Re: Relation bulk write facility

On Sun, Feb 25, 2024 at 07:52:16AM +1300, Thomas Munro wrote:
> On Sun, Feb 25, 2024 at 6:24 AM Noah Misch <noah@leadboat.com> wrote:
> > On Fri, Feb 23, 2024 at 04:27:34PM +0200, Heikki Linnakangas wrote:
> > > Committed this. Thanks everyone!
> >
> > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mandrill&dt=2024-02-24%2015%3A13%3A14 got:
> > TRAP: failed Assert("(uintptr_t) buffer == TYPEALIGN(PG_IO_ALIGN_SIZE, buffer)"), File: "md.c", Line: 472, PID:
43188608
> >
> > with this stack trace:
> > #5  0x10005cf0 in ExceptionalCondition (conditionName=0x1015d790 <XLogBeginInsert+80> "`", fileName=0x0,
lineNumber=16780064)at assert.c:66
 
> > #6  0x102daba8 in mdextend (reln=0x1042628c <PageSetChecksumInplace+44>, forknum=812540744, blocknum=33,
buffer=0x306e6000,skipFsync=812539904) at md.c:472
 
> > #7  0x102d6760 in smgrextend (reln=0x306e6670, forknum=812540744, blocknum=33, buffer=0x306e6000,
skipFsync=812539904)at smgr.c:541
 
> > #8  0x104c8dac in smgr_bulk_flush (bulkstate=0x306e6000) at bulk_write.c:245
> 
> So that's:
> 
> static const PGIOAlignedBlock zero_buffer = {{0}};  /* worth BLCKSZ */
> 
> ...
>                 smgrextend(bulkstate->smgr, bulkstate->forknum,
>                            bulkstate->pages_written++,
>                            &zero_buffer,
>                            true);
> 
> ... where PGIOAlignedBlock is:
> 
> typedef union PGIOAlignedBlock
> {
> #ifdef pg_attribute_aligned
>     pg_attribute_aligned(PG_IO_ALIGN_SIZE)
> #endif
>     char        data[BLCKSZ];
> ...
> 
> We see this happen with both xlc and gcc (new enough to know how to do
> this).  One idea would be that the AIX *linker* is unable to align it,
> as that is the common tool-chain component here (and unlike stack and
> heap objects, this scope is the linker's job).  There is a
> pre-existing example of a zero-buffer that is at file scope like that:
> pg_prewarm.c.  Perhaps it doesn't get tested?
> 
> Hmm.

GCC docs do say "For some linkers, the maximum supported alignment may be very
very small.", but AIX "man LD" says "data sections are aligned on a boundary
so as to satisfy the alignment of all CSECTs in the sections".  It also has -H
and -K flags to force some particular higher alignment.

On GNU/Linux x64, gcc correctly records alignment=2**12 for the associated
section (.rodata for bulk_write.o zero_buffer, .bss for pg_prewarm.o
blockbuffer).  If I'm reading this right, neither AIX gcc nor xlc is marking
the section with sufficient alignment, in bulk_write.o or pg_prewarm.o:

$ /opt/cfarm/binutils-latest/bin/objdump --section-headers
~/farm/*/HEAD/pgsqlkeep.*/src/backend/storage/smgr/bulk_write.o

/home/nm/farm/gcc64/HEAD/pgsqlkeep.2024-02-24_00-03-22/src/backend/storage/smgr/bulk_write.o:     file format
aix5coff64-rs6000

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         0000277c  0000000000000000  0000000000000000  000000f0  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, CODE
  1 .data         000000e4  000000000000277c  000000000000277c  0000286c  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  2 .debug        0001f7ea  0000000000000000  0000000000000000  00002950  2**3
                  CONTENTS

/home/nm/farm/xlc32/HEAD/pgsqlkeep.2024-02-24_15-12-23/src/backend/storage/smgr/bulk_write.o:     file format
aixcoff-rs6000

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000880  00000000  00000000  00000180  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, CODE
  1 .data         0000410c  00000880  00000880  00000a00  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  2 .bss          00000000  0000498c  0000498c  00000000  2**3
                  ALLOC
  3 .debug        00008448  00000000  00000000  00004b24  2**3
                  CONTENTS
  4 .except       00000018  00000000  00000000  00004b0c  2**3
                  CONTENTS, LOAD

$ /opt/cfarm/binutils-latest/bin/objdump --section-headers ~/farm/*/HEAD/pgsqlkeep.*/contrib/pg_prewarm/pg_prewarm.o

/home/nm/farm/gcc32/HEAD/pgsqlkeep.2024-01-21_03-16-12/contrib/pg_prewarm/pg_prewarm.o:     file format aixcoff-rs6000

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000a6c  00000000  00000000  000000b4  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, CODE
  1 .data         00000044  00000a6c  00000a6c  00000b20  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  2 .bss          00002550  00000ab0  00000ab0  00000000  2**3
                  ALLOC
  3 .debug        0001c50e  00000000  00000000  00000b64  2**3
                  CONTENTS

/home/nm/farm/gcc64/HEAD/pgsqlkeep.2024-02-15_17-13-04/contrib/pg_prewarm/pg_prewarm.o:     file format
aix5coff64-rs6000

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000948  0000000000000000  0000000000000000  00000138  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, CODE
  1 .data         00000078  0000000000000948  0000000000000948  00000a80  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  2 .bss          00002640  00000000000009c0  00000000000009c0  00000000  2**3
                  ALLOC
  3 .debug        0001d887  0000000000000000  0000000000000000  00000af8  2**3
                  CONTENTS