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

[Dor Laor <dlaor@redhat.com>]

On 01/11/2010 11:03 AM, Dor Laor wrote:
> On 01/11/2010 10:30 AM, Avi Kivity wrote:
>> 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);
>
> The above line should be moved out of the 'if'.
>
> Attached results with rhel5.4 (qemu0.11) for win2k8 32bit guest. Note
> the drastic reduction in cpu consumption.

Attachment did not survive the email server, so you'll have to trust me 
saying that cpu consumption was done from 65% -> 40% for reads and from 
80% --> 30% for writes

>
>>> + }
>>> +
>>
>> 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.
>>
>