> From: Patrick Robb [mailto:pr...@iol.unh.edu] > Sent: Saturday, 6 April 2024 21.21 > > On Sat, Apr 6, 2024 at 4:47 AM Morten Brørup <m...@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.
