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

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:

quoted

On Sun, Sep 11, 2005 at 05:30:46PM +0530, Dipankar Sarma wrote:

quoted

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.

quoted

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);



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