On Mon, Mar 03, 2014 at 03:37:07PM -0800, Andrew Morton wrote: > On Mon, 3 Mar 2014 15:29:00 -0800 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Mon, Mar 3, 2014 at 2:38 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > When the file is uncached, results are peculiar: > > > > > > 0.00user 2.84system 0:50.90elapsed 5%CPU (0avgtext+0avgdata 4198096maxresident)k > > > 0inputs+0outputs (1major+49666minor)pagefaults 0swaps > > > > > > That's approximately 3x more minor faults. > > > > This is not peculiar. > > > > When the file is uncached, some pages will obviously be under IO due > > to readahead etc. And the fault-around code very much on purpose will > > *not* try to wait for those pages, so any busy pages will just simply > > not be faulted-around. > > Of course. > > > So you should still have fewer minor faults than faulting on *every* > > page (ie the non-fault-around case), but I would very much expect that > > fault-around will not see the full "one sixteenth" reduction in minor > > faults. > > > > And the order of IO will not matter, since the read-ahead is > > asynchronous wrt the page-faults. > > When a pagefault hits a locked, not-uptodate page it is going to block. > Once it wakes up we'd *like* to find lots of now-uptodate pages in > that page's vicinity. Obviously, that is happening, but not to the > fullest possible extent. We _could_ still achieve the 16x if readahead > was cooperating in an ideal fashion. > > I don't know what's going on in there to produce this consistent 3x > factor. In my VM numbers are different (fault in 1G): cold cache: 2097352inputs+0outputs (2major+25048minor)pagefaults 0swaps hot cache: 0inputs+0outputs (0major+16450minor)pagefaults 0swaps ~1.5x more page faults with cold cache comparing to hot cache. BTW, moving do_fault_around() below __do_fault() doesn't make much better: cold cache: 2097200inputs+0outputs (1major+24641minor)pagefaults 0swaps -- Kirill A. Shutemov -- 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>