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

On 8/30/22 03:55, Tom Lane wrote:
> 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.
>

+1 to that

For aset that's fine not just because of power-of-2 behavior, but
because we use it for chunks of many different sizes - so at least some
of those will have sentinel.

But Slab in used only for changes and txns in reorderbuffer, and it just
so happens both structs are maxaligned on 32-bit and 64-bit machines
(rpi and x86-64). We're unlikely to use slab in many other places, and
the structs don't change very often, and it'd probably grow to another
maxaligned size anyway. So it may be pretty rare to have a sentinel.

regards

--
Tomas Vondra
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