Re: [PATCH] Use cStringIO instead of array in DummyTransaction

On Wed, 14 Oct 2009, Aahz wrote:

> On Wed, Oct 14, 2009, R. Tyler Ballance wrote:
> >
> > +try:
> > +    from cStringIO import StringIO
> > +except ImportError:
> > +    import StringIO
> > +
> 
> It's not clear what the goal is.  IIRC, using ''.join() is roughly
> comparable in performance with cStringIO (which one is faster varies from
> version to version), and if for whatever reason cStringIO is not
> available, StringIO is definitely going to perform worse.

Gahh you're right, what was I thinking. I didn't actually add a
performance test prior to cherry-picking Arun's change. I had *assumed*
(first mistake) that cStringIO would be performing the majority of its
"work" on stream.getvalue() which isn't the case.

Looking at the performance tests that I just ran, as well as the
cStringIO.c code, stream.write() will actually copy (memcpy(2)) the
buffer into the stream's buffer. Whereas list.append() will add a
pointer to a list, periodically over-extending it to accomodate
additional append() calls.

What I find curious is that StringIO *effectively* does a bunch of
list.append() calls, but it is in fact slower than cStringIO (at least
on 2.6) as the code is performing some really silly ''.join() calls
and restructuring/rebuilding the array in the stream.write() call (given
certain conditions).

I'll discuss with Arun where he saw the performance improvements with
cStringIO when he wakes up, but for now I'm just going to partially back
these changes out.


Thanks for pointing the fail out Rahz :)


Cheers,
-R. Tyler Ballance

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