Re: PG15 beta1 sort performance regression due to Generation context change

Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> writes:
> В Вт, 24/05/2022 в 17:39 -0700, Andres Freund пишет:
>> A variation on your patch would be to only store the offset to the block
>> header - that should always fit into 32bit (huge allocations being their own
>> block, which is why this wouldn't work for storing an offset to the
>> context).

> I'm +1 for this.

Given David's results in the preceding message, I don't think I am.
A scheme like this would add more arithmetic and at least one more
indirection to GetMemoryChunkContext(), and we already know that
adding even a test-and-branch there has measurable cost. (I wonder
if using unlikely() on the test would help? But it's not unlikely
in a generation-context-heavy use case.) There would also be a good
deal of complication and ensuing slowdown created by the need for
oversize chunks to be a completely different kind of animal with a
different header.

I'm also not very happy about this:

> And with this change every memory context kind can have same header:

IMO that's a bug not a feature. It puts significant constraints on how
context types can be designed.

regards, tom lane