On Mon, Mar 11, 2024 at 5:55 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > > [...] > > >>>>> we don't want reclamation overhead later. and we want memories immediately > >>>>> available to others. > >>>> > >>>> But by that logic, you also don't want to leave the large folio partially mapped > >>>> all the way until the last subpage is CoWed. Surely you would want to reclaim it > >>>> when you reach partial map status? > >>> > >>> To some extent, I agree. But then we will have two many copies. The last > >>> subpage is small, and a safe place to copy instead. > >>> > >>> We actually had to tune userspace to decrease partial map as too much > >>> partial map both unfolded CONT-PTE and wasted too much memory. if a > >>> vma had too much partial map, we disabled mTHP on this VMA. > >> > >> I actually had a whacky idea around introducing selectable page size ABI > >> per-process that might help here. I know Android is doing work to make the > >> system 16K page compatible. You could run most of the system processes with 16K > >> ABI on top of 4K kernel. Then those processes don't even have the ability to > >> madvise/munmap/mprotect/mremap anything less than 16K alignment so that acts as > >> an anti-fragmentation mechanism while allowing non-16K capable processes to run > >> side-by-side. Just a passing thought... > > > > Right, this project faces a challenge in supporting legacy > > 4KiB-aligned applications. > > but I don't find it will be an issue to run 16KiB-aligned applications > > on a kernel whose > > page size is 4KiB. > > Yes, agreed that a 16K-aligned (or 64K-aligned) app will work without issue on > 4K kernel, but it will also use getpagesize() and know what the page size is. > I'm suggesting you could actually run these apps on a 4K kernel but with a 16K > ABI and potentially get close to the native 16K performance out of them. It's > just a thought though - I don't have any data that actually shows this is better > than just running on a 4K kernel with a 4K ABI, and using 16K or 64K mTHP > opportunistically. I fully agree with this as my Ubuntu filesystem can run on 4KiB, 16KiB and 64KiB basepage size as its elf files are 64KiB aligned. so I would expect new Android apps/middleware move to 64KiB ABI though it might want to change the base page size to 16KiB instead. I believe this is the case. Thanks Barry
