Talk:Real Time Terms

From eLinux.org
Revision as of 18:33, 8 June 2007 by Tim Bird (Talk | contribs) (make note on time)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

These terms do not state a common model for time measurements. I believe it is relatively well-established that the following events and time periods are of interest when measuring realtime performance:

Events

  • A - hardware interrupt assertion
    • This is the time when the hardware interrupt line for an event is raised
  • B - Interrupt service routine starts execution
  • C - Process starts execution
  • D - Result is received from processing the event

Time Periods

Real time performance is often expressed in terms of the maximum, minimum and average duration for certain time periods defined by the events above.

Here are some common terms, expressed relative to those events:

Interrupt latency 
The time from A to B
Scheduling latency 
The time from B to C (this is sometimes referred to as the time from A to C)
Processing time 
The time from C to D
Response latency 
The time from A to D

Often, these or similar terms are used with less accuracy to describe the closest approximation one can get to these, with a particular test framework and instrumentation set.

For example, Response latency may be reported as "response time", and refer to the time from when a host program records the time, previous to transmitting a piece of data which will cause an interrupt on the target machine, to the time when a host program records the time after receiving some signal from the target that processing is completed. Of course, time is taken in the operations of recording time, tranmissing data, and detecting signals on the host machine. But it may be that timing these individual operations at a finer granularity requires instrumentation or hardware support not reasonably available for a test.