linux-kernel - Re: [PATCH v2 0/5] Introduce MADV_COLD and MADV

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Date:   Wed, 12 Jun 2019 12:59:45 +0200
From:   Pavel Machek <pavel@....cz>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>, jannh@...gle.com,
        oleg@...hat.com, christian@...uner.io, oleksandr@...hat.com,
        hdanton@...a.com, lizeb@...gle.com
Subject: Re: [PATCH v2 0/5] Introduce MADV_COLD and MADV_PAGEOUT

Hi!

> - Problem
> 
> Naturally, cached apps were dominant consumers of memory on the system.
> However, they were not significant consumers of swap even though they are
> good candidate for swap. Under investigation, swapping out only begins
> once the low zone watermark is hit and kswapd wakes up, but the overall
> allocation rate in the system might trip lmkd thresholds and cause a cached
> process to be killed(we measured performance swapping out vs. zapping the
> memory by killing a process. Unsurprisingly, zapping is 10x times faster
> even though we use zram which is much faster than real storage) so kill
> from lmkd will often satisfy the high zone watermark, resulting in very
> few pages actually being moved to swap.

Is it still faster to swap-in the application than to restart it?


> This approach is similar in spirit to madvise(MADV_WONTNEED), but the
> information required to make the reclaim decision is not known to the app.
> Instead, it is known to a centralized userspace daemon, and that daemon
> must be able to initiate reclaim on its own without any app involvement.
> To solve the concern, this patch introduces new syscall -
> 
>     struct pr_madvise_param {
>             int size;               /* the size of this structure */
>             int cookie;             /* reserved to support atomicity */
>             int nr_elem;            /* count of below arrary fields */
>             int __user *hints;      /* hints for each range */
>             /* to store result of each operation */
>             const struct iovec __user *results;
>             /* input address ranges */
>             const struct iovec __user *ranges;
>     };
>     
>     int process_madvise(int pidfd, struct pr_madvise_param *u_param,
>                             unsigned long flags);

That's quite a complex interface.

Could we simply have feel_free_to_swap_out(int pid) syscall? :-).

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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