Re: Maximize Number of Instances on RHEL 7.9

2MB, not 2GB.
The latter would be a bit brutal for a 4GB SGA.

Maybe someone could remind me, but I think AIX has 3 possible sizes, with 2GB the biggest and (I think) 4MB as the intermediate.

Regards
Jonathan Lewis

On Sat, 7 Oct 2023 at 19:17, Mladen Gogala <gogala.mladen_at_gmail.com> wrote:

> On 10/7/23 12:40, Jared Still wrote:
>
> One of the things I am curious about is why no hugepages?
>
> I'm not aware of hugepages being non-useful due to small SGA size.
>
> With so many instances I would think it rather a good feature to employ.
>
>
> Well, let's clarify. Huge pages are usually neither swapped out nor paged
> out. Swapping out and paging out are two different processes which can be
> very expensive. Both processes include I/O requests and waiting for the
> process to complete. Second, huge pages are usually 2 GB in size (we're
> talking Linux x86_64) which means that the page table indexing the shared
> segment needs one entry per 2GB page. Without huge pages, page table need
> one entry per 4KB. In other words, huge pages can save quite a bit of
> memory as the page tables indexing huge pages are 524288 times smaller than
> the page tables comprised of 4KB pages. Of course, if the SGA is small,
> savings are a lot smaller, but they still do exist.
>
> Please, do not forget that huge pages are allocated on system startup and
> cannot be used for small pages memory. If you allocate 16GB of pages for
> Oracle, you cannot use it for other, more useful stuff, like Diablo4 or
> GTA5. There is a variety of huge pages, called "transparrent huge pages",
> but Oracle cannot use them. As a matter of fact, it is advised to disable
> transparent huge pages with Oracle because they were causing Linux crashes.
> That is the info from RHEL7. I am not sure about RHEL8 or RHEL9 but I still
> disable them with the following commands:
>
> if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
> echo never > /sys/kernel/mm/transparent_hugepage/enabled
> fi
> if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
> echo never > /sys/kernel/mm/transparent_hugepage/defrag
> fi
>
> I was a VAX/VMS system administrator before I became an Oracle DBA.
> VAX/VMS was very meticulous about the memory parameters and one could tune
> the memory usage quite precisely. Linux is not as good. However, huge pages
> are a good feature, inspired by the VIRTUAL=REAL memory on the MVS
> mainframes. Memory tuning on Linux is still somewhat esoteric discipline,
> not frequently practiced these days. BSD Unix varieties have much more
> configurable memory parameters but, unfortunately, nobody is using BSD,
> ever since the demise of Slowaris 4.
>
>
> --
> Mladen Gogala
> Database Consultant
> Tel: (347) 321-1217https://dbwhisperer.wordpress.com
>
>

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