Re: [PATCH 2/3] tmpfs radix_tree: locate_item to speed up swapoff

[Hugh Dickins]

On Wed, 27 Jul 2011, Hugh Dickins wrote:
> On Wed, 27 Jul 2011, Andrew Morton wrote:
> > On Tue, 19 Jul 2011 15:54:23 -0700 (PDT)
> > Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > 
> > > But it was a shock to find swapoff of a 500MB file 20 times slower
> > > on my laptop, taking 10 minutes; and at that rate it significantly
> > > slows down my testing.
> > 
> > So it used to take half a minute?  That was already awful.
> > Why?  Was it IO-bound?  It doesn't sound like it.
> 
> No, not IO-bound at all.

I oversimplified: about 10 seconds of that was waiting for IO,
the rest (of 10 minutes or of half a minute) was cpu.  It's the cpu
part of it which the change of radix tree has affected, for the worse.

> > How much did that 10 minutes improve?
> 
> To 1 minute: still twice as slow as before.  I believe that's because of
> the smaller nodes and greater height of the generic radix tree.  I ought
> to experiment with a bigger RADIX_TREE_MAP_SHIFT to verify that belief
> (though I don't think tmpfs swapoff would justify raising it): will do.

Yes, raising RADIX_TREE_MAP_SHIFT from 6 to 10 (so on 32-bit the rnode
is just over 4kB, comparable with the old shmem's use of pages for this)
brings the time down considerably: still slower than before, but 12%
slower instead of twice as slow (or 20% slower instead of 3 times as
slow when comparing sys times).

Not that making a radix_tree_node need order:1 page would be sensible.

Hugh
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