RE: [PATCH] dts: Change hugepage runtime config to 2MB Exclusively

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Patrick Robb [mailto:probb@iol.unh.edu]
> Sent: Saturday, 6 April 2024 21.21
> 
> On Sat, Apr 6, 2024 at 4:47 AM Morten Brørup <mb@smartsharesystems.com>
> wrote:
> >
> >
> > This change seems very CPU specific.
> >
> > E.g. in x86 32-bit mode, the hugepage size is 4 MB, not 2 MB.
> >
> > I don't know the recommended hugepage size on other architectures.
> >
> 
> Thanks Morten, that's an important insight which we weren't aware of
> when we initially discussed this ticket.
> 
> We read on some dpdk docs that 1gb hugepages should be set at boot (I
> think the reason is because that's when you can guarantee there is gbs
> of contiguous available memory), and that after boot, only 2gb
> hugepages should be set. I assume that even for other arches which
> don't support the 2mb pages specifically, we still want to allocate
> hugepages using the smallest page size possible per arch (for the same
> reason).

Correct; for very large hugepages, they need to be set at boot.
I don't remember why, but that's the way the Linux kernel works.
2 MB is way below that threshold, and 1 GB is above.

You can also not set nr_overcommit_hugepages for those very large hugepages, only nr_hugepages.

> 
> So I think we can add some dict which stores the smallest valid
> hugepage size per arch. Then during config init, use the config's arch
> value to determine that size, and set the total hugepages allocation
> based on that size and the hugepages count set in the conf.yaml. Or
> maybe we can store the list of all valid hugepgage sizes per arch
> (which are also valid to be set post boot), allow for a new
> hugepage_size value on the conf.yaml, validate the input at config
> init, and then set according to those values. I prefer the former
> option though as I don't think the added flexibility offered by the
> latter seems important.

I agree; the former option suffices.

A tiny detail...
ARM supports multiple (4, I think) different hugepage sizes, where the smallest size is 64 KB.
So you might want to choose another hugepage size than the smallest; but I still agree with your proposed concept of using one specific hugepage size per arch.

Like x86_64, ARM also supports 2 MB and 1 GB hugepage sizes, and 2 MB hugepages is also the typical default on Linux.

I don't know which hugepage sizes are supported by other architectures.
It might only be 32-bit x86 that needs a different hugepage size than 2 MB.