From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tv@fuzzy.cz>
Cc: Andres Freund <andres@anarazel.de>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-committers <pgsql-committers@postgresql.org>
Cc: Andrew Dunstan <andrew@dunslane.net>
Subject: Re: pgsql: Generational memory allocator
Date: Thu, 23 Nov 2017 23:19:14 -0500
Message-ID: <31255.1511497154@sss.pgh.pa.us> (raw)
In-Reply-To: <ce281fac-b6f5-2d86-a9eb-4cc04d5d4bce@fuzzy.cz>
References: <E1eHa4J-0006hI-Q8@gemulon.postgresql.org>
<20171123001649.y2nf7422iay6cw3v@alap3.anarazel.de>
<CANP8+jKoT=Pg5W_3FcKiDf+FMrJhFYacxt_g5E0=tkmb5s8M6g@mail.gmail.com>
<20171123193936.fncfnar5wkpx56mw@alap3.anarazel.de>
<21043.1511467602@sss.pgh.pa.us>
<c09b1f12-8042-1e77-1b3e-ca68a2b44167@fuzzy.cz>
<20171123214636.unwkrkunr7m2bkii@alap3.anarazel.de>
<13547.1511474676@sss.pgh.pa.us>
<ce281fac-b6f5-2d86-a9eb-4cc04d5d4bce@fuzzy.cz>
Tomas Vondra <tv@fuzzy.cz> writes:
> On 11/23/2017 11:04 PM, Tom Lane wrote:
>> 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).
Hm ... the coding in AllocSet is ancient and I have to say that I don't
like it as well as what I put into generation.c. We could not have done
it in that way at the time, because (IIRC) we did not have constant macros
for SIZEOF_SIZE_T, and maybe not SIZEOF_VOID_P either --- and you need
constant macros in order to do the #if calculation, because sizeof() does
not work in preprocessor expressions. But we have 'em now, so I'm tempted
to change aset.c to match generation.c.
As for SlabChunk, I think that's fine as-is (except I wonder why the
"block" field is declared "void *" rather than "SlabBlock *"). Since
the contents are fixed at two pointer fields, it's impossible that
there's any unexpected padding in there --- the struct size is
basically guaranteed to be 2 * sizeof(pointer). Which makes it either 8
or 16 bytes, either one of which is certain to be a multiple of MAXALIGN.
So I think the StaticAssert that that's true is plenty sufficient in
that case.
regards, tom lane
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To: pgsql-committers@postgresql.org
Cc: tgl@sss.pgh.pa.us, tv@fuzzy.cz, andres@anarazel.de, simon@2ndquadrant.com, andrew@dunslane.net
Subject: Re: pgsql: Generational memory allocator
In-Reply-To: <31255.1511497154@sss.pgh.pa.us>
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