Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-13
Data Compression
Data Compression
Data compression indirectly affects Expand line-handler process performance. By
shortening the length of a message, compression can reduce the number of packets
transmitted.
Because the data compression feature has an insignificant impact on the processor,
data compression should always be enabled unless you are certain that no data is
compressible. If compression is enabled and data is not compressible, data
compression actually causes messages to be slightly longer because the Expand
subsystem inserts a compression word every 255 words (510 bytes) of the message.
You can increase network efficiency by analyzing routine data to determine the degree
of compressibility and then setting the frame size to carry the largest data-compressed
message. This technique can be a very effective way to economize processor
resources for point-to-point links with heavy, large-block message traffic.
Data compression is configured using the COMPRESS_ON modifier.
Multi-Line Paths
The multi-line path feature enables you to configure eight parallel lines between the
same two nodes. The advantages of multi-line paths include increased fault-tolerance
and additional bandwidth.
The main disadvantage of multi-line paths is increased processor overhead, which
occurs primarily because extra processing must be done to select the best line for
each frame transmitted and to guarantee sequencing of packets received across
multiple lines. However, the reduction in queuing delays that results from using a multi-
line path usually offsets the extra processor delay.
The interaction of some elements of the Expand network determine the degree of
improvement multiple lines might achieve. These are the elements that control the
service rate of the Expand line-handler processes (including the NAM process, if
used), and the use of the lines in the path. These elements include the following:
•
Processor type
•
Packets per message
•
Window size
•
Variable packet size
Note. Line message rate can also affect the degree of improvement achieved by multiple
lines. For example, if the line message rate is low, multiple lines will not significantly improve
performance. An application, rather than a line, might sometimes be the cause of a bottleneck.