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

On Sun, 7 Apr 2024, 01:59 David Rowley, <dgrowleyml(at)gmail(dot)com> wrote:

> On Sun, 7 Apr 2024 at 05:45, Matthias van de Meent
> <boekewurm+postgres(at)gmail(dot)com> wrote:
> > Malloc's docs specify the minimum chunk size at 4*sizeof(void*) and
> itself uses , so using powers of 2 for chunks would indeed fail to detect
> 1s in the 4th bit. I suspect you'll get different results when you check
> the allocation patterns of multiples of 8 bytes, starting from 40,
> especially on 32-bit arm (where MALLOC_ALIGNMENT is 8 bytes, rather than
> the 16 bytes on i386 and 64-bit architectures, assuming [0] is accurate)

I'd hazard a guess that
> there are more instances of Postgres running on Windows today than on
> 32-bit CPUs and we don't seem too worried about the bit-patterns used
> for Windows.
>

Yeah, that is something I had some thoughts about too, but didn't check if
there was historical context around. I don't think it's worth bothering
right now though.

>> Another reason not to make it 5 bits is that I believe that would make
> >> the mcxt_methods[] array 2304 bytes rather than 576 bytes. 4 bits
> >> makes it 1152 bytes, if I'm counting correctly.
> >
> >
> > I don't think I understand why this would be relevant when only 5 of the
> contexts are actually in use (thus in caches). Is that size concern about
> TLB entries then?
>
> It's a static const array. I don't want to bloat the binary with
> something we'll likely never need. If we one day need it, we can
> reserve another bit using the same overlapping method.
>

Fair points.

>> I revised the patch to simplify hdrmask logic. This started with me
> >> having trouble finding the best set of words to document that the
> >> offset is "half the bytes between the chunk and block". So, instead
> >> of doing that, I've just made it so these two fields effectively
> >> overlap. The lowest bit of the block offset is the same bit as the
> >> high bit of what MemoryChunkGetValue returns.
> >
> >
> > Works for me, I suppose.
>
> hmm. I don't detect much enthusiasm for it.
>

I had a tiring day leaving me short on enthousiasm, after which I realised
there were some things to this patch that would need fixing.

I could've worded this better, but nothing against this code.

-Matthias