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: 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: 2018-01-24T19:54:49Z
Lists: pgsql-hackers 
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). Once that's committed, I can
post a new version of the patch that uses that new infrastructure --
I'll add a call to the new function, without changing anything else.
Failing that, we could actually just use
WaitForParallelWorkersToFinish(). I still don't want to use a barrier,
mostly because it complicates  parallel_leader_participation=off,
something that Amit is in agreement with [2][3].

For now, I am waiting for feedback from Robert on next steps.

[1] https://postgr.es/m/CAH2-Wzm6dF=g9LYwthgCqzRc4DzBE-8Tv28Yvg0XJ8Q6e4+cBQ@mail.gmail.com
[2] https://postgr.es/m/CAA4eK1LEFd28p1kw2Fst9LzgBgfMbDEq9wPh9jWFC0ye6ce62A%40mail.gmail.com
[3] https://postgr.es/m/CAA4eK1+a0OF4M231vBgPr_0Ygg_BNmRGZLiB7WQDE-FYBSyrGg@mail.gmail.com
-- 
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