Re: Estimating HugePages Requirements?

On 8/27/21, 7:01 PM, "Michael Paquier" <michael@paquier.xyz> wrote:
> 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.

Attached is a hacky attempt at adding a shared_memory_size GUC in a
way that could be used with -C.  This should include the amount of
shared memory requested by extensions, too.  As long as huge_page_size
is nonzero, it seems easy enough to provide the number of huge pages
needed as well.

Nathan