* * * * *
                                        
   Programs from the past, Part IV: There Ain't No Such Thing As the Fastest
                                      Code
                                        
> The point I want to make, though, is that the biggest optimization barrier
> that Terje faced was that he thought he had the fastest code possible. Once
> he opened up the possibility that there were faster approaches, and looked
> beyond the specific approach that he had so carefully optimized, he was
> able to come up with code that was a lot faster. Consider the incongruity
> of Terje's willingness to consider a 5 percent speedup significant in the
> light of his later near-doubling of performance.
> 
> “Michael Abrash”, _Zen of Code Optimization_ [1]
> 

I'm probably belaboring the inanimate equus pleonastically, but this isn't
the first time [2] I've done so, so why stop now?

As was pointed out to me [3], I missed an obvious optimization in the map-
generation program [4]. That small change drastically changed the runtime of
the C version of the program:

Table: Latest timings to generate a map file
Version	New Time	Previous Time
------------------------------
Lua [5]	15.50s	16.25s
LuaJIT [6]	0.65s	1.25s
C	0.22s	2.60s

Some other testing revealed that I can now generate in 37 seconds (remember,
four cores running a stupidly parallel problem) that twenty-some years ago
took a year to do—generate 500 map files.

And speaking of 37 seconds … thanks to the Classic Computers Talk Mailing
List [7], I was able to run this latest code on a nearly identical SGI [8]
machine that I used twenty-some years ago—a 33MHz (megaHertz) R3000 [9]
running IRIX 4.0.5.

Back then, the generation of one map took around ten hours, but I had missed
a few optimizations that could have help speed up the program. And yes, it's
true—the optimizations I've since added, plus using a high optimization
setting on the C compiler, resulted in a map file being generated in 37
seconds.

On a twenty-some year old 33MHz machine.

Head. Desk.

The entire program run, which took a year, could have been done in less than
5¼ hours!

I think it's time to stop belaboring this inanimate equus.

[1] http://www.amazon.com/exec/obidos/ASIN/1883577039/conmanlaborat-20
[2] gopher://gopher.conman.org/0Phlog:2008/01/05.1
[3] http://www.reddit.com/r/programming/comments/1jvtkq/hello_jit_world_the_joy_of_simple_jits/cbjald1
[4] gopher://gopher.conman.org/0Phlog:2013/08/06.1
[5] http://www.lua.org/
[6] http://luajit.org/
[7] http://www.classiccmp.org/cctalk.html
[8] http://en.wikipedia.org/wiki/Silicon_Graphics
[9] http://en.wikipedia.org/wiki/R3000

Email Sean Conner at sean@conman.org

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