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

[Bruce Richardson <bruce.richardson@intel.com>]

On Sun, Apr 07, 2024 at 12:04:24AM +0200, Morten Brørup wrote:
> > 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.
>

Even for 32-bit x86, I think most distros now use PAE mode to allow physical
addresses >4GB, so even then 32-bit hugepages are 2MB rather than 4MB.

/Bruce