Date: Thu, 15 Sep 2005 09:58:38 +0530
From: Bharata B Rao <bharata@in.ibm.com>
To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>, Dipankar Sarma <dipankar@in.ibm.com>,
       linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: VM balancing issues on 2.6.13: dentry cache not getting shrunk enough
Message-ID: <20050915042838.GB3869@in.ibm.com>
Reply-To: bharata@in.ibm.com
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On Wed, Sep 14, 2005 at 06:34:04PM -0300, Marcelo Tosatti wrote:
> On Tue, Sep 13, 2005 at 02:17:52PM +0530, Bharata B Rao wrote:
> > On Sun, Sep 11, 2005 at 11:16:36PM -0400, Theodore Ts'o wrote:
> > > On Sun, Sep 11, 2005 at 05:30:46PM +0530, Dipankar Sarma wrote:
> > > > Do you have the /proc/sys/fs/dentry-state output when such lowmem
> > > > shortage happens ?
> > > 
> > > Not yet, but the situation occurs on my laptop about 2 or 3 times
> > > (when I'm not travelling and so it doesn't get rebooted).  So
> > > reproducing it isn't utterly trivial, but it's does happen often
> > > enough that it should be possible to get the necessary data.
> > > 
> > > > This is a problem that Bharata has been investigating at the moment.
> > > > But he hasn't seen anything that can't be cured by a small memory
> > > > pressure - IOW, dentries do get freed under memory pressure. So
> > > > your case might be very useful. Bharata is maintaing an instrumentation
> > > > patch to collect more information and an alternative dentry aging patch 
> > > > (using rbtree). Perhaps you could try with those.
> > > 
> > > Send it to me, and I'd be happy to try either the instrumentation
> > > patch or the dentry aging patch.
> > > 
> > 
> > Ted,
> > 
> > I am sending two patches here.
> > 
> > First is dentry_stats patch which collects some dcache statistics
> > and puts it into /proc/meminfo. This patch provides information 
> > about how dentries are distributed in dcache slab pages, how many
> > free and in use dentries are present in dentry_unused lru list and
> > how prune_dcache() performs with respect to freeing the requested
> > number of dentries.
> 
> Hi Bharata,
> 
> +void get_dstat_info(void)
> +{
> +       struct dentry *dentry;
> +
> +       lru_dentry_stat.nr_total = lru_dentry_stat.nr_inuse = 0;
> +       lru_dentry_stat.nr_ref = lru_dentry_stat.nr_free = 0;
> +
> +       spin_lock(&dcache_lock);
> +       list_for_each_entry(dentry, &dentry_unused, d_lru) {
> +               if (atomic_read(&dentry->d_count))
> +                       lru_dentry_stat.nr_inuse++;
> 
> Dentries on dentry_unused list with d_count positive? Is that possible 
> at all? As far as my limited understanding goes, only dentries with zero 
> count can be part of the dentry_unused list.

As Dipankar mentioned, its now possible to have positive d_count dentires
on unused_list. BTW I think we need a better way to get this data than
going through the entire unused_list linearly, which might not be 
scalable with huge number of dentries.

> 
> +               if (dentry->d_flags & DCACHE_REFERENCED)
> +                       lru_dentry_stat.nr_ref++;
> +       }
> 
> 
> @@ -393,6 +430,9 @@ static inline void prune_one_dentry(stru
> 
>  static void prune_dcache(int count)
>  {
> +       int nr_requested = count;
> +       int nr_freed = 0;
> +
>         spin_lock(&dcache_lock);
>         for (; count ; count--) {
>                 struct dentry *dentry;
> @@ -427,8 +467,13 @@ static void prune_dcache(int count)
>                         continue;
>                 }
>                 prune_one_dentry(dentry);
> +               nr_freed++;
>         }
>         spin_unlock(&dcache_lock);
> +       spin_lock(&prune_dcache_lock);
> +       lru_dentry_stat.dprune_req = nr_requested;
> +       lru_dentry_stat.dprune_freed = nr_freed;
> 
> Don't you mean "+=" ? 

No. Actually here I am capturing the number of dentries freed
per invocation of prune_dcache.

Regards,
Bharata.
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