On 06/14/2016 08:11 AM, Linus Torvalds wrote: > On Mon, Jun 13, 2016 at 5:52 AM, Kirill A. Shutemov > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: >> On Sat, Jun 11, 2016 at 06:02:57PM -0700, Linus Torvalds wrote: >>> >>> I've timed it at over a thousand cycles on at least some CPU's, but >>> that's still peanuts compared to a real page fault. It shouldn't be >>> *that* noticeable, ie no way it's a 6% regression on its own. >> >> Looks like setting accessed bit is the problem. > > Ok. I've definitely seen it as an issue, but never to the point of > several percent on a real benchmark that wasn't explicitly testing > that cost. > > I reported the excessive dirty/accessed bit cost to Intel back in the > P4 days, but it's apparently not been high enough for anybody to care. > >> We spend 36% more time in page walk only, about 1% of total userspace time. >> Combining this with page walk footprint on caches, I guess we can get to >> this 3.5% score difference I see. >> >> I'm not sure if there's anything we can do to solve the issue without >> screwing relacim logic again. :( > > I think we should say "screw the reclaim logic" for now, and revert > commit 5c0a85fad949 for now. > > Considering how much trouble the accessed bit is on some other > architectures too, I wonder if we should strive to simply not care > about it, and always leaving it set. And then rely entirely on just > unmapping the pages and making the "we took a page fault after > unmapping" be the real activity tester. > > So get rid of the "if the page is young, mark it old but leave it in > the page tables" logic entirely. When we unmap a page, it will always > either be in the swap cache or the page cache anyway, so faulting it > in again should be just a minor fault with no actual IO happening. > > That might be less of an impact in the end - yes, the unmap and > re-fault is much more expensive, but it presumably happens to much > fewer pages. FWIW, something like that is what Martin did for s390 3 years ago. We now use invalidation and page faults to implement the *young functions in pgtable.h (basically using a SW young bit). This helped us to get rid of the storage keys (which contain the HW reference bit). The performance did not seem to suffer. See commit 0944fe3f4a323f436180d39402cae7f9c46ead17 s390/mm: implement software referenced bits > > What do you think? Your proposal would be to do the software tracking via invalidation/fault part of the generic mm code and not to hide it in the architecture backend. Correct? > > Linus > -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
