Re: [PATCH 1/2] Update the bash prompt from 'applied' instead of the obsolete 'current'

On 2007-05-21 16:17:22 +0100, Catalin Marinas wrote:

On 21/05/07, Karl Hasselström [off-list ref] wrote:

quoted

On 2007-05-21 12:15:40 +0200, Karl Hasselström wrote:

quoted

We might consider having plumbing written in C or something, and
make sure that the plumbing can be called directly if there's
need, but it's going to complicate things greatly compared to
pure Python.

Yes, it will complicate things. I think StGIT would have progressed at
a much slower pace if written in C :-).

I agree. C is not a good language to write a whole application in. But
for the hot spots, C is a reasonable choice.

quoted

What I'm (foggily) envisioning here is to rewrite parts of StGIT
(as little as possible) as a C library (libstgit.so?), and call
the library both from the Python code, and from a "plumbing" C
program (stgit-helper?). We should not try to make the library API
stable, just like the current git library.

Apart from the start-up time, I don't see other major slowdowns
caused by Python.

I haven't, either.

It would be useful to use a git library directly without invoking
external applications (I'm not sure what's the state of a "libgit.a"
or what improvement we would get).

There is no usable git library yet. But once there is, I agree we
should use it.

As for the start-up time, unless you write most of the commands in C,
we would still have to load Python modules. If you run stg-prof
instead of stg for a simple command like 'top', you can see that the
main function takes about 60-70ms, the rest to 150ms reported by the
external 'time' is Python start-up and module loading.

My suggestion was to have a small stand-alone C program that could do
some operations that need to be really fast, such as
top/applied/unapplied. It need not have a nice user interface since
it's only going to be called by scripts (bash-completion and the
like), and it should only handle those operations that _must- avoid
the Python startup penalty. And for sanity reasons, it should share
code with stgit.

I had a quick try at using "freeze.py" to generate a binary (well,
it includes python bytecodes but it might save time on module
look-up) but it got confused by my optimisation to only load module
commands based on the stg arguments. Maybe we should try this first.

I agree that we should try pure-Python optimizations first.

quoted

There are two kinds of things we'd want to have in the library:
(1) things that are too slow to do in Python, and (2) things that
need to be available from stgit-helper in order to avoid Python's
startup cost, such as top/applied/unapplied for the bash
completion and bash prompt.

As you probably guessed, I'm not really in favour of re-writing
parts of StGIT in C, at least not in the near future, though anyone
can fork and re-implement it :-).

I wouldn't do it just for fun, either. But if it's a prerequisite to
get good enough performance for something we really want, and all else
has failed, I'd be willing to argue for the introduction of a C
library and helper application.

-- 
Karl Hasselström, kha@treskal.com
      www.treskal.com/kalle