On 2017-02-27 22:57:24 -0500, Tom Lane wrote:
> If the slab allocator would be happier with just a MemoryContext pointer
> as chunk header, I think we should push in this direction rather than
> invent some short-term hack.

It would - it really doesn't need the size, because it's the same for
the whole context, and thereby is a waste of space.  Still wondering if
we should band-aid this till that's done.

> One could imagine redefining aset.c's chunk header along the lines of
> typedef struct AllocSetChunkHeader
> {
>     Size        size;             /* size of data space allocated in chunk */
>     Size        requested_size;   /* original request size */
> #if 32-bit-but-maxalign-is-8
>     Size        padding;          /* needed to avoid padding below */
> #endif
> #endif
>     MemoryContext context;        /* owning context */
>     /* there must not be any padding to reach a MAXALIGN boundary here! */
> } AllocSetChunkHeader;
> where we'd possibly need some help from configure to implement that inner
> #if condition, but it seems doable enough.

Hm, that should be doable with something like

which'd probably be better documentation than a macro that hides this
(arguing internally whether SIZEOF_VOID_P or SIZEOF_SIZE_T) is better.

Working on a patch now, will post but not push tonight.


Andres Freund

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