Hi, I got a few questions about the wal stats while working on the shmem stats patch:
1) What is the motivation to have both prevWalUsage and pgWalUsage, instead of just accumulating the stats since the last submission? There doesn't seem to be any comment explaining it? Computing differences to previous values, and copying to prev*, isn't free. I assume this is out of a fear that the stats could get reset before they're used for instrument.c purposes - but who knows? 2) Why is there both pgstat_send_wal() and pgstat_report_wal()? With the former being used by wal writer, the latter by most other processes? There again don't seem to be comments explaining this. 3) Doing if (memcmp(&WalStats, &all_zeroes, sizeof(PgStat_MsgWal)) == 0) just to figure out if there's been any changes isn't all that cheap. This is regularly exercised in read-only workloads too. Seems adding a boolean WalStats.have_pending = true or such would be better. 4) For plain backends pgstat_report_wal() is called by pgstat_report_stat() - but it is not checked as part of the "Don't expend a clock check if nothing to do" check at the top. It's probably rare to have pending wal stats without also passing one of the other conditions, but ... Generally the various patches seems to to have a lot of the boilerplate style comments (like "Prepare and send the message"), but very little in the way of explaining the design. Greetings, Andres Freund
