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

[Andrew Morton <akpm@osdl.org>]

Andi Kleen <ak@suse.de> wrote:
>
> On Tuesday 13 September 2005 23:59, David Chinner wrote:
> > On Tue, Sep 13, 2005 at 02:17:52PM +0530, Bharata B Rao wrote:
> > > Second is Sonny Rao's rbtree dentry reclaim patch which is an attempt
> > > to improve this dcache fragmentation problem.
> >
> > FYI, in the past I've tried this patch to reduce dcache fragmentation on
> > an Altix (16k pages, 62 dentries to a slab page) under heavy
> > fileserver workloads and it had no measurable effect. It appeared
> > that there was almost always at least one active dentry on each page
> > in the slab.  The story may very well be different on 4k page
> > machines, however.
> 
> I always thought dentry freeing would work much better if it
> was turned upside down.
> 
> Instead of starting from the high level dcache lists it could
> be driven by slab: on memory pressure slab tries to return pages with unused 
> cache objects. In that case it should check if there are only
> a small number of pinned objects on the page set left, and if 
> yes use a new callback to the higher level user (=dcache) and ask them
> to free the object.

Considered doing that with buffer_heads a few years ago.  It's impossible
unless you have a global lock, which bh's don't have.  dentries _do_ have a
global lock, and we'd be tied to having it for ever more.

The shrinking code would have be able to deal with a dentry which is going
through destruction by other call paths, so dcache_lock coverage would have
to be extended considerably - it would have to cover the kmem_cache_free(),
for example.   Or we put some i_am_alive flag into the dentry.

> The slab datastructures are not completely suited for this right now,
> but it could be done by using one more of the list_heads in struct page
> for slab backing pages.

Yes, some help would be needed in the slab code.

There's only one list_head in struct page and slab is already using it.