Horton

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

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Rushabh Lathia <rushabh.lathia@gmail.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2018-01-03T03:41:35Z
Lists: pgsql-hackers 
On Tue, Jan 2, 2018 at 1:38 AM, Rushabh Lathia <rushabh.lathia@gmail.com> wrote:
> Need to do after the indexRelation build. So I added after update of
> pg_index,
> as indexRelation needed for plan_create_index_worders().
>
> Attaching the separate patch the same.

This made it so that REINDEX and CREATE INDEX CONCURRENTLY no longer
used parallelism. I think we need to do this very late, just before
nbtree's ambuild() routine is called from index.c.

> So you suggesting that need to do adjustment with the output of
> compute_parallel_worker() by considering parallel_leader_participation?

We know for sure that there is no reason to not use the leader process
as a worker process in the case of parallel CREATE INDEX. So we must
not have the number of participants (i.e. worker Tuplesortstates) vary
based on the current parallel_leader_participation setting. While
parallel_leader_participation can affect the number of worker
processes requested, that's a different thing. There is no question
about parallel_leader_participation ever being relevant to performance
-- it's strictly a testing option for us.

Even after parallel_leader_participation was added,
compute_parallel_worker() still assumes that the sequential scan
leader is always too busy to help. compute_parallel_worker() seems to
think that that's something that the leader does in "rare" cases not
worth considering -- cases where it has no worker tuples to consume
(maybe I'm reading too much into it not caring about
parallel_leader_participation, but I don't think so). If
compute_parallel_worker()'s assumption was questionable before, it's
completely wrong for parallel CREATE INDEX. I think
plan_create_index_workers() needs to count the leader-as-worker as an
ordinary worker, not special in any way by deducting one worker from
what compute_parallel_worker() returns. (This only happens when it's
necessary to compensate -- when leader-as-worker participation is
going to go ahead.)

I'm working on fixing up what you posted. I'm probably not more than a
week away from posting a patch that I'm going to mark "ready for
committer". I've already made the change above, and once I spend time
on trying to break the few small changes needed within buffile.c I'll
have taken it as far as I can, most likely.

-- 
Peter Geoghegan

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