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

[Dor Laor]

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.