Timestamps

Timestamps

The IRIX operating system and Silicon Graphics hardware provide two forms of free-running clock that you can use as a timestamp; that is, as a value establishes the relative time difference between two events. One clock is returned by a standard system call; the other is a hardware device you map into process address space.

Time of Day Timestamp

The BSD-compatible function gettimeofday() returns the time of day as two long integers which together give the time since 1/1/1970 to the microsecond. The resolution of this value is at least 10 milliseconds--that is, it is guaranteed to change at least 100 times a second. The actual resolution depends on the system.

The time of day timestamp is discussed further in Chapter 5, "Managing Time and Time Intervals." The sample program under "Getting the Time of Day Stamp" tests the time-of-day clock to find out its true precision.

Hardware Cycle Counter

The cycle counter is a high-precision hardware counter that is updated continuously. In a Challenge/Onyx machine it is a 64-bit value. In other Silicon Graphics architectures the cycle counter has less precision; for example, in the Indy it is a 32-bit counter.

In the Challenge/Onyx, the cycle counter is incremented every 21 nanoseconds. In other architectures the frequency is lower, although it is always comparable to the instruction execution time. (For example, in the Indy it is incremented every 40 nanoseconds.) Because of the high frequency, the cycle counter is certain to contain a different value every time it is sampled.

Note: Considered as a time standard, the Challenge/Onyx cycle counter is accurate to 1 part in 10,000. If you use it to measure intervals between events, be aware that it can drift by as much as 100 microseconds per second. You sample the cycle counter by mapping it into the process's address space, then reading it as if it were a memory variable. The method is covered in Chapter 5, "Managing Time and Time Intervals." The sample program under "Mapping and Reading the Cycle Counter" also demonstrates its use.