linux-kernel - Re: [PATCH RFC 03/13] vhost: batching fetches

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Date:   Sun, 7 Jun 2020 09:57:10 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Eugenio Pérez <eperezma@...hat.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH RFC 03/13] vhost: batching fetches

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<mst@...hat.com>
> > > > Signed-off-by: Eugenio Pérez<eperezma@...hat.com>
> > > > Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
> > > > Signed-off-by: Michael S. Tsirkin<mst@...hat.com>
> > > > ---
> > > >    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?