Re: Arrays and Performance

[Jon Paris <jon.paris@xxxxxxxxxxxxxx>]

On Aug 4, 2010, at 1:00 PM, rpg400-l-request@xxxxxxxxxxxx wrote:

> > The trouble with this approach is that the arrays are limited in  
> > size, a
> > maximum 9,999 elements in RPG/400 and 32,767 elements in RPG IV.

In the case of RPG IV this is no longer true as of V6.1. For that  
matter it was only true prior to V6.1 if you actually defined the  
arrays to RPG. If you just use a User Space or dynamic memory and  
qsort/bsearch this is not an issue.

For conventional RPG arrays _the_ critical factor is coding the ASCEND  
(or DESCEND) keyword. With these in place %LookUp will do a binary  
search and is very fast - faster than bsearch. Of course you do have  
to be sure that the array is in the sequence you said <grin>. It is  
the old LOOKUP op-code (and %lookup in cases without an ASC/DECend  
keyword) that has poor performance.

In tests I have run, the array approach outperforms a CHAIN or SETLL  
by many orders of magnitude. In one set of tests the difference was in  
excess of 10,000 to 1 and that is 1) after including the time to load  
the array _and_ 2) comparing against a CHAIN scenario where SETOBJACC  
had been used on the file. Remember when comparing the SETOBJACC  
scenario that by using that most of what you save is the physical disk  
reads. The path length through the I/O system is still fairly lengthy.

One approach that I have used in the past is to place the array in a  
User Space. Every time that you get a "miss" then CHAIN to the file.  
If found by CHAIN add it to an array of "updates" for the User Space  
array. At the end of the batch job add the updates to the end of the  
array and sort it. This avoids the overhead of having to load the  
array each day and ensures that your most frequently used products/ 
customers/codes are present in the array. Now it is ready for the next  
day. If you can prime the array by running a quick analysis over the  
last few days transactions you're ahead of the game.




Jon Paris

www.Partner400.com
www.SystemiDeveloper.com