[Qemu-devel] [PATCH v5 3/3] dataplane: submit I/O as a batch

[Ming Lei]

Before commit 580b6b2aa2(dataplane: use the QEMU block
layer for I/O), dataplane for virtio-blk submits block
I/O as a batch.

This commit 580b6b2aa2 replaces the custom linux AIO
implementation(including submit I/O as a batch) with QEMU
block layer, but this commit causes ~40% throughput regression
on virtio-blk performance, and removing submitting I/O
as a batch is one of the causes.

This patch applies the newly introduced bdrv_io_plug() and
bdrv_io_unplug() interfaces to support submitting I/O
at batch for Qemu block layer, and in my test, the change
can improve throughput by ~30% with 'aio=native'.

Following my fio test script:

        [global]
        direct=1
        size=4G
        bsrange=4k-4k
        timeout=40
        numjobs=4
        ioengine=libaio
        iodepth=64
        filename=/dev/vdc
        group_reporting=1

        [f]
        rw=randread

Result on one of my small machine(host: x86_64, 2cores, 4thread, guest: 4cores):
        - qemu master: 65K IOPS
        - qemu master with these patches: 92K IOPS
        - 2.0.0 release(dataplane using custom linux aio): 104K IOPS

Reviewed-by: Paolo Bonzini <address@hidden>
Signed-off-by: Ming Lei <address@hidden>
---
 hw/block/dataplane/virtio-blk.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 6cd75f6..bed9f13 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -83,6 +83,7 @@ static void handle_notify(EventNotifier *e)
     };
 
     event_notifier_test_and_clear(&s->host_notifier);
+    bdrv_io_plug(s->blk->conf.bs);
     for (;;) {
         /* Disable guest->host notifies to avoid unnecessary vmexits */
         vring_disable_notification(s->vdev, &s->vring);
@@ -116,6 +117,7 @@ static void handle_notify(EventNotifier *e)
             break;
         }
     }
+    bdrv_io_unplug(s->blk->conf.bs);
 }
 
 /* Context: QEMU global mutex held */
-- 
1.7.9.5