linux-kernel - Re: [PATCH] mm: vmpressure: make vmpressure

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Message-ID: <20160203161910.GA10440@cmpxchg.org>
Date:	Wed, 3 Feb 2016 11:19:10 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Martijn Coenen <maco@...gle.com>
Cc:	linux-mm@...ck.org, Anton Vorontsov <anton@...msg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.com>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] mm: vmpressure: make vmpressure_window a tunable.

On Wed, Feb 03, 2016 at 11:06:20AM +0100, Martijn Coenen wrote:
> The window size used for calculating vm pressure
> events was previously fixed at 512 pages. The
> window size has a big impact on the rate of notifications
> sent off to userspace, in particular when using the
> "low" level. On machines with a lot of memory, the
> current value may be excessive.
> 
> On the other hand, making the window size depend on
> machine size does not allow userspace to change the
> notification rate based on the current state of the
> system. For example, when a lot of memory is still
> available, userspace may want to increase the window
> since it's not interested in receiving notifications
> for every 2MB scanned.
>
> This patch makes vmpressure_window a sysctl tunable.

If the machine is just cleaning up use-once cache, frequent events
make no sense. And if the machine is struggling, the notifications
better be in time.

That's hardly a tunable. It's a factor that needs constant dynamic
adjustment depending on VM state. The same state this mechanism is
supposed to report. If we can't get this right, how will userspace?

A better approach here would be to 1) find a minimum window size that
makes us confident that there are no false positives - this is likely
to be based on machine size, maybe the low watermark? - and 2) limit
reporting of lower levels, so you're not flooded with ALLGOOD! events.

VMPRESSURE_CRITICAL: report every vmpressure_win
VMPRESSURE_MEDIUM: report every vmpressure_win*2
VMPRESSURE_LOW: report every vmpressure_win*4

Pick your favorite scaling factor here.