On Thu, 4 Apr 2024 at 22:43, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Matthias van de Meent <boekewurm+postgres@gmail.com> writes:
> > On Mon, 25 Mar 2024 at 22:44, Tom Lane <tgl@sss.pgh.pa.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.)
Hmm, it seems like a decent idea, but I didn't want to deal with the
repercussions of that this late in the cycle when these 2 bits were
still relatively easy to get hold of.
> 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.
I didn't do very extensive testing, but the light performance tests
that I did with the palloc performance benchmark patch & script shared
above indicate didn't measure an observable negative effect.
An adapted version of the test that uses repalloc() to check
performance differences in MCXT_METHOD() doesn't show a significant
performance difference from master either. That test case is attached
as repalloc-performance-test-function.patch.txt.
The full set of patches would then accumulate to the attached v5 of
the patchset.
0001 is an update of my patch from yesterday, in which I update
MemoryContextMethodID infrastructure for more IDs, and use a new
naming scheme for unused/reserved IDs.
0002 and 0003 are David's patches, with minor changes to work with
0001 (rebasing, and I moved the location around to keep function
declaration in order with memctx ids)
Kind regards,
Matthias van de Meent
