• RtwgPTSpec_R2

Introduction

The passage of time is, almost by definition, of interest to real-time software. Specific examples include:

• Periodic threads with or without deadlines
• Watchdog timers
• Sporadic servers
• Other CPU-consumption budgeting.
• Deadlines for aperiodic processing

Rationale

The POSIX specification is mature and generally accepted. It includes a set of time-related APIs that provide a strong basis to build software that needs these services. The POSIX timers functions are adopted for this specification without modification.

Specification

1. The kernel MUST conform to the POSIX specification IEEE Std 1003.1-2001 for timers. This includes functions marked with the following margin codes:
   • The TMR margin code for POSIX timers
   • The CS margin code for clock selection
   • The MON margin code for a monotonic clock
2. The CPT margin code for process CPU time clocks is not required.
3. The kernel timer support MAY include support for one or more clocks with high-resolution utilizing hardware counters or timers to achieve timer resolution with 100us or lower.

Notes

The list of required functions includes:

• clock_gettime, clock_settime, and clock_getres
• timer_create, timer_delete, timer_settime, timer_gettime, timer_getoverrun
• nanosleep, clock_nanosleep

References

Support for POSIX timers is included in the CELF kernel. This also includes support for high-resolution timers on selected architectures.

Support for POSIX timers is also included in 2.6 Linux kernels. A patch for high-resolution timers is available at http://high-res-timers.sourceforge.net.