Re: [PATCH v2 0/5] Introduce MADV_COLD and MADV_PAGEOUT

From: Pavel Machek <pavel@ucw.cz>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-api@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Tim Murray <timmurray@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Daniel Colascione <dancol@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Sonny Rao <sonnyrao@google.com>,
	Brian Geffon <bgeffon@google.com>,
	jannh@google.com, oleg@redhat.com, christian@brauner.io,
	oleksandr@redhat.com, hdanton@sina.com, lizeb@google.com
Subject: Re: [PATCH v2 0/5] Introduce MADV_COLD and MADV_PAGEOUT
Date: Wed, 12 Jun 2019 12:59:45 +0200	[thread overview]
Message-ID: <20190612105945.GA16442@amd> (raw)
In-Reply-To: <20190610111252.239156-1-minchan@kernel.org>

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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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