AIX 5L Problem Determination - IBM Redbooks

later). The method used in AIX Version 4.3.2 and earlier versions of the
operating system can, under certain circumstances, give an inflated view of wa
time on SMPs. In AIX Version 4.3.2 and earlier, at each clock interrupt on each
processor (100 times a second per processor), a determination is made as to in
which of the four categories (usr/sys/wio/idle) to place the last 10 ms of time. If
any disk I/O is in progress, the wa category is incremented. For example,
systems with just one thread doing I/O could report over 90 percent wa time
regardless of the number of CPUs it has.
The change in AIX Version 4.3.3 is to only mark an idle CPU as wa if an
outstanding I/O was started on that CPU. This method can report much lower wa
times when just a few threads are doing I/O and the system is otherwise idle. For
example, a system with four CPUs and one thread doing I/O will report a
maximum of 25 percent wa time. A system with 12 CPUs and one thread doing
I/O will report a maximum of 8.3 percent wa time.
Also, NFS now goes through the buffer cache, and waits in those routines are
accounted for in the wa statistics.
A wa value over 25 percent could indicate that the disk subsystem might not be
balanced properly, or it might be the result of a disk-intensive workload.
The faults columns
It may also be worthwhile to look at the faults columns, which provide information
about process control, such as trap and interrupt rate.
The in column
In the in column is the number of device interrupts per second observed in the
interval.
The sy column
In the sy column is the number of system calls per second observed in the
interval. Resources are available to user processes through well-defined system
calls. These calls instruct the kernel to perform operations for the calling process
and exchange data between the kernel and the process. Because workloads and
applications vary widely, and different calls perform different functions, it is
impossible to define how many system calls per-second are too many. But
typically, when the sy column raises over 10000 calls per second on a
uniprocessor, further investigation is called for (on a SMP system, the number is
10000 calls per second per processor). One reason for this high number of calls
per second could be polling subroutines like select(). For this column, it is
advisable to have a baseline measurement that gives a count for a normal sy
value.
240 IBM ^ Certification Study Guide - AIX 5L Problem Determination Tools and Techniques