Todd Miller wrote:
On Wed, 2004-06-30 at 15:57, Tim Hochberg wrote:
After futzing around some more I figured out a way to trick python into using _ndarray_item. I added "type->tp_as_sequence->sq_item = _ndarray_item;" to _ndarray new.
I'm puzzled why you had to do this. You're using Python-2.3.x, right? There's conditionally compiled code which should be doing this statically. (At least I thought so.)
By this do you mean the "#if PY_VERSION_HEX >= 0x02030000 " that is wrapped around _ndarray_item? If so, I believe that it *is* getting compiled, it's just never getting called.
What I think is happening is that the class NumArray inherits its sq_item from PyClassObject. In particular, I think it picks up instance_item from Objects/classobject.c. This appears to be fairly expensive and, I think, ends up calling tp_as_mapping->mp_subscript. Thus, _ndarray's sq_item slot never gets called. All of this is pretty iffy since I don't know this stuff very well and I didn't trace it all the way through. However, it explains what I've seen thus far.
This is why I ended up using the horrible hack. I'm resetting NumArray's sq_item to point to _ndarray_item instead of instance_item. I believe that access at the python level goes through mp_subscrip, so it shouldn't be affected, and only objects at the C level should notice and they should just get the faster sq_item. You, will notice that there are an awful lot of I thinks in the above paragraphs though...
I then optimized _ndarray_item (code at end). This halved the execution time of my arbitrary benchmark. This trick may have horrible, unforseen consequences so use at your own risk.
Right now the sq_item hack strikes me as somewhere between completely unnecessary and too scary for me! Maybe if python-dev blessed it.
Yes, very scary. And it occurs to me that it will break subclasses of NumArray if they override __getitem__. When these subclasses are accessed from C they will see nd_array's sq_item instead of the overridden getitem. However, I think I also know how to fix it. But it does point out that it is very dangerous and there are probably dark corners of which I'm unaware. Asking on Python-List or PyDev would probably be a good idea.
The nonscary, but painful, fix would to rewrite NumArray in C.
This optimization looks good to me.
Unfortunately, I don't think the optimization to sq_item will affect much since NumArray appears to override it with
Finally I commented out the __del__ method numarraycore. This resulted in an additional speedup of 64% for a total speed up of 240%. Still not close to 10x, but a large improvement. However, this is obviously not viable for real use, but it's enough of a speedup that I'll try to see if there's anyway to move the shadow stuff back to tp_dealloc.
FYI, the issue with tp_dealloc may have to do with which mode Python is compiled in, --with-pydebug, or not. One approach which seems like it ought to work (just thought of this!) is to add an extra reference in C to the NumArray instance __dict__ (from NumArray.__init__ and stashed via a new attribute in the PyArrayObject struct) and then DECREF it as the last part of the tp_dealloc.
That sounds promising.
Well, be picking out your beer.
I was only about half right, so I'm not sure I qualify...