Re: VM balancing issues on 2.6.13: dentry cache not getting shrunk enough

Marcelo Tosatti
2005-09-14T21:34:04Z

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.

+ 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 "+=" ?

+ spin_unlock(&prune_dcache_lock);