How to measure cpu time to ~ 1 millisecond?

[Guy Harris]

>>Setup:  Sun 4/330, SunOS 4.1
>>
>>We need to measure cpu time in a program down to about a 1 millisecond
>>resolution.  The clock call says it can only measure to 16.7 ms.  Any
>>other ideas on how cpu time can be measured?
>
>My best guess here is that your machine runs at 60 clock ticks per second.

Actually, it's 100 clock ticks per second.  The reason why "clock()"
says it acts as if it were 60 clock ticks per second is that the code
was taken straight from S5, which uses the "times()" call, which in
SunOS maps the internal CPU time values to values that look as if they
were in 1/60ths of a second (this is inherited from 4.2BSD, which did
this for binary compatibility with 4.1BSD).

>Your system might, however, provide a getrusage() system call (resource.h).

It does, but...

>If you have it, that gives the "best estimation" for your system of CPU
>utilization (system and user modes) with a granularity of microseconds
>(see struct timeval in time.h)

...just because a "struct timeval" has a "tv_usec" field doesn't mean it
has a one-microsecond resolution.  The actual resolution is still the
underlying resolution of whatever clock is used, and on a 4/330 that's
the rate at which clock interrupts are generated, or 100 per second.