Re: Atomics for heap_parallelscan_nextpage()

On August 16, 2017 10:47:23 AM PDT, Robert Haas <robertmhaas@gmail.com> wrote:
>On Wed, Aug 16, 2017 at 1:40 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I was wondering why the shm_toc code was using BUFFERALIGN and not
>> MAXALIGN, and I now suspect that the answer is "it's an entirely
>> undocumented kluge to make the atomics code not crash on 32-bit
>> machines, so long as nobody puts a pg_atomic_uint64 anywhere except
>> in a shm_toc".
>
>Well, shm_toc considerably predates 64-bit atomics, so I think the
>causality cannot run in that direction.  shm_toc.c first appeared in
>the tree in January of 2014.  src/include/port/atomics didn't show up
>until September of that year, and 64-bit atomics weren't actually
>usable in practice until e8fdbd58fe564a29977f4331cd26f9697d76fc40 in
>April of 2017.

Well, not for core code.  I certainly know about production code using it, because crusty platforms are considered irrelevant...

Independent of that, a comment explaining what the BUFFERALIGN is intending would be good.

Andres
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