Initial benchmarks for checkout performance improvements

[Aaron Bentley]

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Aaron Bentley wrote:
> Thanks for the pointer.  I actually used echo 3, because it looks more
> representative of a user with cold cache.

More benchmarks for ya, this time hot cache numbers on a version of
TreeTransform that has been modified to behave like the top-renames
strategy.  Big difference!

normal-checkout:
transform 8
real 25.82
user 6.61
sys 18.24

fast-checkout:
transform 6
real 8.13
user 5.64
sys 2.39

I also tried disabling the inventory update, to see if that gave any gains:
no-inventory:
transform 9
real 8.03
user 5.74
sys 2.19

So the renaming change has really significant effects:
- - It gets us into the same ballpark as cp -a.  cp -a is still 3.35x
faster, but it's not a factor of 10 difference.
- - It gives us roughly equivalent system time to cp -a.  The big
difference is in user time, which we can address through standard
profiling and optimization.  Or even by implementing critical portions
in Pyrex.

Unfortunately, the fast-checkout TreeTransform works only with building
a tree.  It is a quick hack that doesn't work in the general case.  But
now that the performance gains are proven, I'm willing to put in the
time to make top-renames work in the general case.

If you're curious, it's here:
http://code.aaronbentley.com/bzr/bzrrepo/fast-checkout

Aaron
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