Re: dentry bloat.

[Dipankar Sarma <dipankar@in.ibm.com>]

On Sat, May 08, 2004 at 12:13:09PM -0700, Linus Torvalds wrote:
> On Sat, 8 May 2004, Andrew Morton wrote:
> > 
> > I think we can simply take ->d_lock a bit earlier in __d_lookup.  That will
> > serialise against d_move(), fixing the problem which you mention, and also
> > makes d_movecount go away.
> 
> If you do that, RCU basically loses most of it's meaning.
> 
> You'll be taking a lock for - and dirtying in the cache - every single
> dentry on the hash chain, which is pretty much guaranteed to be slower
> than just taking the dcache_lock _once_, even if that one jumps across 
> CPU's a lot.
> 
> In other words, no, I don't think that's a good idea. We really want to
> take the dentry lock only after we're pretty sure we have the right
> dentry. Otherwise the dentry chains will be bouncing from CPU to CPU all
> the time.

Exactly. Taking ->d_lock for every dentry while traversing the hash
chain should be avoided. As such, atomic operations on that path
are getting costly.

Thanks
Dipankar