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

[Peter Zijlstra]

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 :-)