Re: Performance Improvement by reducing WAL for Update Operation

The v3 patch applies perfectly here now.  Attached is a spreadsheet with
test results from two platforms, a Mac laptop and a Linux server.  I
used systems with high disk speed because that seemed like a worst case
for this improvement.  The actual improvement for shrinking WAL should
be even better on a system with slower disks.

There are enough problems with the "hundred tiny fields" results that I
think this not quite ready for commit yet.  More comments on that below.
  This seems close though, close enough that I am planning to follow up
to see if the slow disk results are better.

Reviewing the wal-update-testsuite.sh test program, I think the only
case missing that would be useful to add is "ten tiny fields, one
changed".  I think that one is interesting to highlight because it's
what benchmark programs like pgbench do very often.

The GUC added by the program looks like this:

postgres=# show wal_update_compression_ratio ;
  wal_update_compression_ratio
------------------------------
  25

Is possible to add a setting here that disables the feature altogether?
  That always makes it easier to consider a commit, knowing people can
roll back the change if it makes performance worse.  That would make
performance testing easier too.  wal-update-testsuit.sh takes as long as
13 minutes, it's long enough that I'd like the easier to automate
comparison GUC disabling adds.  If that's not practical to do given the
intrusiveness of the code, it's not really necessary.  I haven't looked
at the change enough to be sure how hard this is.

On the Mac, the only case that seems to have a slowdown now is "hundred
tiny fields, half nulled".  It would be nice to understand just what is
going on with that one.  I got some ugly results in "two short fields,
no change" too, along with a couple of other weird results, but I think
those were testing procedure issues that can be ignored.  The pgbench
throttle work I did recently highlights that I can't really make a Mac
quiet/consistent for benchmarking very well.  Note that I ran all of the
Mac tests with assertions on, to try and catch platform specific bugs.
The Linux ones used the default build parameters.

On Linux "hundred tiny fields, half nulled" was also by far the worst
performing one, with a >30% increase in duration despite the 14% drop in
WAL.  Exactly what's going on there really needs to be investigated
before this seems safe to commit.  All of the "hundred tiny fields"
cases seem pretty bad on Linux, with the rest of them running about a
11% duration increase.

This doesn't seem ready to commit for this CF, but the number of problem
cases is getting pretty small now.  Now that I've gotten more familiar
with the test programs and the feature, I can run more performance tests
on this at any time really.  If updates addressing the trouble cases are
ready from Amit or Hari before the next CF, send them out and I can look
at them without waiting until that one starts.  This is a very promising
looking performance feature.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com