PTR Overview



Objectives

Goal: Provide platform dependent timing routines that share a common Application Programming Interface (API). Ptools will provide a central access point for platform-specific implementations of the routines.


What We Are Not Trying to Do


Basic Terms Used for PTR

The following terms are used in establishing the semantics of the portable timing routines. The exact units measured and the size of the values measured may differ across platforms.

Timer value
The value returned by a call to a timer routine. This represents the current value of the timer (e.g. an appropriate register). There are three types of timer values:
a) User timer value. This is the value of the timer that measures the CPU time billed to the user (platform-dependent).
b) System timer value. The value of the timer tracking the CPU time used for system activities (platform-dependent).
c) Wallclock timer value. The value of the wallclock, or time-of-day, timer (may be measured in a platform-dependent way such as relative to some previous moment).

Timer values are only meaningful when used to calculate elapsed time, or the interval between the points at which timer values are obtained.
Ticks
The smallest unit of user, system, or wallclock time measurable on a system. It is a measure of the underlying clock rate and will be system dependent.
Tick frequency
A measure of the tick granularity (e.g. number of ticks per second) for user, system, or wallclock time on a given platform. There are two types of tick sizes:
a) Nominal tick frequency. The reference value supplied by the vendor.
b) Calibrated tick frequency. The value obtained from a calibration program. This is the value for a particular instance of a specific platform, and takes into account variation in hardware and software versions on a given platform.
Resolution/Accuracy
The minimal time unit that can be measured with confidence. It will be at least as coarse as the tick size, and probably coarser since many timers operate as step, rather than continuous functions. This value will be nominal (i.e., supplied by the vendor) indicating the number of bits that should be considered valid when calculating elapsed time.
Rollover frequency
The number of ticks that can be accommodated before a timer rolls over or is reset. This value will be nominal (i.e., supplied by the vendor).
Overhead (or Intrusion)
The cost (intrusiveness) of using one of the timer routines to capture a timer value. This cost will be calibrated (i.e., obtained from a calibration program).


Use of Portable Timing Routines


Other Options Considered

  1. Timers return meaningful units at each call ( the appropriate register is read and the value is converted to meaningful time units).
    - Pros: this would eliminates the problem with overlapping intervals
    - Cons: Too intrusive - user can't control where overhead was incurred
  2. Software timers: user instantiates/starts/stops/reads the timer (stopwatch metaphor)
    - Pros: conversion not needed for start/stop
    - Cons: need multiple timers if intervals overlap
  3. OUR CHOICE: Use opaque timer values.
    - Pros: user controls overhead by making explicit conversion calls
    - Cons: timer values of themselves are meaningless

Portable Timing Routines home page
Parallel Tools at OSU home page
Parallel Tools Consortium home page

For further information, contact kennino@cs.orst.edu.