Slow inventory extraction from weavefile

[John A Meinel]

I don't know what the target speed is, but with the latest bzr.newformat
tree, on my old 450MHz celeron, it takes >1.5 seconds per inventory that
I want to extract. (For 20 revision, it took 36 seconds, over the course
of all revisions, it took between 1.8-2.0s per revision).

Now, I'm not loading the weave directly, and then just asking it
repeatedly for just a section, so there is some time which is spent
loading from the disk, but after 20 requests, I'm pretty sure everything
is cached by the OS.

On my Mac 1.5GHz, it is 13s for 20 revisions, so closer to 0.6s per
inventory (about 0.71-0.74s per rev for all 1728 revisions). Also on the
Mac, it takes 3m5s to just do "weave.py stats .bzr/inventory.weave",
which is 1782 versions, or 0.1s per weave (not bad, but 3m for stats
seems a little long).

Is there a specific performance desired? Because a commit is going to
need to insert an entry into the weave, which at the very least means
extracting the old inventory, so this adds a sizeable amount of time to
the commit, just to update the inventory.

John
=:->
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