linux-kernel - Re: meminfo Committed

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Message-ID: <20090415084713.GU7082@balbir.in.ibm.com>
Date:	Wed, 15 Apr 2009 14:17:13 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Eric B Munson <ebmunson@...ibm.com>,
	Mel Gorman <mel@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: meminfo Committed_AS underflows

* KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> [2009-04-15 13:10:06]:

> > * KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> [2009-04-15 11:04:59]:
> > 
> > >  	committed = atomic_long_read(&vm_committed_space);
> > > +	if (committed < 0)
> > > +		committed = 0;
> > 
> > Isn't this like pushing the problem under the rug?
> 
> global_page_state() already has same logic.
> IOW almost meminfo filed has this one (except Commited_AS).
>

OK
 
> 
> > >  	allowed = ((totalram_pages - hugetlb_total_pages())
> > >  		* sysctl_overcommit_ratio / 100) + total_swap_pages;
> > > 
> > > Index: b/mm/swap.c
> > > ===================================================================
> > > --- a/mm/swap.c
> > > +++ b/mm/swap.c
> > > @@ -519,7 +519,7 @@ EXPORT_SYMBOL(pagevec_lookup_tag);
> > >   * We tolerate a little inaccuracy to avoid ping-ponging the counter between
> > >   * CPUs
> > >   */
> > > -#define ACCT_THRESHOLD	max(16, NR_CPUS * 2)
> > > +#define ACCT_THRESHOLD	max_t(long, 16, num_online_cpus() * 2)
> > >
> > 
> > Hmm.. this is a one time expansion, free of CPU hotplug.
> > 
> > Should we use nr_cpu_ids or num_possible_cpus()?
> 
> #define num_online_cpus()       cpumask_weight(cpu_online_mask)
> #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
> 
> num_possible_cpus() have the same calculation cost.
> nr_cpu_ids isn't proper value.
> it point to valid cpu-id range, no related number of online nor possible cpus.
>

Since the value is just a basis for thresholds, num_online_cpus()
might work. 

-- 
	Balbir
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