Expand Configuration and Management Manual (H06.21+, J06.10+)

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 14
Multi-CPU Paths
Window Size
If a line is added to a path using a different protocol or telecommunications network at
Layer 1, the path might have different delay characteristics from the original single line.
It might be necessary to change the Layer 2 window size (TXWINDOW modifier) to
minimize any delays introduced by switches or new protocols.
In some situations, the HDLC Extended protocol might be advantageous on terrestrial
links. Some terrestrial networks, especially those that include private switches and
CBXs, can have variable delays that at times are as great as a 0.25-second satellite
hop. If queuing for an Expand line-handler process occurs in such a network, yet the
process and communications use is low, switching delay might be the cause and using
the HDLC Extended protocol might be advisable.
Variable Packet Size
Variable packet size effectively increases the Expand packet size to 1024 bytes or
greater. Multi-line paths can distribute data across available lines more evenly when
the packet size is smallest. Applications that send messages just below the size of the
configured variable packet size over multi-line paths might notice an increase in
latency because all data is sent across only one line.
For example, a four-line path with a variable packet size (PATHPACKETBYTES
modifier) of 4095 bytes will only use the first line for all requests of 4 Kbytes or less
unless multiple requests are received simultaneously. If the PATHPACKETBYTES
modifier is configured at the default (1024 bytes), requests larger than 4 Kbytes will
use all four lines more evenly.
The general formula for configuring the variable packet size for multi-line paths using
low to medium bandwidth lines is:
If the average message size is not known, you can use the Exp
and subsystem SCF
command PATH STATS to display a histogram of message sizes.
Multi-CPU Paths
The multi-CPU path is the fundamental component of the Expand multi-CPU feature. A
multi-CPU path can consist of up to 16 individual Expand paths, including multi-line
paths. Expand-over-ServerNet line-handler processes cannot participate as members
of a Superpath.
The main advantages of the Expand multi-CPU feature include:
•
Higher total throughput
•
More even spreading of the communications load over multiple processors
•
Reduced message system interprocessor traffic
PATHPACKETBYTES = average_message_size / number_of_lines