Hi Andrey, On Sat, Oct 19, 2024 at 5:39 PM Andrey M. Borodin <x4...@yandex-team.ru> wrote: > > Hi hackers! > > On a recent hacking workshop [0] Thomas mentioned that patches using new API > would be welcomed. > So I prototyped streamlining of B-tree vacuum for a discussion. > When cleaning an index we must visit every index tuple, thus we uphold a > special invariant: > After checking a trailing block, it must be last according to subsequent > RelationGetNumberOfBlocks(rel) call. > > This invariant does not allow us to completely replace block loop with > streamlining. That's why streamlining is done only for number of blocks > returned by first RelationGetNumberOfBlocks(rel) call. A tail is processed > with regular ReadBufferExtended().
I'm wondering why is the case, ISTM that we can do *p.current_blocknum = scanblkno* and *p.last_exclusive = num_pages* in each loop of the outer for? + /* We only streamline number of blocks that are know at the beginning */ know -> known + * However, we do not depent on it much, and in future ths + * expetation might change. depent -> depend ths -> this expetation -> expectation > > Also, it's worth mentioning that we have to jump to the left blocks from a > recently split pages. We also do it with regular ReadBufferExtended(). That's > why signature btvacuumpage() now accepts a buffer, not a block number. > > > I've benchmarked the patch on my laptop (MacBook Air M3) with following > workload: > 1. Initialization > create unlogged table x as select random() r from generate_series(1,1e7); > create index on x(r); > create index on x(r); > create index on x(r); > create index on x(r); > create index on x(r); > create index on x(r); > create index on x(r); > vacuum; > 2. pgbench with 1 client > insert into x select random() from generate_series(0,10) x; > vacuum x; > > On my laptop I see ~3% increase in TPS of the the pgbench (~ from 101 to > 104), but statistical noise is very significant, bigger than performance > change. Perhaps, a less noisy benchmark can be devised. > > What do you think? If this approach seems worthwhile, I can adapt same > technology to other AMs. > I think this is a use case where the read stream api fits very well, thanks. > > Best regards, Andrey Borodin. > > [0] > https://rhaas.blogspot.com/2024/08/postgresql-hacking-workshop-september.html > -- Regards Junwang Zhao
