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:

quoted

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.

quoted

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 <redacted>
Signed-off-by: Vinayak Menon <redacted>

-- 
Michal Hocko
SUSE Labs

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>