Messages in this thread | | | Subject | Re: [PATCH 1/3] ptr_ring: batch ring zeroing | From | Jason Wang <> | Date | Wed, 12 Apr 2017 16:03:13 +0800 |
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On 2017年04月07日 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@redhat.com> > --- > > 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
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