Horton

Re: Large writable variables

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
Date: 2018-10-15T23:45:03Z
Lists: pgsql-hackers 
Hi,

I've a patch making .data variables constant, fwiw. Will post in a
bit. Just so we don't duplicate work too much.


On 2018-10-15 19:41:49 -0400, Tom Lane wrote:
> I wrote:
> > Andres Freund <andres@anarazel.de> writes:
> >> 0000000008560224 0000000000023440 b tzdefrules_s
> >> 0000000008536704 0000000000023440 b gmtmem.7009
> 
> > I think that tzdefrules_s is not used in common cases (though I could be
> > wrong about that), so we could win by alloc-on-first-use.  The same might
> > be true for gmtmem, but there's a sticking point: there is no provision
> > for failure there, so I'm unsure how we avoid crashing on OOM.
> 
> So my intuition was exactly backwards on this.
> 
> 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.

Ok, that makes sense.


> 2. gmtmem, on the other hand, is only used if somebody calls pg_gmtime,
> which already has a perfectly good error-report convention, cf gmtime(3).
> We have exactly one caller, which was not bothering to test for error :-(
> and would dump core on failure.  And that caller isn't reached in most
> processes, at least not in the regression tests.  So this side of it is
> easy to improve.
> 
> Hence I propose the attached patch for point 2, which I'd want to
> backpatch to keep our copies of the IANA code in sync.

That makes sense, too.

Greetings,

Andres Freund

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.