Re: Parallel Seq Scan

On Fri, Sep 18, 2015 at 9:45 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Fri, Sep 18, 2015 at 1:33 PM, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
> wrote:
>>
>> On Thu, Sep 3, 2015 at 8:21 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
>> wrote:
>> >
>> > Attached, find the rebased version of patch.
>>
>> Here are the performance test results:
>>
>> Query selectivity HashAgg HashAgg
>> (million) + seqscan(ms) +
>> parallel seq scan(ms)
>> 2
>> workers 4 workers 8 workers
>> $1 <= '001' 0.1 16717.00 7086.00
>> 4459.00 2912.00
>> $1 <= '004' 0.4 17962.00 7410.00
>> 4651.00 2977.00
>> $1 <= '008' 0.8 18870.00 7849.00
>> 4868.00 3092.00
>> $1 <= '016' 1.5 21368.00 8645.00
>> 6800.00 3486.00
>> $1 <= '030' 2.7 24622.00 14796.00 13108.00
>> 9981.00
>> $1 <= '060' 5.4 31690.00 29839.00 26544.00
>> 23814.00
>> $1 <= '080' 7.2 37147.00 40485.00 35763.00
>> 32679.00
>>
>
> I think here probably when the selectivity is more than 5, then it should
> not have selected Funnel plan. Have you by any chance changed
> cpu_tuple_comm_cost? If not, then you can try by setting value of
> parallel_setup_cost (may be 10) and then see if it selects the Funnel
> Plan. Is it possible for you to check the cost difference of Sequence
> and Funnel plan, hopefully explain or explain analyze should be sufficient?

Yes, I changed cpu_tuple_comm_cost to zero to observe how parallel seq scan
performs in high selectivity. Forgot to mention in the earlier mail.
Overall the
parallel seq scan performance is good.

>> And also attached perf results for selectivity of 0.1 million and 5.4
>> million cases for analysis.
>>
>
> I have checked perf reports and it seems that when selectivity is more, it
> seems to be spending time in some kernel calls which could be due
> communication of tuples.

Yes. And also in low selectivity with increase of workers, tas and
s_lock functions usage
is getting increased. May be these are also one of the reasons for
scaling problem.

Regards,
Hari Babu
Fujitsu Australia