On Mon, Nov 4, 2019 at 1:00 PM Masahiko Sawada <masahiko.saw...@2ndquadrant.com> wrote: > > On Mon, 4 Nov 2019 at 14:02, Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > On Fri, Nov 1, 2019 at 2:21 PM Masahiko Sawada <sawada.m...@gmail.com> > > wrote: > > > > > > I think that two approaches make parallel vacuum worker wait in > > > different way: in approach(a) the vacuum delay works as if vacuum is > > > performed by single process, on the other hand in approach(b) the > > > vacuum delay work for each workers independently. > > > > > > Suppose that the total number of blocks to vacuum is 10,000 blocks, > > > the cost per blocks is 10, the cost limit is 200 and sleep time is 5 > > > ms. In single process vacuum the total sleep time is 2,500ms (= > > > (10,000 * 10 / 200) * 5). The approach (a) is the same, 2,500ms. > > > Because all parallel vacuum workers use the shared balance value and a > > > worker sleeps once the balance value exceeds the limit. In > > > approach(b), since the cost limit is divided evenly the value of each > > > workers is 40 (e.g. when 5 parallel degree). And suppose each workers > > > processes blocks evenly, the total sleep time of all workers is > > > 12,500ms (=(2,000 * 10 / 40) * 5 * 5). I think that's why we can > > > compute the sleep time of approach(b) by dividing the total value by > > > the number of parallel workers. > > > > > > IOW the approach(b) makes parallel vacuum delay much more than normal > > > vacuum and parallel vacuum with approach(a) even with the same > > > settings. Which behaviors do we expect? > > > > > > > Yeah, this is an important thing to decide. I don't think that the > > conclusion you are drawing is correct because it that is true then the > > same applies to the current autovacuum work division where we divide > > the cost_limit among workers but the cost_delay is same (see > > autovac_balance_cost). Basically, if we consider the delay time of > > each worker independently, then it would appear that a parallel vacuum > > delay with approach (b) is more, but that is true only if the workers > > run serially which is not true. > > > > > I thought the vacuum delay for > > > parallel vacuum should work as if it's a single process vacuum as we > > > did for memory usage. I might be missing something. If we prefer > > > approach(b) I should change the patch so that the leader process > > > divides the cost limit evenly. > > > > > > > I am also not completely sure which approach is better but I slightly > > lean towards approach (b). > > Can we get the same sleep time as approach (b) if we divide the cost > limit by the number of workers and have the shared cost balance (i.e. > approach (a) with dividing the cost limit)? Currently the approach (b) > seems better but I'm concerned that it might unnecessarily delay > vacuum if some indexes are very small or bulk-deletions of indexes > does almost nothing such as brin.
Are you worried that some of the workers might not have much I/O to do but still we divide the cost limit equally? If that is the case then that is the case with the auto vacuum workers also right? -- Regards, Dilip Kumar EnterpriseDB: http://www.enterprisedb.com
