Contents Telektronikk - Telenor

ent on the window size the user data
size giving peak performance varies.
- The form of the throughput graph
shows that the measured throughput
increases with increasing user data
sizes. Thus, the segment dependent
overhead is reduced when transmitting
fewer segments. After an initial phase,
the measured throughput is more or
less independent of increasing the user
data size. However, for larger window
sizes, the measured throughput decreases
with larger user data sizes.
- All window sizes have their anomalies,
i.e. peaks and drops, for certain user
data sizes.
- The receiver is heavier loaded than the
sender.
5.1 Throughput dependent on
user data size
From Figure 7 (a) it is evident that the
throughput is not indifferent to the user
data size. For a given number of bytes to
be transferred, the smaller the user data
size of the write system call, the more
system calls need to be performed. This
affects the achievable throughput in two
ways. One is due to the processing
resources to do many system calls, the
other is due to a lower average size of the
protocol segments transmitted on the
connection. Therefore, for small user
data sizes increasing the user data size
will increase the throughput. The increase
in throughput flattens when an
increase in user data size does not significantly
influence the average segment size
of the connection.
For 4k, 8k, and 16k window sizes the
throughput is more or less independent of
large user data sizes. It is the window
size and host processing capacity which
are the decisive factors on the overall
throughput. On the contrary, larger window
sizes experience a light degradation
in throughput. The throughput degradation
is caused by acknowledgments being
returned relatively late. The larger the
receive user buffer, the more bytes from
the socket buffer can be copied before an
acknowledgment is returned. Therefore,
the average time to acknowledge each
byte is a little longer, and the throughput
degrades.
For small and large user data sizes the
throughput may be increased if, independent
of the send user data size, the
size of the receive user buffer is fixed.
For small user data sizes a larger receive
buffer reduces the number of read system
Throughput [Mbit/s]
Throughput [Mbit/s]
Throughput [Mbit/s]
30
25
20
15
30
20
10
0
30
20
10
Points discussed in the text
0 4096 8192 12288 16384 20480 24576 28672 32768
0
0 16384 32768 49152 65536
Window size [byte]
(b)Througput dependent on window size
User data size [byte]
(a) TCP throughput dependent on window and user data size
CPU utilization [%]
100
75
50
25
Transmit
Receive
4096 byte window
8192 byte window
16384 byte window
24576 byte window
32768 byte window
40960 byte window
52428 byte window
0
0 16384 32768 49152 65536
Window size [byte]
(c) CPU utilization
Figure 7 Sparc2 SBA-100, network driver version 2.2.6
32768 byte window
32768 byte window, 8192 byte user data receive buffer
0 4096 8192 12288 16384 20480 24576 28672 32768
User data size [byte]
Figure 8 Throughput
changes with a fixed size
receive user buffer
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