Hello, On Tue, Mar 18, 2014 at 10:55:24AM -0700, Andy Lutomirski wrote: > On 03/13/2014 11:37 PM, Minchan Kim wrote: > > This patch is an attempt to support MADV_FREE for Linux. > > > > Rationale is following as. > > > > Allocators call munmap(2) when user call free(3) if ptr is > > in mmaped area. But munmap isn't cheap because it have to clean up > > all pte entries, unlinking a vma and returns free pages to buddy > > so overhead would be increased linearly by mmaped area's size. > > So they like madvise_dontneed rather than munmap. > > > > "dontneed" holds read-side lock of mmap_sem so other threads > > of the process could go with concurrent page faults so it is > > better than munmap if it's not lack of address space. > > But the problem is that most of allocator reuses that address > > space soonish so applications see page fault, page allocation, > > page zeroing if allocator already called madvise_dontneed > > on the address space. > > > > For avoidng that overheads, other OS have supported MADV_FREE. > > The idea is just mark pages as lazyfree when madvise called > > and purge them if memory pressure happens. Otherwise, VM doesn't > > detach pages on the address space so application could use > > that memory space without above overheads. > > I must be missing something. > > If the application issues MADV_FREE and then writes to the MADV_FREEd > range, the kernel needs to know that the pages are no longer safe to > lazily free. This would presumably happen via a page fault on write. > For that to happen reliably, the kernel has to write protect the pages > when MADV_FREE is called, which in turn requires flushing the TLBs. It could be done by pte_dirty bit check. Of course, if some architectures don't support it by H/W, pte_mkdirty would make it CoW as you said. > > How does this end up being faster than munmap? MADV_FREE doesn't need to return back the pages into page allocator compared to MADV_DONTNEED and the overhead is not small when I measured that on my machine.(Roughly, MADV_FREE's cost is half of DONTNEED through avoiding involving page allocator.) But I'd like to clarify that it's not MADV_FREE's goal that syscall itself should be faster than MADV_DONTNEED but major goal is to avoid unnecessary page fault + page allocation + page zeroing + garbage swapout. > > --Andy > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>