Horton

Re: Large writable variables

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@postgresql.org
Date: 2018-10-16T22:40:18Z
Lists: pgsql-hackers

Attachments

I wrote:
> 1. tzdefrules_s is filled in the postmaster, and if it's not filled there
> it'd be filled by every child process, so there's zero point in converting
> it to malloc-on-the-fly style.  This is because tzparse() always wants
> it filled.  That's actually a tad annoying, because it looks to me like
> with many timezone settings the data will not get used, but I'm hesitant
> to mess with the IANA code's logic enough to improve that.  Maybe I'll try
> submitting an upstream patch and see what they think of it first.

I spent some time studying this, and realized that the reason that they
always try to load the TZDEFRULES zone data is that they want to absorb
whatever leap-seconds data it has.  We don't want that and would be glad
to just drop said data, but I'm sure that's intentional on their end,
so they are not going to be interested in a patch here.

However ... if you assume that any leap-seconds data in that zone can
be ignored, then the only case where the data need be loaded is when
parsing a POSIX-style zone name that has a DST component but does not
specify a POSIX-style transition date rule.  That's possible in our code
but I do not think it is a mainstream case; in particular, that seems
never to be reached when loading a real zone data file.

Hence, the attached patch postpones the TZDEFRULES load until we find
we actually need it, and ignores any leap-second data therein, and
incidentally makes the data space be malloc-on-demand rather than static.

This is actually less of a hack compared to the upstream code than what
we were doing before, so I feel pretty pleased with it.

			regards, tom lane

Commits

  1. Apply unconstify() in more places

  2. Improve unconstify() documentation

  3. Drop const cast from dlsym() calls

  4. Const-ify a few more large static tables.

  5. Improve tzparse's handling of TZDEFRULES ("posixrules") zone data.

  6. Avoid statically allocating statement cache in ecpglib/prepare.c.

  7. Reorder FmgrBuiltin members, saving 25% in size.

  8. Add macro to cast away const without allowing changes to underlying type.

  9. Mark constantly allocated dest receiver as const.

  10. Avoid statically allocating formatting.c's format string caches.

  11. Correct constness of system attributes in heap.c & prerequisites.

  12. Avoid statically allocating gmtsub()'s timezone workspace.

  13. Correct constness of a few variables.

  14. Move the replication lag tracker into heap memory.