Improve Overall I/O Performance

Improving overall I/O performance can minimize both device I/O and actual CPU time. The techniques listed in this section can greatly improve performance in many applications.

A bottleneck limits the maximum speed of execution by being the slowest process in an executing program. In some programs, I/O is the bottleneck that prevents an improvement in run-time performance. The key to relieving I/O bottlenecks is to reduce the actual amount of CPU and I/O device time involved in I/O. Bottlenecks may be caused by one or more of the following:

• A dramatic reduction in CPU time without a corresponding improvement in I/O time results in an I/O bottleneck.
• By such coding practices as:
  • Unnecessary formatting of data and other CPU-intensive processing
  • Unnecessary transfers of intermediate results
  • Inefficient transfers of small amounts of data
  • Application requirements

Improved coding practices can minimize actual device I/O, as well as the actual CPU time.

You can also consider solutions to system-wide problems like minimizing device I/O delays.

The following sections discuss I/O performance considerations in more detail:

• Use Unformatted Files Instead of Formatted Files
• Write Whole Arrays or Strings
• Write Array Data in the Natural Storage Order
• Use Memory for Intermediate Results
• Enable Implied-DO Loop Collapsing
• Use of Variable Format Expressions
• Efficient Use of Record Buffers and Disk I/O
• Specify RECL
• Use the Optimal Record Type