Horton

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

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Peter Geoghegan <pg@heroku.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Cc: Corey Huinker <corey.huinker@gmail.com>
Date: 2016-09-22T19:51:51Z
Lists: pgsql-hackers 
On 08/02/2016 01:18 AM, Peter Geoghegan wrote:
> No merging in parallel
> ----------------------
>
> Currently, merging worker *output* runs may only occur in the leader
> process. In other words, we always keep n worker processes busy with
> scanning-and-sorting (and maybe some merging), but then all processes
> but the leader process grind to a halt (note that the leader process
> can participate as a scan-and-sort tuplesort worker, just as it will
> everywhere else, which is why I specified "parallel_workers = 7" but
> talked about 8 workers).
>
> One leader process is kept busy with merging these n output runs on
> the fly, so things will bottleneck on that, which you saw in the
> example above. As already described, workers will sometimes merge in
> parallel, but only their own runs -- never another worker's runs. I
> did attempt to address the leader merge bottleneck by implementing
> cross-worker run merging in workers. I got as far as implementing a
> very rough version of this, but initial results were disappointing,
> and so that was not pursued further than the experimentation stage.
>
> Parallel merging is a possible future improvement that could be added
> to what I've come up with, but I don't think that it will move the
> needle in a really noticeable way.

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.

- Heikki

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