[Freeciv-Dev] Path finding: to cache or not to cache.

[Gregory Berkolaiko <Gregory.Berkolaiko@xxxxxxxxxxxx>]

Hi,

I did some experimenting with caching of various values.
Here are some basic observations:

* Caching leads to increase in the size of the basic node struct which 
results in a slow down.

* Caching is done to optional values, like extra_cost and is_my_zoc.  Even 
if the corresponding call-back or option is not enable, the memory 
allocation is performed.

I did the following experiment: 
code version 1: extra cost of the tile is cached in the struct node
code version 2: extra cost of the tile is computed each time it is needed

setup a: no get_ECOT call-back is supplied
setup b: a get_ECOT callback static to path_finding.c and rather slim is 
supplied

Results (average time over about 8 runs):
1a  0.94 sec
2a  0.91 sec

1b  0.99 sec
2b  0.99 sec


I tend to make following conclusions: if get_ECOT call-back is a little 
more substantial, the caching will give speed.  But how often a call-back 
would be supplied?  Probably often enough.  But it's worth thinking about 
zoc-related cached values (2 bools).  AI largely ignores ZOC in it's 
distance-estimations and AI is the biggest customer for path-finding, at 
least now.

G.