Re: MultiXact\SLRU buffers configuration

Hi Andrey, all,

I propose some changes, and I'm attaching a new version:

I renamed the GUCs as clog_buffers etc (no "_slru_").  I fixed some
copy/paste mistakes where the different GUCs were mixed up.  I made
some changes to the .conf.sample.  I rewrote the documentation so that
it states the correct unit and defaults, and refers to the
subdirectories that are cached by these buffers instead of trying to
give a new definition of each of the SLRUs.

Do you like those changes?

Some things I thought about but didn't change:

I'm not entirely sure if we should use the internal and historical
names well known to hackers (CLOG), or the visible directory names (I
mean, we could use pg_xact_buffers instead of clog_buffers).  I am not
sure why these GUCs need to be PGDLLIMPORT, but I see that NBuffers is
like that.

I wanted to do some very simple smoke testing of CLOG sizes on my
local development machine:

  pgbench -i -s1000 postgres
  pgbench -t4000000 -c8 -j8 -Mprepared postgres

I disabled autovacuum after running that just to be sure it wouldn't
interfere with my experiment:

  alter table pgbench_accounts set (autovacuum_enabled = off);

Then I shut the cluster down and made a copy, so I could do some
repeated experiments from the same initial conditions each time.  At
this point I had 30 files 0000-001E under pg_xact, holding 256kB = ~1
million transactions each.  It'd take ~960 buffers to cache it all.
So how long does VACUUM FREEZE pgbench_accounts take?

I tested with just the 0001 patch, and also with the 0002 patch
(improved version, attached):

clog_buffers=128:  0001=2:28.499, 0002=2:17:891
clog_buffers=1024: 0001=1:38.485, 0002=1:29.701

I'm sure the speedup of the 0002 patch can be amplified by increasing
the number of transactions referenced in the table OR number of
clog_buffers, considering that the linear search produces
O(transactions * clog_buffers) work.  That was 32M transactions and
8MB of CLOG, but I bet if you double both of those numbers once or
twice things start to get hot.  I don't see why you shouldn't be able
to opt to cache literally all of CLOG if you want (something like 50MB
assuming default autovacuum_freeze_max_age, scale to taste, up to
512MB for the theoretical maximum useful value).

I'm not saying the 0002 patch is bug-free yet though, it's a bit finickity.