An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 88
granularity of 1.024µsec time slices. This value was varied to investigate
the effect upon CPU utilisation which was measured using the Unix time
command. Figure 5.4 shows the effects of varying the interrupt values on the
Intel e1000 (version 5.2.20) driver on back-to-back tests at 1Gb/sec using
TCP and iperf on the MB-NG testbed network (See Appendix C.2). In all
tests, it can be seen that the most intensive component of CPU utilisation
is that of the kernel process, which involves both the memory handling and
packet interrupts.
Figure 5.4(a) shows the effect of changing the coalescing values on the
sender only, with the receiver set to interrupt on every packet (RXIntValue=0).
It clearly shows the reduction of CPU load on the sending machine as it approaches
23µsec - which is equivalent to approximately 2 packets (in this
case acks) that are received back-to-back due to the use of ABC (See Section
5.2.4) which causes two data packets to be sent back-to-back. As the
receiver is constantly interrupting on every (data) packet, its CPU utilisation
remains constant.
Figure 5.4(b) shows the effects of adjusting the receiver’s RXIntValue. It
can be seen that a small change of this value to approximately the time taken
to process one packet can reduce the CPU utilisation of the kernel by about
half. As it is now the sender which is interrupting on each packet, it can be
seen that there is also the benefit of lower CPU utilisation on the sender at
the same settings.
Figure 5.5 shows similar tests but varying the transmitting TXIntValue
coalesce value rather than the receiving.
It was observed that altering the TXIntValue on the receiver has no effect
on the CPU utilisation of either system. This is due to the fact that even
though ack clocking is important to TCP, it does not affect the number