IRQ latency and context switching on mvme5500 (was Re: RTEMS mvme5500 bsp)

Peter Dufault dufault at
Thu Jul 28 17:10:48 UTC 2005

On Jul 27, 2005, at 10:26 PM, Kate Feng wrote:

> Based on the test  result, it shows that  RTEMS-mvme5500 is
> more deterministic  and  steadier than vxWorks-mvme5500
> for the highest priority task.  For  both  the idle  and loaded  
> system,
> RTEMS-mvme5500 "GUARANTEED"  twice or three times
> faster response time in a steadier state.     The "worst case" is a
> critical factor  in consideration of the real-time  system.

Can you also include all the identifying information about the  
vxWorks version, and whether you've modified anything from the  

I've also got a fairly big application running on RTEMS-mvme5500 and  
now vxWorks 5.5.  I don't know why Wind River sold my client 5.5, I  
wasn't involved in that, maybe 6.x doesn't run on the MVME5500?   
Maybe 6.x is more up-to-date?  If anyone knows, please let me know.

FIRST, THOUGH, RTEMS doesn't do anything with the 64-bit counter on  
the PPC, right?  That's where I get my timings.

I can't get to the system to provide full details, but here are a few  
notes from memory.

1. The vxWorks distribution has L2 and L3 instruction cache disabled  
from the factory, with a note such as "per Wind River support" in the  
config.h Motorola header file where it's disabled.  I've enabled  
instruction cache without noting any problems.  Timings are  
particularly bad before that.

2. The vxWorks header files are ANTIQUE, and my client just bought it  
last month!  gcc is "2.96".  The claimed POSIX compliance is circa  
1995 and even then it doesn't come close.  The networking headers are  
particularly bad.  I had to put a whole lot of work-arounds in to get  
code that runs on Linux, FreeBSD, RTEMS, Cygwin, and OS/X to work on  
the vxWorks "POSIX" interface.

3. With cache enabled the context switch times are still surprisingly  
slow on vxWorks.  I'll quantify "a lot" when I can dig up some notes,  
but my biggest surprise was the time between the interrupt semaphore  
post in my ISR and when I was finally in the high-priority task (set  
at PRIOMAX) activated by that semaphore.

4. My networking code needed tweeking to get it to work reliably and  
fast on vxWorks.  I never could get receives to work in a timely  
fashion unless the receiving end knew ahead of time how much to  
receive, even setting the low watermarks, send and receive buffer  
sizes, etc wouldn't get things working well.  I finally gave up and  
started sending a fixed minimum packet size that told what would come  
in the next packet if it didn't fit in the first one. The same code  
is run in all places so that isn't a variable, it probably improves  
it a bit everywhere.

I'm running a multi-axis control application and I've been doing life  
testing with both RTEMS and vxWorks at the following rates.  I'm  
using my own drivers for analog I/O.  The first number is RTEMS, the  
second vxWorks.  I arrived at these numbers by detecting analog input  
overruns and then backing off a bit and also being sure the UDP data  
is coming up OK (I live with occasional drops).

Data acquisition interrupt (copies data off PCI board, posts  
semaphore): 22KHz 15KHz
Inner loop (fields semaphore, does inner loop, outputs to DACs,  
safety checks, periodically posts semaphore for outer loop, assembles  
data for UDP upload, posts upload semaphore): 22KHz 15KHz
Outer loop (Fields semaphore, does outer loop processing, profiles  
motions): 2.2KHz  1.875KHz
IP Command exerciser:  (Receives new position command, returns  
status) 50Hz 50Hz
Command monitor: Runs when a command is typed in.
UDP Data Upload task: Runs continuously when it can, priority set  
below system network tasks.
System tasks: The network tasks, and whatever else is started up out- 

Finally, let me note that this was a good exercise - the changes I  
made to get vxWorks faster (networking changes, custom versions of  
semaphores with timeouts since there is no sem_timedwait and POSIX  
timers are of little use on vxWorks) also folded into RTEMS and  
improved my life test rate there from 20KHz to 22KHz.


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