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

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 29
Example 1: Changing Packet Size
The key statistics in Example 19-6 are the Level 4/Level 3 Sent and Rcvd packets and
links. Each link reflects a network request. Each request requires at least one packet to
be sent, plus one to be received. Both packets are seen in the history.
In this example, the frame size (FRAMESIZE modifier) was 132 words and the variable
packet size (PATHPACKETBYTES modifier) was not available. The extended packet
format was enabled (L4EXTPACKETS_ON modifier), which means that 75 percent of
each packet was Expand subsystem overhead. Assuming 192 bytes of user data per
packet, the average message sizes are easily calculated using this formula:
This average message sizes are derived from the data shown:
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). This 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 regardless of what packet size is used
Example 19-6. SCF PATH STATS Display
-------------------- LEVEL 4 MESSAGE HISTOGRAM ---------------------------
<= 64 .. 71027 <= 128 .. 25609 <= 256.. 4211
<= 512 .. 1676 <= 1024 .. 1466 <= 2048.. 370
<= 4096 .. 179 > 4096 .. 2288
-------------------- LEVEL 4 / LEVEL 3------------Average--------Average---
Packets Forwards Links Packets/Block Bytes/Block
Sent 230144 0 24888 1.0 238
Rcvd 94921 0 28559 1.0 178
L4 Packets Discarded......... 0
average_message_size = (packets / links) * 192
Average message size sent: (230144/24888) * 192 = 9.25 * 192 = 1776
Average message size received: (94921/28559) * 192 = 3.32 * 192 = 638
Average overall message size: (325065/53477) * 192 = 6.08 * 192 = 1167
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