Re: Estimating HugePages Requirements?

On Fri, Aug 27, 2021 at 08:16:40PM +0000, Bossart, Nathan wrote:
> On 8/27/21, 12:39 PM, "Andres Freund" <andres@anarazel.de> wrote:
>> One thing I wonder is if this wouldn't better be dealt with in a more generic
>> way. While this is the most problematic runtime computed GUC, it's not the
>> only one. What if we introduced a new shared_memory_size GUC, and made
>> --describe-config output it? Perhaps adding --describe-config=guc-name?
>>
>> I also wonder if we should output the number of hugepages needed instead of
>> the "raw" bytes of shared memory. The whole business about figuring out the
>> huge page size, dividing the shared memory size by that and then rounding up
>> could be removed in that case. Due to huge_page_size it's not even immediately
>> obvious which huge page size one should use...
>
> I like both of these ideas.

That pretty much looks like -C in concept, isn't it?  Except that you
cannot get the actual total shared memory value because we'd do this
operation before loading shared_preload_libraries and miss any amount
asked by extensions.  There is a problem similar when attempting to do
postgres -C data_checksums, for example, which would output an
incorrect value even if the cluster has data checksums enabled.
--
Michael