Re: [PATCH 3/4] Count the number of times zone_reclaim() scans andfails

[Wu Fengguang]

On Wed, Jun 10, 2009 at 01:01:43AM +0800, Mel Gorman wrote:
> On NUMA machines, the administrator can configure zone_reclaim_mode that
> is a more targetted form of direct reclaim. On machines with large NUMA
> distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> unmapped pages will be reclaimed if the zone watermarks are not being met.
> 
> There is a heuristic that determines if the scan is worthwhile but it is
> possible that the heuristic will fail and the CPU gets tied up scanning
> uselessly. Detecting the situation requires some guesswork and experimentation
> so this patch adds a counter "zreclaim_failed" to /proc/vmstat. If during
> high CPU utilisation this counter is increasing rapidly, then the resolution
> to the problem may be to set /proc/sys/vm/zone_reclaim_mode to 0.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> ---
>  include/linux/vmstat.h |    3 +++
>  mm/vmscan.c            |    4 ++++
>  mm/vmstat.c            |    3 +++
>  3 files changed, 10 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index ff4696c..416f748 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -36,6 +36,9 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		FOR_ALL_ZONES(PGSTEAL),
>  		FOR_ALL_ZONES(PGSCAN_KSWAPD),
>  		FOR_ALL_ZONES(PGSCAN_DIRECT),
> +#ifdef CONFIG_NUMA
> +		PGSCAN_ZONERECLAIM_FAILED,
> +#endif

I'd rather to refine the zone accounting (ie. mapped tmpfs pages)
so that we know whether a zone scan is going to be fruitless.  Then
we can get rid of the remedy patches 3 and 4.

We don't have to worry about swap cache pages accounted as file pages.
Since there are no double accounting in NR_FILE_PAGES for tmpfs pages.

We don't have to worry about MLOCKED pages, because they may defeat
the estimation temporarily, but after one or several more zone scans,
MLOCKED pages will go to the unevictable list, hence this cause of
zone reclaim failure won't be persistent.

Any more known accounting holes?

Thanks,
Fengguang

>  		PGINODESTEAL, SLABS_SCANNED, KSWAPD_STEAL, KSWAPD_INODESTEAL,
>  		PAGEOUTRUN, ALLOCSTALL, PGROTATED,
>  #ifdef CONFIG_HUGETLB_PAGE
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e862fc9..8be4582 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2489,6 +2489,10 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	ret = __zone_reclaim(zone, gfp_mask, order);
>  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
>  
> +	if (!ret) {
> +		count_vm_events(PGSCAN_ZONERECLAIM_FAILED, 1);
> +	}
> +
>  	return ret;
>  }
>  #endif
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 1e3aa81..02677d1 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -673,6 +673,9 @@ static const char * const vmstat_text[] = {
>  	TEXTS_FOR_ZONES("pgscan_kswapd")
>  	TEXTS_FOR_ZONES("pgscan_direct")
>  
> +#ifdef CONFIG_NUMA
> +	"zreclaim_failed",
> +#endif
>  	"pginodesteal",
>  	"slabs_scanned",
>  	"kswapd_steal",
> -- 
> 1.5.6.5
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