Hi, Thanks for working on this.
I’ve just come across this thread and haven’t had a chance to dig into the patch yet, but I’m keen to review it soon. In the meantime, I have a quick question: is WAIT FOR REPLY intended mainly for user-defined functions, or can internal code invoke it as well? During a recent performance run [1] I noticed heavy polling in read_local_xlog_page_guts(). Heikki’s comment from a few months ago also hints that we could replace this check–sleep–repeat loop with the condition-variable (CV) infrastructure used by walsender: /* * Loop waiting for xlog to be available if necessary * * TODO: The walsender has its own version of this function, which uses a * condition variable to wake up whenever WAL is flushed. We could use the * same infrastructure here, instead of the check/sleep/repeat style of * loop. */ Because read_local_xlog_page_guts() waits for a specific flush or replay LSN, polling becomes inefficient when the wait is long. I built a POC patch that swaps polling for CVs, but a single global CV (or even separate “flush” and “replay” CVs) isn’t ideal: The wake-up routines don’t know which LSN each waiter cares about, so they’d have to broadcast on every flush/replay. Caching the minimum outstanding LSN could reduce spuriously awakened waiters, yet wouldn’t eliminate them—multiple backends might wait for different LSNs simultaneously. A more precise solution would require a request queue that maps waiters to target LSNs and issues targeted wake-ups, adding complexity. Walsender accepts the potential broadcast overhead by using two cvs for different waiters, so it might be acceptable for read_local_xlog_page_guts() as well. However, if WAIT FOR REPLY becomes available to backend code, we might leverage it to eliminate the polling for waiting replay in read_local_xlog_page_guts() without introducing a bespoke dispatcher. I’d appreciate any thoughts on whether that use case is in scope. Best, Xuneng [1] https://www.postgresql.org/message-id/cabptf7vufym9tta9vy8zts77qst+yl_htsdxufnw3xsdb5b...@mail.gmail.com
