[Please cc linux-api when proposing user interface] On Mon 22-01-18 11:10:14, Vinayak Menon wrote: > Based on Kirill's patch [1]. > > Currently, faultaround code produces young pte. This can screw up > vmscan behaviour[2], as it makes vmscan think that these pages are hot > and not push them out on first round. > > During sparse file access faultaround gets more pages mapped and all of > them are young. Under memory pressure, this makes vmscan swap out anon > pages instead, or to drop other page cache pages which otherwise stay > resident. > > Modify faultaround to produce old ptes if sysctl 'want_old_faultaround_pte' > is set, so they can easily be reclaimed under memory pressure. > > This can to some extend defeat the purpose of faultaround on machines > without hardware accessed bit as it will not help us with reducing the > number of minor page faults. So we just want to add a knob to cripple the feature? Isn't it better to simply disable it than to have two distinct implementation which is rather non-intuitive and I would bet that most users will be clueless about how to set it or when to touch it at all. So we will end up with random cargo cult hints all over internet giving you your performance back... I really dislike this new interface. If the fault around doesn't work for you then disable it. > Making the faultaround ptes old results in a unixbench regression for some > architectures [3][4]. But on some architectures like arm64 it is not found > to cause any regression. > > unixbench shell8 scores on arm64 v8.2 hardware with CONFIG_ARM64_HW_AFDBM > enabled (5 runs min, max, avg): > Base: (741,748,744) > With this patch: (739,748,743) > > So by default produce young ptes and provide a sysctl option to make the > ptes old. > > [1] http://lkml.kernel.org/r/1463488366-47723-1-git-send-email-kirill.shutemov@xxxxxxxxxxxxxxx > [2] https://lkml.kernel.org/r/1460992636-711-1-git-send-email-vinmenon@xxxxxxxxxxxxxx > [3] https://marc.info/?l=linux-kernel&m=146582237922378&w=2 > [4] https://marc.info/?l=linux-mm&m=146589376909424&w=2 > > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx> -- Michal Hocko SUSE Labs -- 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>
