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 >> questions: >> >> 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 >> play. >> 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. -- Thanks & Regards, Kuntal Ghosh EnterpriseDB: http://www.enterprisedb.com
0001-Fix-parallel-worker-counts-after-a-crash.patch
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0002-Add-GUC-for-max_parallel_workers-upper-limit.patch
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