On 04/10/2017 01:39 PM, Kuntal Ghosh wrote:
On Thu, Apr 6, 2017 at 6:50 AM, Robert Haas <robertmh...@gmail.com> wrote:
On Wed, Apr 5, 2017 at 8:17 PM, Neha Khatri <nehakhat...@gmail.com> wrote:
The problem here seem to be the change in the max_parallel_workers value
while the parallel workers are still under execution. So this poses two

1. From usecase point of view, why could there be a need to tweak the
max_parallel_workers exactly at the time when the parallel workers are at
2. Could there be a restriction on tweaking of max_parallel_workers while
the parallel workers are at play? At least do not allow setting the
max_parallel_workers less than the current # of active parallel workers.

Well, that would be letting the tail wag the dog.  The maximum value
of max_parallel_workers is only 1024, and what we're really worried
about here is seeing a value near PG_UINT32_MAX, which leaves a lot of
daylight.  How about just creating a #define that's used by guc.c as
the maximum for the GUC, and here we assert that we're <= that value?

I've added a GUC parameter named MAX_PARALLEL_WORKER_LIMIT and
asserted that the absolute difference between the two counts <= that
value, i.e.,
abs((int)(register_count - terminate_count)) <= MAX_PARALLEL_WORKER_LIMIT;

Because, it's possible that register count has overflowed but
terminate count has not yet overflowed. As a result, the difference in
unsigned integer will be near UINT32_MAX. Hence, we need the absolute
difference after typecasting the same to integer. This value should be
less than the max_parallel_workers upper limit.

I've attached both the patches for better accessibility. PFA.

At first I was like 'WTF? Why do we need a new GUC just becase of an assert?' but you're actually not adding a new GUC parameter, you're adding a constant which is then used as a max value for max for the two existing parallel GUCs.

I think this is fine.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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