Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-30
Example 1: Changing Packet Size
These average message sizes suggest the use of a larger packet size on the path.
Changing the packet size to 1024 bytes would greatly reduce the processor cost per
request and reduce the Expand subsystem overhead required per message.
Messages up to 960 bytes will fit within a 1024-byte packet (with 64 bytes of
overhead). The following compares the packets per link required for 256-byte packets
(frame size equal to 132 words) and 1024-byte packets:
These calculations suggest that it would be very effective to increase the Expand
packet size to 1024 bytes on this path. Expand subsystem processor cost could be
reduced significantly, probably to less than 50 percent of the previous Expand
subsystem processor cost. The savings would be in the processor cost per packet. The
per-message processor cost is a constant no matter what packet size is used since the
number of messages and the size of the messages is the same in both cases.
For messages sent, changing the packet size to 1024 bytes improves the packets-per-
link ratio by about five times. Increasing the packet size to 2048 bytes could further
improve efficiency on the path (but this is not possible for all line types):
Increasing the packet size to 2048 bytes should increase efficiency slightly more since
it would only reduce the number of packets by an additional 40 percent, possibly
reducing Expand subsystem processor cost another 10 to 20 percent.
For each packet saved, 64 bytes of Expand header is no longer needed. The
percentage of bandwidth saved can be estimated. First, choose the busy direction. In
the example, the outgoing direction has the largest average message size (1776
bytes). If in doubt, select the direction where links times message size is greatest.
Calculate the bandwidths required for the 1024-byte packet size and the original 256-
byte size using the following formula:
The bandwidth used for 1024-byte packets would be
1776 + (2 * 64) = 1904 = 15323 bits/message
256-Byte Packets 1024-Byte Packets
Messages Sent
(average message size=1776 bytes)
230144/24888 = 9.25 46043/24888 = 1.85
Messages Received
(average message size = 638 bytes)
94921/28559 = 3.32 28559/28559 = 1.0
Overall 325065/53477 = 6.08 74602/53477 = 1.4
1024-Byte Packets 2048-Byte Packets
Messages Sent
(average message size=1776 bytes)
46043/24888 = 1.85 24888/24888 = 1.0
Messages Received
(average message size=638 bytes)
28559/28559 = 1.0 28559/28559 = 1.0
Overall 74602/53477 = 1.4 53477/53477 = 1.0
average_message_size + (number_of_packets * 64)