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, Thomas Munro <thomas.munro@enterprisedb.com>
Date: 2018-10-16T20:11:45Z
Lists: pgsql-hackers

Attachments

On 2018-10-16 10:16:33 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2018-10-16 01:59:00 -0400, Tom Lane wrote:
> >> Also, I noticed that the biggest part of those structs are arrays of
> >> FormatNode, which has been designed with complete lack of thought about
> >> size or padding issues.  We can very easily cut it in half on 64-bit
> >> machines.
> 
> > Heh, neat. I feel like we've paid very little attention to that in a
> > myriad of places :(
> 
> Most of the time, we probably *shouldn't* pay attention to it; logical
> field ordering is worth a good deal IMO.

Sure. But there's plenty structs which we allocate a bunch off, that are
frequently accessed, where a lot of space is wasted to padding.  I agree
that we don't need to contort many structs, but there's plenty where we
should.   Often enough it's possible to reorder without making things
make meaningfully less sense.

> But in a case like this,
> where there are large arrays of the things and it's not very painful
> to avoid padding waste, it's worth the trouble.

Attached is a patch that shrinks fmgr_builtins by 25%. That seems
worthwhile, it's pretty frequently accessed, making it more dense is
helpful.  Unless somebody protests soon, I'm going to apply that...

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.