On Sun, 7 Apr 2024, 01:59 David Rowley, <dgrowle...@gmail.com> wrote:
> On Sun, 7 Apr 2024 at 05:45, Matthias van de Meent > <boekewurm+postg...@gmail.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
