Re: WIP: WAL prefetch (another approach)

Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> FWIW I also chewed through many megawatts trying to reproduce this on
> a PowerPC system in 64 bit big endian mode, with an emulator. No
> cigar. However, it's so slow that I didn't make it to 10 runs...

Speaking of megawatts ... my G4 has now finished about ten cycles of
installcheck-parallel without a failure, which isn't really enough
to draw any conclusions yet. But I happened to notice the
accumulated CPU time for the background processes:

USER PID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND
tgl 19048 0.0 4.4 229952 92196 ?? Ss 3:19PM 19:59.19 postgres: startup recovering 000000010000001400000022
tgl 19051 0.0 0.1 229656 1696 ?? Ss 3:19PM 27:09.14 postgres: walreceiver streaming 14/227D8F14
tgl 19052 0.0 0.1 229904 2516 ?? Ss 3:19PM 17:38.17 postgres: walsender tgl [local] streaming 14/227D8F14

IOW, we've spent over twice as many CPU cycles shipping data to the
standby as we did in applying the WAL on the standby. Is this
expected? I've got wal_consistency_checking = all, which is bloating
the WAL volume quite a bit, but still it seems like the walsender and
walreceiver have little excuse for spending more cycles per byte
than the startup process.

(This is testing b3ee4c503, so if Thomas' WAL changes improved
efficiency of the replay process at all, the discrepancy could be
even worse in HEAD.)

regards, tom lane