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

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2022-08-09 15:21:57 -0400, Tom Lane wrote:
>> Do we really need it to be that tight? I know we only have 3 methods today,
>> but 8 doesn't seem that far away. If there were six bits reserved for
>> this I'd be happier.

> We only have so many bits available, so that'd have to come from some other
> resource. The current division is:

> + * 1. 3-bits to indicate the MemoryContextMethodID
> + * 2. 1-bit to indicate if the chunk is externally managed (see below)
> + * 3. 30-bits for the amount of memory which was reserved for the chunk
> + * 4. 30-bits for the number of bytes that must be subtracted from the chunk
> + * to obtain the address of the block that the chunk is stored on.

> I suspect we could reduce 3) here a bit, which I think would end up with slab
> context's max chunkSize shrinking further. Which should still be fine.

Hmm, I suppose you mean we could reduce 4) if we needed to. Yeah, that
seems like a reasonable place to buy more bits later if we run out of
MemoryContextMethodIDs. Should be fine then.

regards, tom lane