linux-kernel - Re: [PATCH 14/50] sched: Set the scan rate proportional to the memory usage of the task being scanned

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Date:	Mon, 16 Sep 2013 17:18:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/50] sched: Set the scan rate proportional to the
 memory usage of the task being scanned

On Tue, Sep 10, 2013 at 10:31:54AM +0100, Mel Gorman wrote:
> @@ -860,9 +908,14 @@ void task_numa_fault(int node, int pages, bool migrated)
>  	 * If pages are properly placed (did not migrate) then scan slower.
>  	 * This is reset periodically in case of phase changes
>  	 */
> -        if (!migrated)
> -		p->numa_scan_period = min(sysctl_numa_balancing_scan_period_max,
> +        if (!migrated) {
> +		/* Initialise if necessary */
> +		if (!p->numa_scan_period_max)
> +			p->numa_scan_period_max = task_scan_max(p);
> +
> +		p->numa_scan_period = min(p->numa_scan_period_max,
>  			p->numa_scan_period + jiffies_to_msecs(10));

So the next patch changes the jiffies_to_msec() thing.. is that really
worth a whole separate patch?

Also, I really don't believe any of that is 'right', increasing the scan
period by a fixed amount for every !migrated page is just wrong.

Firstly; there's the migration throttle which basically guarantees that
most pages aren't migrated -- even when they ought to be, thus inflating
the period.

Secondly; assume a _huge_ process, so large that even a small fraction
of non-migrated pages will completely clip the scan period.
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