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

On Fri, 7 Oct 2022 at 09:05, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Here's a v2 incorporating discussed changes.
>
> In reordering enum MemoryContextMethodID, I arranged to avoid using
> 000 and 111 as valid IDs, since those bit patterns will appear in
> zeroed and wipe_mem'd memory respectively. Those should probably be
> more-or-less-permanent exclusions, so I added comments about it.

I'm just considering some future developer here that is writing a new
MemoryContext type and there's no more space left and she or he needs
to either use 000 or 111. I think if that was me, I might be unsure if
I should be looking to expand the bit-space to make room. I might
think that based on the word "avoid" in:

> + MCTX_UNUSED1_ID, /* avoid: 000 occurs in never-used memory */
> + MCTX_UNUSED5_ID /* avoid: 111 occurs in wipe_mem'd memory */

but the final sentence in:

> + * dummy entries for unused IDs in the mcxt_methods[] array. We also try
> + * to avoid using bit-patterns as valid IDs if they are likely to occur in
> + * garbage data.

leads me to believe we're just *trying* to avoid using these bit-patterns.

Also, the comment in mcxt_methods[] might make me believe that it's ok
for me to use them if I really need to.

> + * Unused (as yet) IDs should have dummy entries here. This allows us to

Based on these comments, I'm not quite sure if I should be completely
avoiding using 000 and 111 or I should just use those last when there
are no other free slots in the array. It might be quite a long time
before someone is in this situation, so should we be more clear?

However, maybe you've left it this way as you feel it's a decision
that must be made in the future, perhaps based on how difficult it
would be to free up another bit?

David