Performance requirements for bzr checkout --lightweight

[James Westby]

Hi,

I wasn't sure what the best way to raise this issue was, so I thought
I would at least start with a mail to the list.

First of all, let me say that I realise that performance has been
a major focus for a long while now, and things have improved
a lot. I do not wish this email to insult anybody's work.

There has been a discussion on ubuntu-devel recently about
DistributedDevelopment, the plan to make all of Ubuntu available
in bzr. One of the use cases that people are concerned about
is sponsorhip of a package that they have not touched before,
which is a fairly common occurrence. This currently involves
getting the latest version of the code, applying a diff, building
an testing, and them uploading.

In this discussion it was noted that the first time looking at
a package is one area where bzr is going to be slower than
current tools, as there is more to do than a simple "apt-get
source". Some timings were done, and people had widely
varying results, but testing with the latest and greatest
showed that the times for "checkout --lightweight" were
heading towards those of "apt-get source", but still
too far away for comfort.

I realise that emphasising one operation doesn't necessarily
help with performance optimisation, but I would like to
ask for some attention to be paid to this operation. Ideally
a lightweight checkout of a packaging branch would take less
than 150% or 200% of the time for an "apt-get source" of the
same package.

I am happy to provide branches and traces on demand, and I can
ask others to provide them as well where they are seeing worse
performance than I am.

One slightly concerning this is that bzr+ssh:// is consistently
slower than http:// for this case. I think this is partly this is
due to the SSH negotiation at the start, and partly due to the
number of smart server calls before the data is streamed out.
I say concerning as documentation will specify lp: transports,
assuming that they will work well for most cases, and I would
rather not have to confuse the situation by trying to
explain how to checkout over http and then switch to bzr+ssh://.

You can find the root of the discussion at

https://lists.ubuntu.com/archives/ubuntu-devel/2008-August/026249.html

and the most interesting part for this mail starting at

https://lists.ubuntu.com/archives/ubuntu-devel/2008-August/026249.html

The timings from Emmet are at

https://lists.ubuntu.com/archives/ubuntu-devel/2008-August/026249.html


Thanks,

James