AIX Version 4.3 Differences Guide

performing a read operation, therefore allowing greater throughput on multiple
concurrent reads.
If a thread attempting to read from a file cannot get the inode lock and there is a
write operation in progress, then the thread blocks on the lock waiting for the
write operation to complete.
2.11 Increase in the Upper Limit of Trace Buffer (4.3.1)
The current upper limit for a trace buffer produced by the trace command with -T
option is around 55 MB on SMP systems. This only allows a few seconds of a
performance benchmark execution to be recorded. As a consequence, only a
fraction of the data needed can be collected, since the benchmarks can take up
to several minutes.
For AIX Version 4.3.1, the upper limit is increased to the size of a segment and
two segments when double buffering is used or as close to that as possible,
allowing the amount of information collected to be more complete and useful.
2.12 Kernel Scaling Enhancements (4.3.2)
With increasing demands being placed on machines acting as busy network
servers, it is possible that in certain situations some kernel resources may
become exhausted. As machines supporting larger amounts of physical memory,
adapters, and devices are introduced, it makes sense for the kernel to be able to
use larger resource pools when required. Therefore, the crash utility, used for
examining system images and the running kernel, is enhanced to understand the
new increased system resources.
The following sections describe the major enhancements.
2.12.1 Network Memory Buffer Pool
The kernel allocates memory from the network memory buffer pool, commonly
called the mbuf pool, to be used as buffers by the networking subsystem. The
size of the mbuf pool is a tunable parameter and is changed using the thewall
option of the no command.
2.12.1.1 Network Memory Buffer Pool Size Increase
The maximum size of the mbuf pool is now hardware dependent. Previous
versions of AIX allocated the mbuf pool from the kernel heap. AIX 4.3.2 now uses
a dedicated memory segment for the mbuf pool on most machines, and four
contiguous memory segments on CHRP machines. This allows a maximum mbuf
pool of 256 MB on most machines and 1 GB on CHRP hardware. The kernel will
allocate an amount of virtual memory equal to one half the amount of physical
memory, or the maximum value allowed for the hardware type, whichever is
smaller. For example, on a machine with 128 MB of memory, the default value of
thewall will be 64 MB. On a CHRP machine with 16 GB of memory, the default
value will be 1 GB.
The larger mbuf pool will allow greater network throughput on large SMP
systems.
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