64-bit SuperCore Timestamp Per Target Evaluation Request

Joel Sherrill joel.sherrill at OARcorp.com
Tue Dec 9 21:26:59 UTC 2008


This is long but important so please take the time
to read to the bottom.

I just committed the last of a series of patches
which adds the SuperCore Timestamp handler and
the capability for a CPU port to pick one of three
implementations of SuperCore Timestamps:

+ struct timespec (current implementation
+ int64_t (method not inlined)
+ int64_t (methods inlined)

The int64_t is used for nanoseconds since POSIX epoch (1970)
and can represent ~200 years.

I have done extensive testing on SPARC/sis and PowerPC/psim
to determine the impact of each implementation.  I hope the
committed code reflects this. :)  I used tm26 (Thread Dispatch)
and tm02 (semaphore obtain blocking as reference performance

On PowerPC/psim the following numbers are in instructions:
                         timespec  int64  inlined int64
    dispatch:              446      446      400
    blocking sem obtain:   627      626      581
On SPARC/sis, the following numbers are in microseconds:
                         timespec  int64  inlined int64
    dispatch:               59       61       53
    blocking sem obtain:    98      100       92

This means that (on THESE TARGETS) inlined int64_t results
in ~7.5% faster blocking semaphore obtains and about 10% off
any thread dispatch that context switches.  This impact a lot
of thread operations.

Since inlined int64_t operations appear to be faster, they
may be a better choice on some targets.  However, there is
a size issue I need to look into.  The minimum executable is
slightly smaller with int64_t but ticker is larger. The timespec
math was optimized for clock tick so maybe the int64_t math
just is heavier there.  I don't know.

The code is now committed to the CVS trunk.  It would be
helpful to get feedback on other targets as to the impact.
For now, you will have edit
cpukit/score/include/rtems/score/timestamp.h to switch
between the alternatives.

I am convinced that the best choice for each target will
need to be selected.  On targets like the PowerPC where
code space is not an issue, inlined int64_t is probably
the best choice.  But others may not show the performance
benefit or memory may be at a premium (e.g. Thumb) so
a different choice may be apprpriate.

Please pitch in and provide feedback.

Joel Sherrill, Ph.D.             Director of Research & Development
joel.sherrill at OARcorp.com        On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
   Support Available             (256) 722-9985

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