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

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Date:   Tue, 28 May 2019 11:58:49 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Michal Hocko <mhocko@...nel.org>
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 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@...er@...her@ 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.
>