linux-kernel - Re: [PATCH 17/63] sched: Set the scan rate proportional to the memory usage of the task being scanned

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Message-ID: <5252F2FA.1020201@redhat.com>
Date:	Mon, 07 Oct 2013 13:44:26 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 17/63] sched: Set the scan rate proportional to the memory
 usage of the task being scanned

On 10/07/2013 06:28 AM, Mel Gorman wrote:
> The NUMA PTE scan rate is controlled with a combination of the
> numa_balancing_scan_period_min, numa_balancing_scan_period_max and
> numa_balancing_scan_size. This scan rate is independent of the size
> of the task and as an aside it is further complicated by the fact that
> numa_balancing_scan_size controls how many pages are marked pte_numa and
> not how much virtual memory is scanned.
> 
> In combination, it is almost impossible to meaningfully tune the min and
> max scan periods and reasoning about performance is complex when the time
> to complete a full scan is is partially a function of the tasks memory
> size. This patch alters the semantic of the min and max tunables to be
> about tuning the length time it takes to complete a scan of a tasks occupied
> virtual address space. Conceptually this is a lot easier to understand. There
> is a "sanity" check to ensure the scan rate is never extremely fast based on
> the amount of virtual memory that should be scanned in a second. The default
> of 2.5G seems arbitrary but it is to have the maximum scan rate after the
> patch roughly match the maximum scan rate before the patch was applied.
> 
> On a similar note, numa_scan_period is in milliseconds and not
> jiffies. Properly placed pages slow the scanning rate but adding 10 jiffies
> to numa_scan_period means that the rate scanning slows depends on HZ which
> is confusing. Get rid of the jiffies_to_msec conversion and treat it as ms.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.com>


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