How user exit routines affect DFSORT performance

Before writing a user exit routine, consider the following factors:

Your routines occupy main storage that would otherwise be available to DFSORT. Because its main storage is restricted, DFSORT might need to perform extra passes to sort the data. This, of course, increases sorting time.
User exit routines increase the overall run-time.
Attention: Several of the user exits give your routine control once for each record until you pass a "do not return" return code to DFSORT. You must remember this when designing your routines.
Using INCLUDE, OMIT, INREC, OUTFIL, OUTREC, and SUM instead of user exit routines allows DFSORT to perform more efficiently.