An Investigation into Transport Protocols and Data Transport ...

5.1. TCP Hardware Requirements 83
CPU Load (fraction)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
CPU Requirements for Various TCP Throughputs
Sender Kernel
Receiver Kernel
Sender User
Receiver User
0
0 100 200 300 400 500 600 700 800 900 1000
Throuhgput (mbit/sec)
Figure 5.3: CPU utilisation with different TCP throughputs.
identical Dual Xeon 2.0Ghz PCs over the MB-NG private WAN (See Appendix
C.2). The configuration of both machines is presented in Table B.2.
The receiver is configured to simulate packet loss by dropping every n th
packet to achieve a desired throughput as determined by the response function
which limits the throughput of TCP based upon the loss rate experienced
(See Equation 5.7). As each TCP connection under Linux is handled under a
single processor thread, the benefits of SMP systems are limited, and this is
demonstrated by nearly 100% utilisation of a single processor at high speeds.
It was observed that the TCP receiver requires a higher CPU load than
that of the sender, with an almost linear relation. The sender, on the other
hand, requires more CPU at higher speeds (with a higher gradient at higher
throughputs), most likely to due to influx of ack packets that require extra
processing overheads (such as duplicate acks and SACKs).
Other research [tePM] confirms the approximate calculation that 1Ghz
CPU is required for each Gigabit/sec throughput when the performance is
restricted to end-node hardware rather than Internet performance.