Horton

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

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Peter Geoghegan <pg@heroku.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2016-09-27T14:08:14Z
Lists: pgsql-hackers 
On Mon, Sep 26, 2016 at 3:40 PM, Peter Geoghegan <pg@heroku.com> wrote:
> On Mon, Sep 26, 2016 at 6:58 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> That requires some kind of mutual exclusion mechanism, like an LWLock.
>>
>> No, it doesn't.  Shared memory queues are single-reader, single-writer.
>
> The point is that there is a natural dependency when merging is
> performed eagerly within the leader. One thing needs to be in lockstep
> with the others. That's all.

I don't know what any of that means.  You said we need something like
an LWLock, but I think we don't.  The workers just write the results
of their own final merges into shm_mqs.  The leader can read from any
given shm_mq until no more tuples can be read without blocking, just
like nodeGather.c does, or at least it can do that unless its own
queue fills up first.  No mutual exclusion mechanism is required for
any of that, as far as I can see - not an LWLock, and not anything
similar.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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