Expand Configuration and Management Manual (H06.03+)

Tuning
Expand Configuration and Management Manual—529522-002
19-13
Multi-Line Paths
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
Processor Type
The processor overhead for serving multiple lines is greater than the processor
overhead for serving one line per path for an equivalent volume of throughput. The
degree of increased cost depends on the processor type, the version of the Expand
software used, and the speed differences (if any) between the lines.
Packets Per Message
While a large message fragmented into small packets might make more efficient use of
the communications bandwidth than a message in a single large packet, it takes more
processor time for the fragmentation and the reassembly.
A single high-speed line might be a better solution than multiple lines, especially from
the standpoint of processor efficiency, when a single important application dominates
the performance considerations. If the variable packet-size feature is used with multiple
lines, the PATHPACKETBYTES modifier value should be configured to use all lines in
the multi-line path equally.
Refer to Variable Packet-Size Configuration on page 19-6 for more information about
the variable packet-size feature.
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.