Re: Reducing the chunk header sizes on all memory context types

Hi,

On 2022-07-12 17:01:18 +1200, David Rowley wrote:
> I've taken Andres' patch and made some quite significant changes to
> it. In the patch's current state, the sort performance regression in
> PG15 vs PG14 is fixed. The generation.c context chunk header has been
> reduced to 8 bytes from the previous 24 bytes as it is in master.
> aset.c context chunk header is now 8 bytes instead of 16 bytes.

I think this is *very* cool. But I might be biased :)

> There is at least one. It might be major; to reduce the AllocSet chunk
> header from 16 bytes down to 8 bytes I had to get rid of the freelist
> pointer that was reusing the "aset" field in the chunk header struct.
> This works now by storing that pointer in the actual palloc'd memory.
> This could lead to pretty hard-to-trace bugs if we have any code that
> accidentally writes to memory after pfree.

Can't we use the same trick for allcations in the freelist as we do for the
header in a live allocation? I.e. split the 8 byte header into two and use
part of it to point to the next element in the list using the offset from the
start of the block, and part of it to indicate the size?

Greetings,

Andres Freund