On 04/05/2017 04:15 PM, Robert Haas wrote:
On Wed, Apr 5, 2017 at 10:09 AM, Kuntal Ghosh
<kuntalghosh.2...@gmail.com> wrote:
Did you intend to attach that patch to this email?
Actually, I'm confused how we should ensure (register_count >
terminate_count) invariant. I think there can be a system crash what
Tomas has suggested up in the thread.
Assert(parallel_register_count - parallel_terminate_count <=
max_parallel_workers);
Backend 1 > SET max_parallel_worker = 8;
Backend 1 > Execute a long running parallel query q1 with number of
parallel worker spawned is say, 4.
At this point, parallel_register_count should be equal to
parallel_terminate_count. 4 workers were started, and 4 have
terminated.
No, the workers were started, but are still running, so
register_count = 4
terminate_count = 0
Backend 2> SET max_parallel_worker = 3;
Backend 2 > Try to execute any parallel query q2 with number of
parallel worker spawned > 0.
Now here parallel_register_count should get bumped up to 4+(# of
workers now launched) at the beginning and then
parallel_terminate_count at the end. No problem.
What's going wrong, here?
Well, assuming the other workers are still running, you get
register_count = 4
terminate_count = 0
and the difference is greater than max_parallel_workers = 3. Which
violates the assert.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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