Re: [PATCH 1/3] ptr_ring: batch ring zeroing

[Jason Wang]

On 2017å04æ12æ 16:03, Jason Wang wrote:
> On 2017å04æ07æ 13:49, Michael S. Tsirkin wrote:
> > A known weakness in ptr_ring design is that it does not handle well the
> > situation when ring is almost full: as entries are consumed they are
> > immediately used again by the producer, so consumer and producer are
> > writing to a shared cache line.
> >
> > To fix this, add batching to consume calls: as entries are
> > consumed do not write NULL into the ring until we get
> > a multiple (in current implementation 2x) of cache lines
> > away from the producer. At that point, write them all out.
> >
> > We do the write out in the reverse order to keep
> > producer from sharing cache with consumer for as long
> > as possible.
> >
> > Writeout also triggers when ring wraps around - there's
> > no special reason to do this but it helps keep the code
> > a bit simpler.
> >
> > What should we do if getting away from producer by 2 cache lines
> > would mean we are keeping the ring moe than half empty?
> > Maybe we should reduce the batching in this case,
> > current patch simply reduces the batching.
> >
> > Notes:
> > - it is no longer true that a call to consume guarantees
> >    that the following call to produce will succeed.
> >    No users seem to assume that.
> > - batching can also in theory reduce the signalling rate:
> >    users that would previously send interrups to the producer
> >    to wake it up after consuming each entry would now only
> >    need to do this once in a batch.
> >    Doing this would be easy by returning a flag to the caller.
> >    No users seem to do signalling on consume yet so this was not
> >    implemented yet.
> >
> > Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
> > ---
> >
> > Jason, I am curious whether the following gives you some of
> > the performance boost that you see with vhost batching
> > patches. Is vhost batching on top still helpful?
>
> The patch looks good to me, will have a test for vhost batching patches.
>
> Thanks

Still helpful:

before this patch: 1.84Mpps
with this patch: 2.00Mpps
with batch dequeuing: 2.30Mpps

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Thanks