Re: [POC] Faster processing at Gather node

On Sat, Sep 9, 2017 at 8:14 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Fri, Sep 8, 2017 at 11:07 PM, Alexander Kuzmenkov
> <a(dot)kuzmenkov(at)postgrespro(dot)ru> wrote:
>> Hi Rafia,
>>
>> I like the idea of reducing locking overhead by sending tuples in bulk. The
>> implementation could probably be simpler: you could extend the API of shm_mq
>> to decouple notifying the sender from actually putting data into the queue
>> (i.e., make shm_mq_notify_receiver public and make a variant of shm_mq_sendv
>> that doesn't send the notification).
>>
>
> Rafia can comment on details, but I would like to bring it to your
> notice that we need kind of local buffer (queue) for gathermerge
> processing as well where the data needs to be fetched in order from
> queues. So, there is always a chance that some of the workers have
> filled their queues while waiting for the master to extract the data.
> I think the patch posted by Rafia on the nearby thread [1] addresses
> both the problems by one patch.
>
>
> [1] - https://www.postgresql.org/message-id/CAOGQiiNiMhq5Pg3LiYxjfi2B9eAQ_q5YjS%3DfHiBJmbSOF74aBQ%40mail.gmail.com
>

Thanks Alexander for your interest in this work. As rightly pointed
out by Amit, when experimenting with this patch we found that there
are cases when master is busy and unable to read tuples in
shared_queue and the worker get stuck as it can not process tuples any
more. When experimenting aong these lines, I found that Q12 of TPC-H
is showing great performance improvement when increasing
shared_tuple_queue_size [1].
It was then we realised that merging this with the idea of giving an
illusion of larger tuple queue size with a local queue[1] could be
more beneficial. To precisely explain the meaning of merging the two
ideas, now we write tuples in local_queue once shared_queue is full
and as soon as we have filled some enough tuples in local queue we
copy the tuples from local to shared queue in one memcpy call. It is
giving good performance improvements for quite some cases.

I'll be glad if you may have a look at this patch and enlighten me
with your suggestions. :-)

[1] - https://www.postgresql.org/message-id/CAOGQiiNiMhq5Pg3LiYxjfi2B9eAQ_q5YjS%3DfHiBJmbSOF74aBQ%40mail.gmail.com

--
Regards,
Rafia Sabih
EnterpriseDB: http://www.enterprisedb.com/