I have been playing around with setting vfs_cache_pressure=0 on our v2.4.1 MDS and noticed that the dentry usage as reported from slabtop looks very odd. It seems to remain pretty much the same for long periods of time and then jumps up suddenly. I have attached a graph of ldiskfs_inode_cache,dentry and buffer_head.
Is the dentry usage tracked in some other slab? The server ran out of memory not long after the dentry count jumped up. What I'm interesting in doing is track slab usage and toggle vfs_cache_pressure to avoid running out of memory whilst maximising the inode/dentry cache.