On Tue, 9 Oct 2018 13:14:00 -0700 "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> wrote: > Android needs to mremap large regions of memory during memory management > related operations. The mremap system call can be really slow if THP is > not enabled. The bottleneck is move_page_tables, which is copying each > pte at a time, and can be really slow across a large map. Turning on THP > may not be a viable option, and is not for us. This patch speeds up the > performance for non-THP system by copying at the PMD level when possible. > > The speed up is three orders of magnitude. On a 1GB mremap, the mremap > completion times drops from 160-250 millesconds to 380-400 microseconds. > > Before: > Total mremap time for 1GB data: 242321014 nanoseconds. > Total mremap time for 1GB data: 196842467 nanoseconds. > Total mremap time for 1GB data: 167051162 nanoseconds. > > After: > Total mremap time for 1GB data: 385781 nanoseconds. > Total mremap time for 1GB data: 388959 nanoseconds. > Total mremap time for 1GB data: 402813 nanoseconds. > > Incase THP is enabled, the optimization is skipped. I also flush the > tlb every time we do this optimization since I couldn't find a way to > determine if the low-level PTEs are dirty. It is seen that the cost of > doing so is not much compared the improvement, on both x86-64 and arm64. Looks tasty. > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -191,6 +191,54 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, > drop_rmap_locks(vma); > } > > +bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr, I'll park this for now, shall plan to add a `static' in there then merge it up after 4.20-rc1.
