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

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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.

M.
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