On Thu, 5 Dec 2019 at 19:54, Robert Haas <robertmh...@gmail.com> wrote: > > [ Please trim excess quoted material from your replies. ] > > On Thu, Dec 5, 2019 at 12:27 AM Dilip Kumar <dilipbal...@gmail.com> wrote: > > I agree that there is no point is first to spawn more workers to get > > the work done faster and later throttle them. Basically, that will > > lose the whole purpose of running it in parallel. > > Right. I mean if you throttle something that would have otherwise > kept 3 workers running full blast back to the point where it uses the > equivalent of 2.5 workers, that might make sense. It's a little > marginal, maybe, but sure. But once you throttle it back to <= 2 > workers, you're just wasting resources. > > I think my concern here is ultimately more about usability than > whether or not we allow throttling. I agree that there are some > possible cases where throttling a parallel vacuum is useful, so I > guess we should support it. But I also think there's a real risk of > people not realizing that throttling is happening and then being sad > because they used parallel VACUUM and it was still slow. I think we > should document explicitly that parallel VACUUM is still potentially > throttled and that you should consider setting the cost delay to a > higher value or 0 before using it. > > We might even want to add a FAST option (or similar) to VACUUM that > makes it behave as if vacuum_cost_delay = 0, and add something to the > examples section for VACUUM that suggests e.g. > > VACUUM (PARALLEL 3, FAST) my_big_table > Vacuum my_big_table with 3 workers and with resource throttling > disabled for maximum performance. >
Please find some review comments for v35 patch set 1. + /* Return immediately when parallelism disabled */ + if (max_parallel_maintenance_workers == 0) + return 0; + Here, we should add check of max_worker_processes because if max_worker_processes is set as 0, then we can't launch any worker so we should return from here. 2. + /* cap by max_parallel_maintenace_workers */ + parallel_workers = Min(parallel_workers, max_parallel_maintenance_workers); + Here also, we should consider max_worker_processes to calculate parallel_workers. (by default, max_worker_processes = 8) Thanks and Regards Mahendra Thalor EnterpriseDB: http://www.enterprisedb.com
