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

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Date:	Fri, 30 May 2014 15:35:41 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ming Lei <ming.lei@...onical.com>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org
Cc:	Ming Lei <ming.lei@...onical.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] block: virtio_blk: don't hold spin lock during world switch

Ming Lei <ming.lei@...onical.com> 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@...tcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: virtualization@...ts.linux-foundation.org
> Signed-off-by: Ming Lei <ming.lei@...onical.com>

Acked-by: Rusty Russell <rusty@...tcorp.com.au>

Thanks!
Rusty.
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