[maemo-developers] Performance of floating point instructions

[From: Bernd Stramm]

On Wed, 2010-03-10 at 20:29 +0200, Alberto Mardegan wrote:
> Alberto Mardegan wrote:
> > Does one have any figure about how the performance of the FPU is, 
> > compared to integer operations?
> 
> I added some profiling to the code, and I measured the time spent by a 
> function which is operating on an array of points (whose coordinates are 
> integers) and trasforming each of them into a geographic coordinates 
> (latitude and longitude, floating point) and calculating the distance 
> from the previous point.
> 
> http://vcs.maemo.org/git?p=maemo-mapper;a=shortlog;h=refs/heads/gps_control
> map_path_calculate_distances() is in path.c,
> calculate_distance() is in utils.c,
> unit2latlon() is a pointer to unit2latlon_google() in tile_source.c
> 
> 
> The output (application compiled with -O0):
> 
> 
> double:
> 
> map_path_calculate_distances: 110 ms for 8250 points
> map_path_calculate_distances: 5 ms for 430 points
> 
> map_path_calculate_distances: 109 ms for 8250 points
> map_path_calculate_distances: 5 ms for 430 points
> 
> 
> float:
> 
> map_path_calculate_distances: 60 ms for 8250 points
> map_path_calculate_distances: 3 ms for 430 points
> 
> map_path_calculate_distances: 60 ms for 8250 points
> map_path_calculate_distances: 3 ms for 430 points
> 
> 
> float with fast FPU mode:
> 
> map_path_calculate_distances: 50 ms for 8250 points
> map_path_calculate_distances: 2 ms for 430 points
> 
> map_path_calculate_distances: 50 ms for 8250 points
> map_path_calculate_distances: 2 ms for 430 points
> 
> 
> So, it seems that there's a huge improvements when switching from 
> doubles to floats; although I wonder if it's because of the FPU or just 
> because the amount of data passed around is smaller.

Right, is your experiment actually measuring floating point performance,
or is that swamped out by memory accesses, or some bus transfers or
something like that?

> On the other hand, the improvements obtained by enabling the fast FPU 
> mode is rather small -- but that might be due to the fact that the FPU 
> operations are not a major player in this piece of code.
> 
> One curious thing is that while making these changes, I forgot to change 
> the math functions to there float version, so that instead of using:
> 
> float x, y;
> x = sinf(y);
> 
> I was using:
> 
> float x, y;
> x = sin(y);
> 
> The timings obtained this way are surprisingly (at least to me) bad:
> 
> map_path_calculate_distances: 552 ms for 8250 points
> map_path_calculate_distances: 92 ms for 430 points
> 
> map_path_calculate_distances: 552 ms for 8250 points
> map_path_calculate_distances: 91 ms for 430 points
> 
> Much worse than the double version. The only reason I can think of, is 
> the conversion from float to double and vice versa, but is it really 
> that expensive?
> 
> Anyway, I'll stick to using 32bit floats. :-)
> 

It is often hard to tell how much difference optimizing a particular
operation makes. If the setup is cheaper for the slower operation, do
you gain anything by using faster ops? Hard to measure sometimes.

Like racing, it's not how fast you go, it's when you get there.

Bernd