Compaq COBOL AAQ2G1FTK - Manual (Page 500)

Optimizing Your Compaq COBOL Program
15.5 Choices in Procedure Division Statements
15.5.5 Minimizing USE Procedures with LINKAGE SECTION References
Compaq COBOL can perform certain optimizations if a program unit does
not contain USE procedures that reference LINKAGE SECTION items. Note
that USE procedures implicitly reference the following variables for any files
associated with the USE procedures:
FILE STATUS
DEPENDING ON
RELATIVE KEY
LINAGE-COUNTER
Record buffer
If you need to reference LINKAGE SECTION items in a USE procedure, try
to limit the size of the program unit containing that USE procedure. Compaq
COBOL will be able to perform more aggressive optimizations on all the other
program units that do not contain the LINKAGE SECTION references in any
USE procedures.
15.6 I/O Operations
Compaq COBOL provides methods of controlling actions taken by the I/O system
during I/O operations. You have the choice of accepting the defaults the I/O
system provides or using these optional methods to make your program more
efficient.
The Compaq COBOL language elements that can specify alternatives to the I/O
system defaults are as follows:
• The APPLY clause in the I-O-CONTROL paragraph
• The RESERVE n AREAS clause in the FILE-CONTROL paragraph
• The SAME RECORD AREA clause in the I-O-CONTROL paragraph
• The BLOCK CONTAINS clause in the FD entry
On OpenVMS, for additional information on the RMS terms and concepts
included in this section, see the OpenVMS Record Management Utilities Reference
Manual and the OpenVMS Record Management Services Reference Manual.
♦
15.6.1 Using the APPLY Clause
On OpenVMS, the APPLY clause in the I-O-CONTROL paragraph of the
Environment Division provides phrases that you can use to improve I/O
processing. Examine the following example of the APPLY clause:
APPLY

CONTIGUOUS
CONTIGUOUS-BEST-TRY

PREALLOCATION preall-amt ON { file-name } ...
15.6.1.1 Using the PREALLOCATION Phrase of the APPLY Clause
By default, the system does not preallocate disk blocks. As a result, files
can require multiple extensions of disk blocks. Although file extension is
transparent to your program, it can reduce performance. To avoid a degradation
in performance, use the APPLY PREALLOCATION clause to preallocate disk
blocks.
15–10 Optimizing Your Compaq COBOL Program