Re: [PATCH RFC] mm/madvise: implement MADV_STOCKPILE (kswapd from user space)

[Konstantin Khlebnikov]

On 28.05.2019 11:42, Michal Hocko wrote:
> On Tue 28-05-19 11:04:46, Konstantin Khlebnikov wrote:
> > On 28.05.2019 10:38, Michal Hocko wrote:
> [...]
> > > Could you define the exact semantic? Ideally something for the manual
> > > page please?
> >
> > Like kswapd which works with thresholds of free memory this one reclaims
> > until 'free' (i.e. memory which could be allocated without invoking
> > direct recliam of any kind) is lower than passed 'size' argument.
>
> s@lower@higher@ I guess

Yep. My wording still bad.
'size' argument should be called 'watermark' or 'threshold'.

I.e. reclaim while 'free' memory is lower passed 'threshold'.

> > Thus right after madvise(NULL, size, MADV_STOCKPILE) 'size' bytes
> > could be allocated in this memory cgroup without extra latency from
> > reclaimer if there is no other memory consumers.
> >
> > Reclaimed memory is simply put into free lists in common buddy allocator,
> > there is no reserves for particular task or cgroup.
> >
> > If overall memory allocation rate is smooth without rough spikes then
> > calling MADV_STOCKPILE in loop periodically provides enough room for
> > allocations and eliminates direct reclaim from all other tasks.
> > As a result this eliminates unpredictable delays caused by
> > direct reclaim in random places.
>
> OK, this makes it more clear to me. Thanks for the clarification!
> I have clearly misunderstood and misinterpreted target as the reclaim
> target rather than free memory target.  Sorry about the confusion.
> I sill think that this looks like an abuse of the madvise but if there
> is a wider consensus this is acceptable I will not stand in the way.