Re: [PATCH] mm: vmpressure: make vmpressure_window a tunable.

[Johannes Weiner]

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.