Horton

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

Peter Geoghegan <pg@heroku.com>

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From: Peter Geoghegan <pg@heroku.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2016-09-24T13:07:21Z
Lists: pgsql-hackers 
On Thu, Sep 22, 2016 at 8:57 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Sep 22, 2016 at 3:51 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> It'd be good if you could overlap the final merges in the workers with the
>> merge in the leader. ISTM it would be quite straightforward to replace the
>> final tape of each worker with a shared memory queue, so that the leader
>> could start merging and returning tuples as soon as it gets the first tuple
>> from each worker. Instead of having to wait for all the workers to complete
>> first.
>
> If you do that, make sure to have the leader read multiple tuples at a
> time from each worker whenever possible.  It makes a huge difference
> to performance.  See bc7fcab5e36b9597857fa7e3fa6d9ba54aaea167.

That requires some kind of mutual exclusion mechanism, like an LWLock.
It's possible that merging everything lazily is actually the faster
approach, given this, and given the likely bottleneck on I/O at htis
stage. It's also certainly simpler to not overlap things. This is
something I've read about before [1], with "eager evaluation" sorting
not necessarily coming out ahead IIRC.

[1] http://digitalcommons.ohsu.edu/cgi/viewcontent.cgi?article=1193&context=csetech
-- 
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