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

On Tue, 19 Jul 2011 15:54:23 -0700 (PDT)
Hugh Dickins [off-list ref] wrote:

We have already acknowledged that swapoff of a tmpfs file is slower
than it was before conversion to the generic radix_tree: a little
slower there will be acceptable, if the hotter paths are faster.

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.

Now, most of that turned out to be overhead from PROVE_LOCKING and
PROVE_RCU: without those it was only 4 times slower than before;
and more realistic tests on other machines don't fare as badly.

What's unrealistic about doing swapoff of a 500MB tmpfs file?

Also, confused.  You're talking about creating a regular file on tmpfs
and then using that as a swapfile?  If so, that's a
kernel-hacker-curiosity only?

I've tried a number of things to improve it, including tagging the
swap entries, then doing lookup by tag: I'd expected that to halve
the time, but in practice it's erratic, and often counter-productive.

The only change I've so far found to make a consistent improvement,
is to short-circuit the way we go back and forth, gang lookup packing
entries into the array supplied, then shmem scanning that array for the
target entry.  Scanning in place doubles the speed, so it's now only
twice as slow as before (or three times slower when the PROVEs are on).

So, add radix_tree_locate_item() as an expedient, once-off, single-caller
hack to do the lookup directly in place.  #ifdef it on CONFIG_SHMEM and
CONFIG_SWAP, as much to document its limited applicability as save space
in other configurations.  And, sadly, #include sched.h for cond_resched().

How much did that 10 minutes improve?

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