Re: dentries: dentry defragmentation

[Andi Kleen <andi@firstfloor.org>]

On Mon, Feb 01, 2010 at 09:16:45PM +1100, Nick Piggin wrote:
> On Mon, Feb 01, 2010 at 11:10:13AM +0100, Andi Kleen wrote:
> > On Mon, Feb 01, 2010 at 06:08:35PM +1100, Nick Piggin wrote:
> > > I always preferred to do defrag in the opposite way. Ie. query the
> > > slab allocator from existing shrinkers rather than opposite way
> > > around. This lets you reuse more of the locking and refcounting etc.
> > 
> > I looked at this for hwpoison soft offline.
> > 
> > But it works really badly because the LRU list ordering 
> > has nothing to do with the actual ordering inside the slab pages.
> 
> No, you don't *have* to follow LRU order. The most important thing

What list would you follow then?

There's LRU, there's hast (which is as random) and there's slab
itself. The only one who is guaranteed to match the physical
layout in memory is slab. That is what this patchkit is trying
to attempt.

> is if you followed what I wrote is to get a pin on the objects and

Which objects? You first need to collect all that belong to a page.
How else would you do that?

> > > whether we are in this slab defrag mode, and if so, whether the slab
> > > is very sparse. If yes, then reclaim aggressively.
> > 
> > The typical result is that you need to get through most of the LRU
> > list (and prune them all) just to free the page.
> 
> Really? If you have a large proportion of slabs which are quite
> internally fragmented, then I would have thought it would give a
> significant improvement (aggressive reclaim, that is)


You wrote the same as me?


> 
> 
> > > If that doesn't perform well enough and you have to go further and
> > 
> > It doesn't.
> 
> Can we see your numbers? And the patches you tried?

What I tried (in some dirty patches you probably don't want to see)
was to just implement slab shrinking for a single page for soft hwpoison.
But it didn't work too well because it couldn't free the objects
still actually in the dcache.

Then I called the shrinker and tried to pass in the page as a hint
and drop only objects on that page, but I realized that it's terrible
inefficient to do it this way.

Now soft hwpoison doesn't care about a little inefficiency, but I still
didn't like to be terrible inefficient.

That is why I asked Christoph to repost his old patchkit that can 
do the shrink from the slab side (which is the right order here)

BTW the other potential user for this would be defragmentation
for large page allocation.

-Andi


-- 
ak@linux.intel.com -- Speaking for myself only.