[MERGE] Benchmark caching intermediate trees

[John Arbash Meinel]

John Arbash Meinel wrote:

...

> I think dropping the setup time from almost 90ms down to
> 7-15s for each test that wants to use a real kernel tree seems
> worthwhile to me.
> 
> John
> =:->

The attached patch extends the previous work just a little bit. It makes
it so that the trees used by the 'bench_log.py' tests are also cached
(make_many_commits_tree and make_heavily_merged_tree).

It also updates the rest of the tests to use the cached trees. In the
end, this has a rather large effect on the total time to run the test suite.

The current 'bzr selftest --benchmark' takes 516s to run on my machine.

In my caching branch, I added a few tests (which take an extra 177s to
run uncached), but still the total time drops to: 420s. (So the real
comparison time would be 516-117=339s).

And if I re-use a cache directory it only takes 320s to run everything.
(The extra tests still take 130s, because some of them explicitly
disable the cache to make a comparison).

So in a general sense, caching the intermediate results inbetween runs
of the benchmark suite drops the current suite time from 516s down to
310s - 130s = 180s.

Also, I don't know if it really matters, but it is a lot friendlier to
the OS caching system. Running the benchmark suite can generate around a
gig of data. With caching, most of this is now hardlinked against about
140MB of cached data.

I don't know if there is a need to keep the uncached tests around, since
they do take a while, and they are only testing that it was worthwhile
to do the caching. But I have them there for now.

John
=:->
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