Re: Attempt to consolidate reading of XLOG page

On Thu, Nov 21, 2019 at 05:05:50PM +0900, Michael Paquier wrote:
> And with WAL segments at 1MB, I was seeing quite a slowdown with the
> patch... Then I have done an extra test with pg_waldump with the
> segments generated previously with the output redirected to /dev/null.
> Going through 512 segments takes 15.730s with HEAD (average of 3 runs)
> and 15.851s with the patch.

Here are more tests with pg_waldump and 1MB/1GB segment sizes with
records generated from pgbench, (7 runs, eliminated the two highest
and two lowest, these are the remaining 3 runs as real time):
1) 1MB segment size, 512 segments:
time pg_waldump 000000010000000100000C00 000000010000000100000F00 > /dev/null
- HEAD: 0m4.512s, 0m4.446s, 0m4.501s
- Patch + system's pg_read: 0m4.495s, 0m4.502s, 0m4.486s
- Patch + fallback pg_read: 0m4.505s, 0m4.527s, 0m4.495s
2) 1GB segment size, 3 segments:
time pg_waldump 000000010000000200000001 000000010000000200000003 > /dev/null
- HEAD: 0m11.802s, 0m11.834s, 0m11.846s
- Patch + system's pg_read: 0m11.939s, 0m11.991s, 0m11.966s
- Patch + fallback pg_read: 0m12.054s, 0m12.066s, 0m12.159s
So there is a tendency for a small slowdown here. Still it is not
that much, so I withdraw my concerns.

Another thing:
+void WALReadRaiseError(WALReadError *errinfo);
This is missing an "extern" declaration.

Alvaro, you are marked as a committer of this CF entry. Are you
planning to look at it again? Sorry for the delay from my side.
--
Michael