Subject: Re: pgsql: Generational memory allocator
To: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>
Cc: Simon Riggs <simon@2ndquadrant.com>,
 pgsql-committers <pgsql-committers@postgresql.org>,
 Andrew Dunstan <andrew@dunslane.net>
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 <20171123193936.fncfnar5wkpx56mw@alap3.anarazel.de>
 <21043.1511467602@sss.pgh.pa.us>
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From: Tomas Vondra <tv@fuzzy.cz>
Message-ID: <ce281fac-b6f5-2d86-a9eb-4cc04d5d4bce@fuzzy.cz>
Date: Thu, 23 Nov 2017 23:51:03 +0100
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On 11/23/2017 11:04 PM, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
>> On 2017-11-23 22:34:57 +0100, Tomas Vondra wrote:
>>> Hmmm, I see. Presumably adding this to GenerationChunk (similarly to what we
>>> do in AllocChunkData) would address the issue:
>>>
>>> #if MAXIMUM_ALIGNOF > 4 && SIZEOF_VOID_P == 4
>>> Size		padding;
>>> #endif
>>>
>>> but I have no way to verify that (no access to such machine). I wonder why
>>> SlabChunk doesn't need to do that (perhaps a comment explaining that would
>>> be appropriate?).
> 
>> Can't you just compile pg on a 32bit system and manually define MAXALIGN
>> to 8 bytes?
> 
> I pushed a patch that computes how much padding to add and adds it.
> (It might not really work if size_t and void * are different sizes,
> because then there could be additional padding in the struct; but
> that seems very unlikely.)
> 

Thanks. Do we need to do something similar to the other memory contexts? 
I see Slab does not do this at all (assuming it's not necessary), and 
AllocSet does this in a different way (which seems a bit strange).

regards
Tomas