On 15 February 2017 at 18:40, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Feb 15, 2017 at 4:43 AM, Amit Khandekar <amitdkhan...@gmail.com> > wrote: >>> On 14 February 2017 at 22:35, Robert Haas <robertmh...@gmail.com> wrote: >>>> For example, suppose that I have a scan of two children, one >>>> of which has parallel_workers of 4, and the other of which has >>>> parallel_workers of 3. If I pick parallel_workers of 7 for the >>>> Parallel Append, that's probably too high. >> >> In the patch, in such case, 7 workers are indeed selected for Parallel >> Append path, so that both the subplans are able to execute in parallel >> with their full worker capacity. Are you suggesting that we should not >> ? > > Absolutely. I think that's going to be way too many workers. Imagine > that there are 100 child tables and each one is big enough to qualify > for 2 or 3 workers. No matter what value the user has selected for > max_parallel_workers_per_gather, they should not get a scan involving > 200 workers. > > What I was thinking about is something like this: > > 1. First, take the maximum parallel_workers value from among all the children. > > 2. Second, compute log2(num_children)+1 and round up. So, for 1 > child, 1; for 2 children, 2; for 3-4 children, 3; for 5-8 children, 4; > for 9-16 children, 5, and so on. > > 3. Use as the number of parallel workers for the children the maximum > of the value computed in step 1 and the value computed in step 2.
Ah, now that I closely look at compute_parallel_worker(), I see what you are getting at. For plain unpartitioned table, parallel_workers is calculated as roughly equal to log(num_pages) (actually it is log3). So if the table size is n, the workers will be log(n). So if it is partitioned into p partitions of size n/p each, still the number of workers should be log(n). Whereas, in the patch, it is calculated as (total of all the child workers) i.e. n * log(n/p) for this case. But log(n) != p * log(x/p). For e.g. log(1000) is much less than log(300) + log(300) + log(300). That means, the way it is calculated in the patch turns out to be much larger than if it were calculated using log(total of sizes of all children). So I think for the step 2 above, log(total_rel_size) formula seems to be appropriate. What do you think ? For compute_parallel_worker(), it is actually log3 by the way. BTW this formula is just an extension of how parallel_workers is calculated for an unpartitioned table. >>> For example, suppose that I have a scan of two children, one >>> of which has parallel_workers of 4, and the other of which has >>> parallel_workers of 3. If I pick parallel_workers of 7 for the >>> Parallel Append, that's probably too high. Had those two tables been >>> a single unpartitioned table, I would have picked 4 or 5 workers, not >>> 7. On the other hand, if I pick parallel_workers of 4 or 5 for the >>> Parallel Append, and I finish with the larger table first, I think I >>> might as well throw all 4 of those workers at the smaller table even >>> though it would normally have only used 3 workers. >> >>> Having the extra 1-2 workers exit does not seem better. >> >> It is here, where I didn't understand exactly why would we want to >> assign these extra workers to a subplan which tells use that it is >> already being run by 'parallel_workers' number of workers. > > The decision to use fewer workers for a smaller scan isn't really > because we think that using more workers will cause a regression. > It's because we think it may not help very much, and because it's not > worth firing up a ton of workers for a relatively small scan given > that workers are a limited resource. I think once we've got a bunch > of workers started, we might as well try to use them. One possible side-effect I see due to this is : Other sessions might not get a fair share of workers due to this. But again, there might be counter argument that, because Append is now focussing all the workers on a last subplan, it may finish faster, and release *all* of its workers earlier. BTW, there is going to be some logic change in the choose-next-subplan algorithm if we consider giving extra workers to subplans. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
