
> + notify_guest(s);
> +}
> void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *conf,
> VirtIOBlockDataPlane **dataplane,
> Error **errp)
> @@ -122,7 +119,6 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev,
> VirtIOBlkConf *conf,
> } else {
> s->ctx = qemu_get_aio_context();
> }
> - s->bh = aio_bh_new(s->ctx, notify_guest_bh, s);
> s->batch_notify_vqs = bitmap_new(conf->num_queues);
>
> *dataplane = s;
> @@ -140,7 +136,6 @@ void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane
> *s)
> vblk = VIRTIO_BLK(s->vdev);
> assert(!vblk->dataplane_started);
> g_free(s->batch_notify_vqs);
> - qemu_bh_delete(s->bh);
> if (s->iothread) {
> object_unref(OBJECT(s->iothread));
> }
>
