AIX Version 4.3 Differences Guide

2.12.1.2 Network Memory Buffer Pool Allocation Algorithm
The algorithm used by the net_malloc kernel service for allocating buffers of
various sizes has been changed. The high-water marks for various buffer sizes
have been increased, particularly the 2 KB size used by the Ethernet and token
ring device drivers.
Requests for memory buffers between 16 KB and 128 KB are now rounded to the
nearest power of 2. Allocations in this range were rounded to the nearest page
size on prior versions of AIX. This change reduces the number of different sizes
of buffers available, which in turn reduces the number of free lists the algorithm is
required to manage.
The method used by net_malloc to maintain free list information has been
changed. On prior versions of AIX, each CPU maintained a free list for each
different size of buffer. There is now one central free list for each buffer size
between 16 KB and 128 KB in size. Each CPU still maintains a free list for each of
the smaller buffer sizes. This change minimizes the amount of memory used by
unallocated large buffers, while at the same time still allowing each CPU to
maintain a list of smaller buffers for faster allocation.
2.12.1.3 Network Memory Buffer Pool Statistics
The kernel maintains usage statistics for the buffers allocated from the network
memory buffer pool. The information contains details of the number of buffers of
each size, and for each size, information on the number of buffers in use and the
number of failed requests. This information can be displayed using the netstat -m
command. In addition to maintaining information indexed by buffer size, the
kernel also maintains information indexed by the purpose the buffer is being used
for. The type indexed information can be expensive to maintain, so AIX Version
4.3.2 has introduced a method to disable it.
The new extendednetstats network variable, which is altered using the no
command, determines whether the by-type statistical information should be
collected by the kernel. The variable has a default value of 1, which means that
the kernel will collect the information. In order to improve networking
performance, the default AIX installation disables collection of this information by
changing the value of extendednetstats to 0. This is performed in the file
/etc/rc.net that is run during system start up. If you want to view the by-type
statistics, you should comment out the section of /etc/rc.net that changes the
value of extendednetstats.
The following section is near the end of the file and appears as:
##################################################################
# This disables extended netstat statistics for performance
# reasons. To have extended netstat statistics enabled on
# future reboots, comment out the following three lines.
##################################################################
if [ -f /usr/sbin/no ] ; then
/usr/sbin/no -o extendednetstats=0 >>/dev/null 2>&1
fi
The information collected when extendednetstats is set to a value of 1 is
displayed near the end of the output produced by the netstat -m command. An
example of the output is shown below.
# netstat -m
22 AIX Version 4.3 Differences Guide