On Wed, Oct 10, 2018 at 01:02:22AM +0300, Kirill A. Shutemov wrote: > On Tue, Oct 09, 2018 at 01:14:00PM -0700, Joel Fernandes (Google) 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. > > Okay. That's interesting. > > It makes me wounder why do we pass virtual address to pte_alloc() (and > pte_alloc_one() inside). > > If an arch has real requirement to tight a page table to a virtual address > than the optimization cannot be used as it is. Per-arch should be fine > for this case, I guess. > > If nobody uses the address we should just drop the argument as a > preparation to the patch. I couldn't find any use of the address. But I am wondering why you feel passing the address is something that can't be done with the optimization. The pte_alloc only happens if the optimization is not triggered. Also the clean up of the argument that you're proposing is a bit out of scope of this patch but yeah we could clean it up in a separate patch if needed. I don't feel too strongly about that. It seems cosmetic and in the future if the address that's passed in is needed, then the architecture can use it. > Anyway, I think the optimization requires some groundwork before it can be > accepted. At least some explanation why it is safe to move page table from > one spot in virtual address space to another. So I did go through several scenarios and its fine to my eyes. I tested it too and couldn't find any issue. Could you describe your concern a bit more? The mm->mmap_sem lock is held through out the mremap. Further we are acquiring needed rmap locks if needed and the ptl locks of the old new page-table pages. And this same path is already copying pmds for hugepages. thanks, - Joel
