Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On Fri, Jan 26, 2018 at 12:36 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Fri, Jan 26, 2018 at 12:00 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>> On Thu, Jan 25, 2018 at 10:00 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>> At this point, my preferred solution is for someone to go implement
>>>> Amit's WaitForParallelWorkersToAttach() idea [1] (Amit himself seems
>>>> like the logical person for the job).
>>>>
>>>
>>> I can implement it and share a prototype patch with you which you can
>>> use to test parallel sort stuff.
>>
>> That would be great. Thank you.
>>
>>> I would like to highlight the
>>> difference which you will see with WaitForParallelWorkersToAttach as
>>> compare to WaitForParallelWorkersToFinish() is that the former will
>>> give you how many of nworkers_launched workers are actually launched
>>> whereas latter gives an error if any of the expected workers is not
>>> launched. I feel former is good and your proposed way of calling it
>>> after the leader is done with its work has alleviated the minor
>>> disadvantage of this API which is that we need for workers to startup.
>>
>> I'm not sure that it makes much difference, though, since in the end
>> WaitForParallelWorkersToFinish() is called anyway, much like
>> nodeGather.c. Have I missed something?
>>
>
> Nopes, you are right. I had in my mind that if we have something like
> what I am proposing, then we don't even need to detect failures in
> WaitForParallelWorkersToFinish and we can finish the work without
> failing.
>
>> I had imagined that WaitForParallelWorkersToAttach() would give me an
>> error in the style of WaitForParallelWorkersToFinish(), without
>> actually waiting for the parallel workers to finish.
>>
>
> I think that is also doable. I will give it a try and report back if
> I see any problem with it.
>

I have posted the patch for the above API and posted it on a new
thread [1]. Do let me know either here or on that thread if the patch
suffices your need?

[1] - https://www.postgresql.org/message-id/CAA4eK1%2Be2MzyouF5bg%3DOtyhDSX%2B%3DAo%3D3htN%3DT-r_6s3gCtKFiw%40mail.gmail.com

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com