Re: [PATCH v3] mm: make faultaround produce old ptes

Re: [PATCH v3] mm: make faultaround produce old ptes

[Michal Hocko]

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@linux.intel.com
> > [2] https://lkml.kernel.org/r/1460992636-711-1-git-send-email-vinmenon@codeaurora.org
> > [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@linux.intel.com>
> > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs

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