This helpfile describes some of the factors to consider when planning a         
large TURF run.  This section was last updated on Sept 9,2012.                  
              *  limitations on run size  *                                     
ITEMS: Using lots of items can be done, but only with sensible combina-         
tion sizes, or by using the STEP option (see below).                            
For example, one might look at 4 items out of 200 (64.7 million passes),        
but even 6 out of 200 would become excessive (82.4 billion passes).             
COMBINATION SIZE: the maximum combination size is 60 items.  One could          
look at 16 out of 24, for example, or even 40 out of 45.  However, 60           
out of 210 is so large that it will never finish.  This P-STAT command,         
PUT( COMBINATIONS(50,7))$  would return the number of combinations that         
7 out of 50 would require, for example, and may be useful in estimating         
the time of a prospective run.                                                  
HOW LARGE A RUN CAN BE DONE: as described above, it depends on the              
number of cases, the number of combinations, the options used, and the          
speed of the PC itself.                                                         
If you are considering a large run, 10 billion combinations or more, you        
might try smaller run first, get it's time, and use the ratio of the            
combinations to estimate the time needed for the larger run.                    
IS 10 OUT OF 50 POSSIBLE: this is 10.27 billion combinations and would          
undoubtedly take quite a few hours, but is possible.  The progress meter        
ticks every million combinations, so you can easily tell how long a run         
will take once it starts.                                                       
              *  doing very large runs  *                                       
              *  using the STEP option  *                                       
STEP, followed by two or more sizes, processes the sizes in a stepwise          
manner in one collective run. The sizes must ascend.                            
The first step size is done normally, i.e., all possible combinations           
are evaluated, exactly like a SIZE run.  Then, the items in its best            
combination for reach (or optionally for frequency) are fed into the            
second size, and so on.                                                         
A STEP run is usually much faster than a SIZE run because it is trying          
fewer combinations.  However, it will probably not provide the abso-            
lutely best combination.                                                        
The STEP option is fully described in the TURF.OPTIONS helpfile.                
              *  limitations on memory size  *                                  
MEMORY CONSTRAINTS FOR INPUT: the input data must fit in memory.  (We           
really don't want to read the data afresh from disk for each of 50 mil-         
lion different combinations.)                                                   
In most situations memory should not be a problem because the data is           
usually stored very compactly.                                                  
The final report shows how much of the input data area was used; that           
line however is omitted when less than 50% was needed.                          
MEMORY CONSTRAINTS FOR OUTPUT: the output files (except for the                 
FULL.OUTPUT file) are collected in memory in sort order as the run              
progresses. The default sizes cause no problems.                                
If one asks for 20,000 results in the REACH.RESULTS or FREQ.RESULTS             
files, the run will be slightly slower but the file should fit.