Messages in this thread | | | Date | Sat, 15 Apr 2017 01:50:48 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH 1/3] ptr_ring: batch ring zeroing |
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On Fri, Apr 14, 2017 at 03:52:23PM +0800, Jason Wang wrote: > > > On 2017年04月12日 16:03, Jason Wang wrote: > > > > > > 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 > > Still helpful: > > before this patch: 1.84Mpps > with this patch: 2.00Mpps > with batch dequeuing: 2.30Mpps
Just a thought: could you test dropping the consumer spinlock completely? Just around the peek?
As I said previously, perf c2c tool should be helpful to locate sources latency related to cache.
> Acked-by: Jason Wang <jasowang@redhat.com> > > Thanks
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