Re: WIP: WAL prefetch (another approach)

On Mon, Apr 4, 2022 at 3:12 PM Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote:
> [review]

Thanks! I took almost all of your suggestions about renaming things,
comments, docs and moving a magic number into a macro.

Minor changes:

1. Rebased over the shmem stats changes and others that have just
landed today (woo!). The way my simple SharedStats object works and
is reset looks a little primitive next to the shiny new stats
infrastructure, but I can always adjust that in a follow-up patch if
required.

2. It was a bit annoying that the pg_stat_recovery_prefetch view
would sometimes show stale numbers when waiting for WAL to be
streamed, since that happens at arbitrary points X bytes apart in the
WAL. Now it also happens before sleeping/waiting and when recovery
ends.

3. Last year, commit a55a9847 synchronised config.sgml with guc.c's
categories. A couple of hunks in there that modified the previous
version of this work before it all got reverted. So I've re-added the
WAL_RECOVERY GUC category, to match the new section in config.sgml.

About test coverage, the most interesting lines of xlogprefetcher.c
that stand out as unreached in a gcov report are in the special
handling for the new CREATE DATABASE in file-copy mode -- but that's
probably something to raise in the thread that introduced that new
functionality without a test. I've tested that code locally; if you
define XLOGPREFETCHER_DEBUG_LEVEL you'll see that it won't touch
anything in the new database until recovery has replayed the
file-copy.

As for current CI-vs-buildfarm blind spots that recently bit me and
others, I also tested -m32 and -fsanitize=undefined,unaligned builds.

I reran one of the quick pgbench/crash/drop-caches/recover tests I had
lying around and saw a 17s -> 6s speedup with FPW off (you need much
longer tests to see speedup with them on, so this is a good way for
quick sanity checks -- see Tomas V's results for long runs with FPWs
and curved effects).

With that... I've finally pushed the 0002 patch and will be watching
the build farm.