On Tue 23-01-18 15:55:06, Michal Hocko wrote: > [Please cc linux-api when proposing user interface] now for real... > 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 -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
