The x-Kernel: An architecture for implementing network protocols - IDA

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Figure 5: Message Manager
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cost in Unix varies significantly. In particular, the Unix curve can be divided into four distinct parts: (1)
the incremental cost of going from 200 bytes to 500 bytes is 0.57 msec per 100 bytes; (2) the cost of
sending 600 bytes is over 1 msec less than the cost of sending 500 bytes; (3) the incremental cost of
sending 600 to 1000 bytes is 0.25 msec per 100 bytes (same as the x-kernel); and (4) the incremental
cost of sending between 1100 and 1400 bytes is again 0.57 msec per 100 bytes.
The reason for this wide difference of behavior is that Unix does not provide a uniform message/buffer
management system. Instead, it is the responsibility of each protocol to represent a message as a linked
list of two different storage units: mbufs which hold up to 118 bytes of data and pages which hold up to
1024 bytes of data [18]. Thus, the difference between the four parts of the Unix curve can be explained
as follows: (1) a new mbuf is allocated for each 118 bytes (i.e., 0.57msec/2 = 0.28msec is the cost of
using an mbuf); (2) a page is allocated when the message size reaches 512 bytes (half a page); (3) the rest
of the page is filled without the need to allocate additional memory; and (4) additional mbufs are used.
Thus, the difference in the cost of sending 500 bytes and 600 bytes in Unix concretely demonstrates the
performance penalty involved in using the “wrong” buffer management strategy; in this case, the penalty
is 14%. Perhaps just as important as this quantitative impact is the “qualitative” difference between the
buffer management scheme offered by the two systems: someone had to think about and write the data
buffering code that results in the Unix performance curve.
Second, Figure 6 gives the performance of the x-kernel and Unix as a function of the number of open
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