[MERGE] log performance

[Aaron Bentley]

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John Arbash Meinel wrote:
> Martin commented on this, but it is true that you don't have to build up
> the list. If you use:
> rev_nos = dict((k, v) for v, k in cut_revs)
> 
> the dict() can accept a generator. I don't know if it saves a lot, but
> it seems like it should.

I would have thought that generator expressions would be slower than
list comprehensions because of all the function call overhead.  But in
fact, you're right.  It's faster.

> I also wanted to comment on the 'which have a 1:1 merge ratio'. I think
> this is a good start. The current bzr.dev actually has 6500 revisions,
> and only 1800 are on mainline. Which gives us 3.5:1.

Not too surprising.

> In my testing, your changes have a large effect on --short performance.

Yeah, I use --short too, and I can feel the difference.

> As a reference point, we also have --log-format=long, the time didn't
> change with or without your patch.
> $ time ./bzr log --log-format=long > /dev/null
> real    0m4.501s

Yeah, the only chance of changing --long would be by avoiding 6500
hasattr calls.

> Anyway, its already merged, but kudos for the improvement. (2.3x faster)

Thanks.

Aaron
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