Horton

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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Robert Haas <robertmhaas@gmail.com>, Rushabh Lathia <rushabh.lathia@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2018-01-26T07:06:14Z
Lists: pgsql-hackers 
On Fri, Jan 26, 2018 at 12:00 PM, Peter Geoghegan <pg@bowt.ie> wrote:
> On Thu, Jan 25, 2018 at 10:00 PM, Amit Kapila <amit.kapila16@gmail.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.  However, it might take me some time as I
am busy with few other things and I am planning to take two days off
for some personal reasons, OTOH if it turns out to be a simple (which
I expect it should be), then I will report back early.

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

Commits

  1. Support parallel btree index builds.

  2. Report an ERROR if a parallel worker fails to start properly.

  3. Transfer state pertaining to pending REINDEX operations to workers.

  4. Add a barrier primitive for synchronizing backends.

  5. Allow DML commands that create tables to use parallel query.

  6. Refactor GetOldestXmin() to use flags

  7. Fix regression in parallel planning against inheritance tables.

  8. Don't create "holes" in BufFiles, in the new logtape code.

  9. Simplify the code for logical tape read buffers.

  10. Fix excessive memory consumption in the new sort pre-reading code.

  11. Implement binary heap replace-top operation in a smarter way.

  12. Cosmetic code cleanup in commands/extension.c.

  13. Speed up planner's scanning for parallel-query hazards.

  14. Read from the same worker repeatedly until it returns no tuple.

  15. Improve tuplesort.c to support variable merge order. The original coding