UNIX Process Accounting

There is a general and deep-rooted belief that UNIX Process Accounting causes a significant overhead to a system. We set out to see if this was true and found that running UNIX Acquire for Process Accounting actually has a low overhead on the UNIX host system.

It may be of interest to understand that in UNIX and Linux the operating system is doing all the capture of the metrics for accounting all the time, and the only part not done when accounting is switched off is the writing of the account record to disk.  Running accounting is very valuable in order to obtain workload-level data with which to build athene® Planner models, or to break out usage of a system by services or users.  It is also the only source of data that splits out system CPU time from user CPU time, or provide a view on the I/O load from a given user or command.

The Benchmark

A Perl script was used to run a benchmark. In essence this script repeatedly creates a new process to run the “uname –a” command. The process creation occurs within a loop structure which means that it can be called for n iterations before the script terminates. On a partition of a pSeries p720 machine configured with 2 x 3 GHz Power 7 processors and running AIX v7.1, 10000 iterations took about 58 seconds to complete on an otherwise empty machine. This represents a command termination rate of over 172 per second.

One major design requirement was to use a command that should not cause or require disk activity, so that any increase that was observed across the tests would be a reflection of the disk I/Os generated directly by the system’s accounting file handling routines. 

The script was run on a quiet machine to ensure that the conditions both with and without process accounting enabled were the same.

The script was run 5 times for each of the three tests, as noted further on, all on an otherwise quiet machine. First the script was run with process accounting switched off (i.e. no pacct file). The benchmark was then repeated with process accounting switched on (i.e. with a growing pacct file), and then with adding AIX’s Advanced Accounting as well as process accounting.

You can see the results this Friday…………

Nick Varley

Principal Consultant