Re: [PATCH v2 4/5] mm/madvise: allow KSM hints for remote API

[Oleksandr Natalenko <oleksandr@redhat.com>]

Hi.

On Fri, Jan 17, 2020 at 01:13:14PM +0300, Kirill Tkhai wrote:
> On 17.01.2020 02:59, Minchan Kim wrote:
> > From: Oleksandr Natalenko <oleksandr@redhat.com>
> > 
> > It all began with the fact that KSM works only on memory that is marked
> > by madvise(). And the only way to get around that is to either:
> > 
> >   * use LD_PRELOAD; or
> >   * patch the kernel with something like UKSM or PKSM.
> > 
> > (i skip ptrace can of worms here intentionally)
> > 
> > To overcome this restriction, lets employ a new remote madvise API. This
> > can be used by some small userspace helper daemon that will do auto-KSM
> > job for us.
> > 
> > I think of two major consumers of remote KSM hints:
> > 
> >   * hosts, that run containers, especially similar ones and especially in
> >     a trusted environment, sharing the same runtime like Node.js;
> > 
> >   * heavy applications, that can be run in multiple instances, not
> >     limited to opensource ones like Firefox, but also those that cannot be
> >     modified since they are binary-only and, maybe, statically linked.
> > 
> > Speaking of statistics, more numbers can be found in the very first
> > submission, that is related to this one [1]. For my current setup with
> > two Firefox instances I get 100 to 200 MiB saved for the second instance
> > depending on the amount of tabs.
> > 
> > 1 FF instance with 15 tabs:
> > 
> >    $ echo "$(cat /sys/kernel/mm/ksm/pages_sharing) * 4 / 1024" | bc
> >    410
> > 
> > 2 FF instances, second one has 12 tabs (all the tabs are different):
> > 
> >    $ echo "$(cat /sys/kernel/mm/ksm/pages_sharing) * 4 / 1024" | bc
> >    592
> > 
> > At the very moment I do not have specific numbers for containerised
> > workload, but those should be comparable in case the containers share
> > similar/same runtime.
> > 
> > [1] https://lore.kernel.org/patchwork/patch/1012142/
> > 
> > Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
> > Signed-off-by: Minchan Kim <minchan@google.com>
> > ---
> >  mm/madvise.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 84cffd0900f1..89557998d287 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1000,6 +1000,8 @@ process_madvise_behavior_valid(int behavior)
> >  	switch (behavior) {
> >  	case MADV_COLD:
> >  	case MADV_PAGEOUT:
> > +	case MADV_MERGEABLE:
> > +	case MADV_UNMERGEABLE:
> >  		return true;
> >  	default:
> >  		return false;
> 
> Remote madvise on KSM parameters should be OK.
> 
> One thing is madvise_behavior_valid() places MADV_MERGEABLE/UNMERGEABLE
> in #ifdef brackes, so -EINVAL is returned by madvise() syscall if KSM
> is not enabled. Here we should follow the same way for symmetry.
> 

Thanks for the suggestion.

Minchan, shall you adopt it directly, or I should send a separate patch?

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Senior Software Maintenance Engineer