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

From: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
To: Peter Geoghegan <pg(at)bowt(dot)ie>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Subject: Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)
Date: 2018-01-25 01:31:19
Message-ID: CAEepm=0Tz5_PbUFo+8nyvOoBhQTPV_H1Mu3=cOri6wVkET+Pnw@mail.gmail.com
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On Thu, Jan 25, 2018 at 9:28 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Wed, Jan 24, 2018 at 12:13 PM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> On Thu, Jan 25, 2018 at 8:54 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>>> 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).
>>
>> Just curious: does the attached also help?
>
> I can still reproduce the problem without the fix I described (which
> does work), using your patch instead.
>
> Offhand, I suspect that the way you set ParallelMessagePending may not
> always leave it set when it should be.

Here's a version that works, and a minimal repro test module thing.
Without 0003 applied, it hangs. With 0003 applied, it does this:

postgres=# call test_fork_failure();
CALL
postgres=# call test_fork_failure();
CALL
postgres=# call test_fork_failure();
ERROR: lost connection to parallel worker
postgres=# call test_fork_failure();
ERROR: lost connection to parallel worker

I won't be surprised if 0003 is judged to be a horrendous abuse of the
interrupt system, but these patches might at least be useful for
understanding the problem.

--
Thomas Munro
http://www.enterprisedb.com

Attachment Content-Type Size
0001-Chaos-monkey-fork-failure.patch application/octet-stream 1.1 KB
0002-A-simple-test-module-that-hangs-on-fork-failure.patch application/octet-stream 4.0 KB
0003-Pessimistic-fork-failure-detector.patch application/octet-stream 4.2 KB

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