Re: [Boost-users] cyl_bessel_j performance

Subject: Re: [Boost-users] cyl_bessel_j performance
From: John Maddock (boost.regex_at_[hidden])
Date: 2011-03-11 14:00:34

> cyl_bessel_j is several times slower than jn from glibc for me. What do I 
> do
> wrong? Bessel function from boost.math seems to be pretty good coded and
> optimized but in a runtime it baffles me.

There are potentially several issue here:

* Are you building with all optimizations on?  If not it will be way way 
slower: remember the code is expanded inline in your program and not in a 
separate lib.
* Are you calling cyl_bessel_j with the first parameter an integer type? 
This saves a few cycles over a floating-point argument call since the 
algorithm doesn't have to figure out whether the parameter is an integer or 
not.
* I confess that's not a class of Math functions I've spent much time 
optimizing for speed.
* There may be a few domains of the function that are deliberately slower in 
order to avoid the worst of the inaccuracies that the bessel functions can 
suffer from.

Even given all of the above, they shouldn't be "several times slower", so if 
you have a specific test case I'll investigate what's wrong.

HTH, John.