[Andres Freund <andres@anarazel.de>]

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

Attachments:

0001-Reorder-FmgrBuiltin-members-saving-25-in-size.patch (text/x-diff, 1.7K) download
From 575a51d94fd4bef6914ba55b33ba96504f64d574 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Tue, 16 Oct 2018 13:05:52 -0700
Subject: [PATCH] Reorder FmgrBuiltin members, saving 25% in size.

That's worth it, as fmgr_builtins is frequently accessed, and as
fmgr_builtins is one of the biggest constant variables in a backend.
---
 src/backend/utils/Gen_fmgrtab.pl | 2 +-
 src/include/utils/fmgrtab.h      | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/backend/utils/Gen_fmgrtab.pl b/src/backend/utils/Gen_fmgrtab.pl
index fa30436895b..ca282913552 100644
--- a/src/backend/utils/Gen_fmgrtab.pl
+++ b/src/backend/utils/Gen_fmgrtab.pl
@@ -230,7 +230,7 @@ my $fmgr_count = 0;
 foreach my $s (sort { $a->{oid} <=> $b->{oid} } @fmgr)
 {
 	print $tfh
-	  "  { $s->{oid}, \"$s->{prosrc}\", $s->{nargs}, $bmap{$s->{strict}}, $bmap{$s->{retset}}, $s->{prosrc} }";
+	  "  { $s->{oid}, $s->{nargs}, $bmap{$s->{strict}}, $bmap{$s->{retset}}, \"$s->{prosrc}\", $s->{prosrc} }";
 
 	$fmgr_builtin_oid_index[ $s->{oid} ] = $fmgr_count++;
 
diff --git a/src/include/utils/fmgrtab.h b/src/include/utils/fmgrtab.h
index d8317eb3eac..8d89d66777b 100644
--- a/src/include/utils/fmgrtab.h
+++ b/src/include/utils/fmgrtab.h
@@ -25,10 +25,10 @@
 typedef struct
 {
 	Oid			foid;			/* OID of the function */
-	const char *funcName;		/* C name of the function */
 	short		nargs;			/* 0..FUNC_MAX_ARGS, or -1 if variable count */
 	bool		strict;			/* T if function is "strict" */
 	bool		retset;			/* T if function returns a set */
+	const char * const funcName;		/* C name of the function */
 	PGFunction	func;			/* pointer to compiled function */
 } FmgrBuiltin;
 
-- 
2.18.0.rc2.dirty