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). I think we need input from some other people as well. I will start a separate thread to discuss this and see if that helps to get the input from others. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
