linux-kernel - Re: [PATCH RFC] mm/madvise: implement MADV

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Date:   Tue, 28 May 2019 10:42:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Roman Gushchin <guro@...com>, linux-api@...r.kernel.org
Subject: Re: [PATCH RFC] mm/madvise: implement MADV_STOCKPILE (kswapd from
 user space)

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@...er@...her@ I guess

> 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.

-- 
Michal Hocko
SUSE Labs