Re: [PATCH] block: virtio_blk: don't hold spin lock during world switch

[Rusty Russell]

Ming Lei <ming.lei@xxxxxxxxxxxxx> writes:
> Firstly, it isn't necessary to hold lock of vblk->vq_lock
> when notifying hypervisor about queued I/O.
>
> Secondly, virtqueue_notify() will cause world switch and
> it may take long time on some hypervisors(such as, qemu-arm),
> so it isn't good to hold the lock and block other vCPUs.
>
> On arm64 quad core VM(qemu-kvm), the patch can increase I/O
> performance a lot with VIRTIO_RING_F_EVENT_IDX enabled:
> 	- without the patch: 14K IOPS
> 	- with the patch: 34K IOPS
>
> fio script:
> 	[global]
> 	direct=1
> 	bsrange=4k-4k
> 	timeout=10
> 	numjobs=4
> 	ioengine=libaio
> 	iodepth=64
>
> 	filename=/dev/vdc
> 	group_reporting=1
>
> 	[f1]
> 	rw=randread
>
> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>

Acked-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

Thanks!
Rusty.
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