[Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device

[Avi Kivity <avi@redhat.com>]

On 01/11/2010 09:40 AM, Vadim Rozenfeld wrote:
> The following patch allows us to improve Windows virtio
> block driver performance on small size requests.
> Additionally, it leads to reducing of cpu usage on write IOs
>
>    

Note, this is not an improvement for Windows specifically.

> diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
> index a2f0639..0e3a8d5 100644
> --- a/hw/virtio-blk.c
> +++ b/hw/virtio-blk.c
> @@ -28,6 +28,7 @@ typedef struct VirtIOBlock
>       char serial_str[BLOCK_SERIAL_STRLEN + 1];
>       QEMUBH *bh;
>       size_t config_size;
> +    unsigned int pending;
>   } VirtIOBlock;
>
>   static VirtIOBlock *to_virtio_blk(VirtIODevice *vdev)
> @@ -87,6 +88,8 @@ typedef struct VirtIOBlockReq
>       struct VirtIOBlockReq *next;
>   } VirtIOBlockReq;
>
> +static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue
> *vq);
> +
>   static void virtio_blk_req_complete(VirtIOBlockReq *req, int status)
>   {
>       VirtIOBlock *s = req->dev;
> @@ -95,6 +98,11 @@ static void virtio_blk_req_complete(VirtIOBlockReq
> *req, int status)
>       virtqueue_push(s->vq,&req->elem, req->qiov.size +
> sizeof(*req->in));
>       virtio_notify(&s->vdev, s->vq);
>
> +    if(--s->pending == 0) {
> +        virtio_queue_set_notification(s->vq, 1);
> +        virtio_blk_handle_output(&s->vdev, s->vq);
> +    }
> +
>    

Coding style: space after if.  See the CODING_STYLE file.

> @@ -340,6 +348,9 @@ static void virtio_blk_handle_output(VirtIODevice
> *vdev, VirtQueue *vq)
>               exit(1);
>           }
>
> +        if(++s->pending == 1)
> +            virtio_queue_set_notification(s->vq, 0);
> +
>           req->out = (void *)req->elem.out_sg[0].iov_base;
>           req->in = (void *)req->elem.in_sg[req->elem.in_num -
> 1].iov_base;
>
>    

Coding style: space after if, braces after if.

Your patch is word wrapped, please send it correctly.  Easiest using git 
send-email.

The patch has potential to reduce performance on volumes with multiple 
spindles.  Consider two processes issuing sequential reads into a RAID 
array.  With this patch, the reads will be executed sequentially rather 
than in parallel, so I think a follow-on patch to make the minimum depth 
a parameter (set by the guest? the host?) would be helpful.

-- 
error compiling committee.c: too many arguments to function