From: Linus Torvalds <firstname.lastname@example.org> To: "Kirill A. Shutemov" <email@example.com> Cc: "Huang, Ying" <firstname.lastname@example.org>, Rik van Riel <email@example.com>, Michal Hocko <firstname.lastname@example.org>, LKML <email@example.com>, Michal Hocko <firstname.lastname@example.org>, Minchan Kim <email@example.com>, Vinayak Menon <firstname.lastname@example.org>, Mel Gorman <email@example.com>, Andrew Morton <firstname.lastname@example.org>, LKP <email@example.com>, Dave Hansen <firstname.lastname@example.org> Subject: Re: [LKP] [lkp] [mm] 5c0a85fad9: unixbench.score -6.3% regression Date: Mon, 13 Jun 2016 23:11:05 -0700 [thread overview] Message-ID: <CA+55aFx2TdqHW5VvirF-fAe4rRtSKK6BH06LyN4Ma3Q7ifJkxA@mail.gmail.com> (raw) In-Reply-To: <20160613125248.GA30109@black.fi.intel.com> On Mon, Jun 13, 2016 at 5:52 AM, Kirill A. Shutemov <email@example.com> 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. What do you think? Linus
next prev parent reply other threads:[~2016-06-14 6:11 UTC|newest] Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-06-06 2:27 kernel test robot 2016-06-06 9:51 ` Kirill A. Shutemov 2016-06-08 7:21 ` [LKP] " Huang, Ying 2016-06-08 8:41 ` Huang, Ying 2016-06-08 8:58 ` Kirill A. Shutemov 2016-06-12 0:49 ` Huang, Ying 2016-06-12 1:02 ` Linus Torvalds 2016-06-13 9:02 ` Huang, Ying 2016-06-14 13:38 ` Minchan Kim 2016-06-15 23:42 ` Huang, Ying 2016-06-13 12:52 ` Kirill A. Shutemov 2016-06-14 6:11 ` Linus Torvalds [this message] 2016-06-14 8:26 ` Kirill A. Shutemov 2016-06-14 16:07 ` Rik van Riel 2016-06-14 14:03 ` Christian Borntraeger 2016-06-14 8:57 ` Minchan Kim 2016-06-14 14:34 ` Kirill A. Shutemov 2016-06-15 23:52 ` Huang, Ying 2016-06-16 0:13 ` Minchan Kim 2016-06-16 22:27 ` Huang, Ying 2016-06-17 5:41 ` Minchan Kim 2016-06-17 19:26 ` Huang, Ying 2016-06-20 0:06 ` Minchan Kim
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