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

Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
> On 8/30/22 02:45, David Rowley wrote:
>> I think the existing sentinel check looks wrong:
>> if (!sentinel_ok(chunk, slab->chunkSize))
>> shouldn't that be passing the pointer rather than the chunk?

> I agree the check in SlabCheck() looks wrong, as it's ignoring the chunk
> header (unlike the other contexts).

> But yeah, I ran "make check-world" and it passed just fine, so my only
> explanation is that the check never actually executes because there's no
> space for the sentinel thanks to alignment, and the tweak you did breaks
> that. Strange ...

A quick code-coverage check confirms that the sentinel_ok() line
is not reached in core or test_decoding tests as of HEAD
(on a 64-bit machine anyway). So we just happen to be using
only allocation requests that are already maxaligned.

I wonder if slab ought to artificially bump up such requests when
MEMORY_CONTEXT_CHECKING is enabled, so there's room for a sentinel.
I think it's okay for aset.c to not do that, because its power-of-2
behavior means there usually is room for a sentinel; but slab's
policy makes it much more likely that there won't be.

regards, tom lane