Subject: Re: [PATCH RFC 03/13] vhost: batching fetches

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Jun 2020 02:01:52 -0400

On Mon, Jun 08, 2020 at 11:35:40AM +0800, Jason Wang wrote:
> 
> On 2020/6/7 下午9:57, Michael S. Tsirkin wrote:
> > On Fri, Jun 05, 2020 at 11:40:17AM +0800, Jason Wang wrote:
> > > On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:
> > > > On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
> > > > > On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> > > > > > With this patch applied, new and old code perform identically.
> > > > > > 
> > > > > > Lots of extra optimizations are now possible, e.g.
> > > > > > we can fetch multiple heads with copy_from/to_user now.
> > > > > > We can get rid of maintaining the log array.  Etc etc.
> > > > > > 
> > > > > > Signed-off-by: Michael S. Tsirkin<[email-masked]>
> > > > > > Signed-off-by: Eugenio Pérez<[email-masked]>
> > > > > > Link:https://lore.kernel.org/r/[email-masked]
> > > > > > Signed-off-by: Michael S. Tsirkin<[email-masked]>
> > > > > > ---
> > > > > >     drivers/vhost/test.c  |  2 +-
> > > > > >     drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
> > > > > >     drivers/vhost/vhost.h |  5 ++++-
> > > > > >     3 files changed, 47 insertions(+), 7 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > > > > > index 9a3a09005e03..02806d6f84ef 100644
> > > > > > --- a/drivers/vhost/test.c
> > > > > > +++ b/drivers/vhost/test.c
> > > > > > @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
> > > > > >     	dev = &n->dev;
> > > > > >     	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
> > > > > >     	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> > > > > > -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> > > > > > +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
> > > > > >     		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
> > > > > >     	f->private_data = n;
> > > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > > index 8f9a07282625..aca2a5b0d078 100644
> > > > > > --- a/drivers/vhost/vhost.c
> > > > > > +++ b/drivers/vhost/vhost.c
> > > > > > @@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> > > > > >     {
> > > > > >     	vq->num = 1;
> > > > > >     	vq->ndescs = 0;
> > > > > > +	vq->first_desc = 0;
> > > > > >     	vq->desc = NULL;
> > > > > >     	vq->avail = NULL;
> > > > > >     	vq->used = NULL;
> > > > > > @@ -367,6 +368,11 @@ static int vhost_worker(void *data)
> > > > > >     	return 0;
> > > > > >     }
> > > > > > +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> > > > > > +{
> > > > > > +	return vq->max_descs - UIO_MAXIOV;
> > > > > > +}
> > > > > 1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
> > > > > length memory regions for us to translate.
> > > > > 
> > > > Yes but I don't see the relevance. This tells us how many descriptors to
> > > > batch, not how many IOVs.
> > > Yes, but questions are:
> > > 
> > > - this introduce another obstacle to support more than 1K queue size
> > > - if we support 1K queue size, does it mean we need to cache 1K descriptors,
> > > which seems a large stress on the cache
> > > 
> > > Thanks
> > > 
> > > 
> > Still don't understand the relevance. We support up to 1K descriptors
> > per buffer just for IOV since we always did. This adds 64 more
> > descriptors - is that a big deal?
> 
> 
> If I understanding correctly, for net, the code tries to batch descriptors
> for at last one packet.
> 
> If we allow 1K queue size then we allow a packet that consists of 1K
> descriptors. Then we need to cache 1K descriptors.
> 
> Thanks

That case is already so pathological, I am not at all worried about
it performing well.

-- 
MST