Re: BF mamba failure

On Thu, Oct 17, 2024 at 01:24:50PM +0900, Michael Paquier wrote:
>> Yeah, FWIW, we would be going from 32 bytes:
>> /* total size (bytes): 32 */
>>
>> to 40 bytes (with the patch applied):
>> /* total size (bytes): 40 */
>>
>> Due to the padding, that's a 8 bytes increase while we're adding "only" 4 bytes.
>
> I have spent some time digging into all the original pgstat threads
> dealing with the shmem implementation or dshash, and I'm not really
> seeing anybody stressing on the size of the individual hash entries.
> This stuff was already wasting space with padding originally, so
> perhaps we're stressing too much here? Would anybody like to comment?

I've been going through this patch again, and the failures that could
be seen because of such failures, like standbys failing in an
unpredictible way is not cool, so I am planning to apply the attached
down to 15 now that the release has colled down. At the end I am not
really stressing about this addition in the structure for the sake of
making the stats entries safe to handle.

I did not like much the name "agecount" though, used to cross-check
how many times an entry is reused in shared memory and in the local
copy we keep in a process, so I've renamed it to "generation".
--
Michael