linux-kernel - Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20101028185839.4b951bdb.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 28 Oct 2010 18:58:39 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Shaohua Li <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter
 threshold when memory is low

On Thu, 28 Oct 2010 10:49:03 +0100
Mel Gorman <mel@....ul.ie> wrote:

> On Thu, Oct 28, 2010 at 10:09:20AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Wed, 27 Oct 2010 09:47:35 +0100
> > Mel Gorman <mel@....ul.ie> wrote:
> > 
> > > Commit [aa45484: calculate a better estimate of NR_FREE_PAGES when
> > > memory is low] noted that watermarks were based on the vmstat
> > > NR_FREE_PAGES. To avoid synchronization overhead, these counters are
> > > maintained on a per-cpu basis and drained both periodically and when a
> > > threshold is above a threshold. On large CPU systems, the difference
> > > between the estimate and real value of NR_FREE_PAGES can be very high.
> > > The system can get into a case where pages are allocated far below the
> > > min watermark potentially causing livelock issues. The commit solved the
> > > problem by taking a better reading of NR_FREE_PAGES when memory was low.
> > > 
> > > <SNIP>
> > > 
> > > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > > index 355a9e6..cafcc2d 100644
> > > --- a/mm/vmstat.c
> > > +++ b/mm/vmstat.c
> > > @@ -81,6 +81,12 @@ EXPORT_SYMBOL(vm_stat);
> > >  
> > >  #ifdef CONFIG_SMP
> > >  
> > > +static int calculate_pressure_threshold(struct zone *zone)
> > > +{
> > > +	return max(1, (int)((high_wmark_pages(zone) - low_wmark_pages(zone) /
> > > +				num_online_cpus())));
> > > +}
> > > +
> > 
> > Could you add background theory of this calculation as a comment to
> > show the difference with calculate_threshold() ?
> > 
> 
> Sure. When writing it, I realised that the calculations here differ from
> what percpu_drift_mark does. This is what I currently have
> 
> int calculate_pressure_threshold(struct zone *zone)
> {
>         int threshold;
>         int watermark_distance;
> 
>         /*
>          * As vmstats are not up to date, there is drift between the estimated
>          * and real values. For high thresholds and a high number of CPUs, it
>          * is possible for the min watermark to be breached while the estimated
>          * value looks fine. The pressure threshold is a reduced value such
>          * that even the maximum amount of drift will not accidentally breach
>          * the min watermark
>          */
>         watermark_distance = low_wmark_pages(zone) - min_wmark_pages(zone);
>         threshold = max(1, watermark_distance / num_online_cpus());
> 
>         /*
>          * Maximum threshold is 125
>          */
>         threshold = min(125, threshold);
> 
>         return threshold;
> }
> 
> Is this better?
> 

sounds nice.

Regards,
-Kame


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/