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

Sonny Rao
2005-09-14T15:37:00Z

On Wed, Sep 14, 2005 at 06:57:56AM -0700, Martin J. Bligh 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.
> >
> > 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.
> >
> > It would probably not be very LRU but a simple hack of having slowly
> > increasing dcache generations. Each dentry use updates the generation.
> > First slab memory freeing pass only frees objects with older generations.
>
> If they're freeable, we should easily be able to move them, and therefore
> compact a fragmented slab. That way we can preserve the LRU'ness of it.
> Stage 1: free the oldest entries. Stage 2: compact the slab into whole
> pages. Stage 3: free whole pages back to teh page allocator.
>
> > Using slowly increasing generations has the advantage of timestamps
> > that you can avoid dirtying cache lines in the common case when
> > the generation doesn't change on access (= no additional cache line bouncing)
> > and it would easily allow to tune the aging rate under stress by changing the
> > length of the generation.
>
> LRU algorithm may need general tweaking like this anyway ... strict LRU
> is expensive to keep.

Based on what I remember, I'd contend it isn't really LRU today, so I
wouldn't try and stick to something that we aren't doing. :)