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

Jason Wang
2017-04-14T07:52:34Z

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

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks