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

[Jason Wang]

On 2017å04æ15æ 05:00, Michael S. Tsirkin wrote:
> On Fri, Apr 14, 2017 at 03:52:23PM +0800, Jason Wang wrote:
> > 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
> Fascinating. How do we explain the gain with batch dequeue?

I count the drop rate (pktgen on tun and count tun tx) and maybe it can 
explain more or less:

Before this patch: TX xmit 1.8Mpps Tx dropped 0.23Mpps Tx total 2.04Mpps 
11% dropped
After this patch: Tx xmit 1.95Mpps Tx dropped 0.33Mpps Tx total 2.28Mpps 
14% dropped
With batched dequeuing: Tx xmit 2.24Mpps Tx dropped 0.01Mpps Tx total 
2.26Mpps ~0% dropped

With this patch, we remove the cache contention by blocking the producer 
more or less. With batch dequeuing, the ring is not full in 99% of the 
cases which probably means the producer is not blocked for most of the time.

> Is it just the lock overhead?

I remove the spinlocks for peeking and dequeuing on top of this patch. 
The tx pps were increased from ~2Mpps to ~2.08Mpps. So it was not only 
the lock overhead.

> Can you pls try to replace
> the lock with a simple non-fair atomic and see what happens?

Not sure I get the idea, we are going for fast path of spinlocks for 
sure which is just a cmpxchg().

Thanks