Horton

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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Robert Haas <robertmhaas@gmail.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: 2018-01-26T06:03:17Z
Lists: pgsql-hackers 
On Fri, Jan 26, 2018 at 11:30 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Thu, Jan 25, 2018 at 1:24 AM, Peter Geoghegan <pg@bowt.ie> wrote:
>> On Tue, Jan 23, 2018 at 9:43 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>>> Right, but what if the worker dies due to something proc_exit(1) or
>>> something like that before calling BarrierArriveAndWait.  I think this
>>> is part of the problem we have solved in
>>> WaitForParallelWorkersToFinish such that if the worker exits abruptly
>>> at any point due to some reason, the system should not hang.
>>
>> I have used Thomas' chaos-monkey-fork-process.patch to verify:
>>
>> 1. The problem of fork failure causing nbtsort.c to wait forever is a
>> real problem. Sure enough, the coding pattern within
>> _bt_leader_heapscan() can cause us to wait forever even with commit
>> 2badb5afb89cd569500ef7c3b23c7a9d11718f2f, more or less as a
>> consequence of the patch not using tuple queues (it uses the new
>> tuplesort sharing thing instead).
>>
>> 2. Simply adding a single call to WaitForParallelWorkersToFinish()
>> within _bt_leader_heapscan() before waiting on our condition variable
>> fixes the problem -- errors are reliably propagated, and we never end
>> up waiting forever.
>>
>> 3. This short term fix works just as well with
>> parallel_leader_participation=off.
>>
>> At this point, my preferred solution is for someone to go implement
>> Amit's WaitForParallelWorkersToAttach() idea [1] (Amit himself seems
>> like the logical person for the job).
>>
>
> I can implement it and share a prototype patch with you which you can
> use to test parallel sort stuff.  I would like to highlight the
> difference which you will see with WaitForParallelWorkersToAttach as
> compare to WaitForParallelWorkersToFinish() is that the former will
> give you how many of nworkers_launched workers are actually launched
> whereas latter gives an error if any of the expected workers is not
> launched.  I feel former is good and your proposed way of calling it
> after the leader is done with its work has alleviated the minor
> disadvantage of this API which is that we need for workers to startup.
>

/we need for workers to startup./we need to wait for workers to startup.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

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