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: Amit Kapila <amit.kapila16@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@enterprisedb.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: 2018-01-23T03:13:10Z
Lists: pgsql-hackers 
On Mon, Jan 22, 2018 at 6:45 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> FWIW, I don't think that that's really much of a difference.
>>
>> ExecParallelFinish() calls WaitForParallelWorkersToFinish(), which is
>> similar to how _bt_end_parallel() calls
>> WaitForParallelWorkersToFinish() in the patch. The
>> _bt_leader_heapscan() condition variable wait for workers that you
>> refer to is quite a bit like how gather_readnext() behaves. It
>> generally checks to make sure that all tuple queues are done.
>> gather_readnext() can wait for developments using WaitLatch(), to make
>> sure every tuple queue is visited, with all output reliably consumed.
>>
>
> The difference lies in the fact that in gather_readnext, we use tuple
> queue mechanism which has the capability to detect that the workers
> are stopped/exited whereas _bt_leader_heapscan doesn't have any such
> capability, so I think it will loop forever.

_bt_leader_heapscan() can detect when workers exit early, at least in
the vast majority of cases. It can do this simply by processing
interrupts and automatically propagating any error -- nothing special
about that. It can also detect when workers have finished
successfully, because of course, that's the main reason for its
existence. What remains, exactly?

I don't know that much about tuple queues, but from a quick read I
guess you might be talking about shm_mq_receive() +
shm_mq_wait_internal(). It's not obvious that that will work in all
cases ("Note that if handle == NULL, and the process fails to attach,
we'll potentially get stuck here forever"). Also, I don't see how this
addresses the parallel_leader_participation issue I raised.

-- 
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