Re: Add bump memory context type and use it for tuplesorts

Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com> writes:
> On Mon, 25 Mar 2024 at 22:44, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Basically, I'm not happy with consuming the last reasonably-available
>> pattern for a memory context type that has little claim to being the
>> Last Context Type We Will Ever Want. Rather than making a further
>> dent in our ability to detect corrupted chunks, we should do something
>> towards restoring the expansibility that existed in the original
>> design. Then we can add bump contexts and whatever else we want.

> So, would something like the attached make enough IDs available so
> that we can add the bump context anyway?

> It extends memory context IDs to 5 bits (32 values), of which
> - 8 have glibc's malloc pattern of 001/010;
> - 1 is unused memory's 00000
> - 1 is wipe_mem's 11111
> - 4 are used by existing contexts (Aset/Generation/Slab/AlignedRedirect)
> - 18 are newly available.

This seems like it would solve the problem for a good long time
to come; and if we ever need more IDs, we could steal one more bit
by requiring the offset to the block header to be a multiple of 8.
(Really, we could just about do that today at little or no cost ...
machines with MAXALIGN less than 8 are very thin on the ground.)

The only objection I can think of is that perhaps this would slow
things down a tad by requiring more complicated shifting/masking.
I wonder if we could redo the performance checks that were done
on the way to accepting the current design.

regards, tom lane