linux-kernel - Re: [RFC 08/10] autonuma, memory tiering: Select hotter pages to promote to fast memory node

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Message-ID: <20191104084404.GA4131@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Nov 2019 09:44:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [RFC 08/10] autonuma, memory tiering: Select hotter pages to
 promote to fast memory node

On Mon, Nov 04, 2019 at 10:41:10AM +0800, Huang, Ying wrote:

> >> +#define NUMA_SCAN_NR_HIST	16
> >> +		int numa_scan_idx;
> >> +		unsigned long numa_scan_jiffies[NUMA_SCAN_NR_HIST];
> >> +		unsigned long numa_scan_starts[NUMA_SCAN_NR_HIST];
> >
> > Why 16? This is 4 cachelines.
> 
> We want to keep the NUMA scanning history reasonably long.  From
> task_scan_min(), the minimal interval between task_numa_work() running
> is about 100 ms by default.  So we can keep 1600 ms history by default
> if NUMA_SCAN_NR_HIST is 16.  If user choose to use smaller
> sysctl_numa_balancing_scan_size, then we can only keep shorter history.
> In general, we want to keep no less than 1000 ms history.  So 16 appears
> like a reasonable choice for us.  Any other suggestion?

This is very good information for Changelogs and comments :-)