[glob2-devel] speedup possible on resource gradients

[Nuage]

* speedup possible on resource gradients:

Just we know how much we can save, I ran the same test (256x256 map, 4 AICastor,
60s, CXXFLAGS="-O3 -march=athlon-xp"), but I "hacked" the code, so that
map.syncStep() is executed twice. This make two resource gradients computed
instead of one, and they are not the same to avoid cache-optimization bias.
(he-he I should not forget not to commit it....)

Simple:
cpu usage graph:
100.0 % | *
 98.5 % |
 95.0 % |
 93.5 % |
 90.0 % |
 88.5 % |
 85.0 % |
 83.5 % |
 80.0 % |
 78.5 % |
 75.0 % |
 73.5 % |
 70.0 % |
 68.5 % |
 65.0 % |
 63.5 % |
 60.0 % |
 58.5 % |
 55.0 % |
 53.5 % | *
 50.0 % | *
 48.5 % | *
 45.0 % | **
 43.5 % | ****
 40.0 % | *****
 38.5 % | ****************
 35.0 % | *********
 33.5 % | *****************
 30.0 % | *******************************
 28.5 % | *********
 25.0 % | *
 23.5 % |
 20.0 % |
 18.5 % |
 15.0 % |
 13.5 % |
 10.0 % |
  8.5 % |
  5.0 % |
  3.5 % |
  0.0 % |

Double:
cpu usage graph:
100.0 % | *
 98.5 % |
 95.0 % |
 93.5 % |
 90.0 % |
 88.5 % |
 85.0 % |
 83.5 % |
 80.0 % |
 78.5 % |
 75.0 % |
 73.5 % |
 70.0 % |
 68.5 % |
 65.0 % | *
 63.5 % |
 60.0 % | *
 58.5 % | *
 55.0 % | *
 53.5 % | **
 50.0 % | ******
 48.5 % | *************
 45.0 % | *****************
 43.5 % | *********
 40.0 % | *******
 38.5 % | *********
 35.0 % | *********************
 33.5 % | *******
 30.0 % | *
 28.5 % |
 25.0 % |
 23.5 % |
 20.0 % |
 18.5 % |
 15.0 % |
 13.5 % |
 10.0 % |
  8.5 % |
  5.0 % |
  3.5 % |
  0.0 % |

It's a bit difficult to interpret, so I took the 90%-quantile. With simple
computation, we reach 40% CPU, and with double we reach 50% CPU. We can then
fairly expect that the resources gradients take 10% of the CPU on my computer.
(AMD Athlon XP 3200+, with low latency but down-clocked RAM and FSB due to a 
bug).