Forums

Any RTOS to report latencies?

Started by Stargazer September 10, 2007
Greetings,

I am wondering are there RTOSes that document interrupt and task
switching latencies on supported platforms and provide with tools to
validate that. I read some advertising hype from OS vendors on how
small their latencies are, but there were no numbers. There were also
several independent studies that got to numbers, which OS salesmen
probably prayed would have never been published :-)

So does anybody knows RTOS(es) that do document and validate the
latencies on supported platforms?


D

On Sep 10, 2:27 pm, Stargazer <stargazer3...@gmail.com> wrote:
> Greetings, > > I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-) > > So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms?
I know that VxWorks has a testsuite that does measure a lot of things including task switches, etc. You can ask for the results for certain boards/processors, but you can also request the test suite to run it on your own hardware. Kind regards, Johan Borkhuis
Stargazer <stargazer3p14@gmail.com> wrote:
> Greetings, > > I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-) > > So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms?
RTA-OSEK from ETAS does. The "Binding Manual" provides full details of all latencies and WCETs in the OS - these are validated on a reference board, and the intention was that the Timing Characterisation Library that enabled these latencies to be measured should be available to customers to determine these figures on their own hardware. Disclaimer - I used to work there... pete -- pete@fenelon.com "how many clever men have called the sun a fool?"
On 2007-09-10, borkhuis@gmail.com <borkhuis@gmail.com> wrote:
> On Sep 10, 2:27 pm, Stargazer <stargazer3...@gmail.com> wrote: >> Greetings, >> >> I am wondering are there RTOSes that document interrupt and task >> switching latencies on supported platforms and provide with tools to >> validate that. I read some advertising hype from OS vendors on how >> small their latencies are, but there were no numbers. There were also >> several independent studies that got to numbers, which OS salesmen >> probably prayed would have never been published :-) >> >> So does anybody knows RTOS(es) that do document and validate the >> latencies on supported platforms? > > I know that VxWorks has a testsuite that does measure a lot of things > including task switches, etc.
As does eCos. -- Grant Edwards grante Yow! If I felt any more at SOPHISTICATED I would DIE visi.com of EMBARRASSMENT!
On Mon, 10 Sep 2007 05:27:44 -0700, Stargazer wrote:

> Greetings, > > I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-) > > So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms? >
I tend to be deeply suspicious of any self-reported OS timing measures, particularly latencies. Generally if I really need to know I instrument the code to toggle a pin when the task starts, then I look at the interrupt source (if it's external) on an oscilloscope and compare it to the task start. If I'm being really thorough I'll instrument the ISR, also. This gives me a trace that goes: _______________________________________ ___| ____|____________|_________________________ Note that if you have something that's _really_ time critical, and if you're blessed by it being simple, then you want to do it in the ISR, or even design it into hardware. Also, should latency be important, you need to be strict about protecting chunks of code -- the execution time of any block of code that's protected from interrupts adds directly to the interrupt latency. -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Stargazer wrote:

> I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-)
Those numbers are VERY dependent on the particular hardware/software, the mode of operation, the number of tasks and critical sections, wait states, DMAs, buses/caches/SDRAMs, etc. It is simple enough to figure out the latencies for the best case, however it is difficult if not impossible to account for the worst case.
> So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms?
You have to check it for yourself anyway, by toggling LEDs and tracing with a scope. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
"Stargazer" <stargazer3p14@gmail.com> wrote in message 
news:1189427264.040957.138540@g4g2000hsf.googlegroups.com...
> Greetings, > > I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-) > > So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms? >
Don't believe anything you read - measure it yourself. Be very careful as to exactly WHAT any measurements are actually measuring, and that the OS state is consistent across all your tests. I receive questionnaires occasionally from researchers, magazines, etc with reference to FreeRTOS.org. My favourite question is "What is the context switch time?". Last time I got this I wrote back and said, "what exactly to you want me to measure?" The answer came back "The time to switch from one task to another". I could not resist "On a PIC18 or a Pentium?" - to which they replied "whatever the average is". Enough said. Another of my favourite quotes from an advert "20% faster context switch time", with no reference to what. LOL. Another, "fastest boot time, only x micro seconds" - without saying what is performed during the boot. Its all bull. BTW, FreeRTOS.org does publish some numbers, but these are rather out of date. SafeRTOS has timing figures for nearly all operations (it has to for safety critical use), but this is only available in the purchased documentation or under NDA. -- Regards, Richard. + http://www.FreeRTOS.org 13 official architecture ports, 1000 downloads per week. + http://www.SafeRTOS.com Certified by T&#2013265948;V as meeting the requirements for safety related systems.
On Sep 10, 8:27 am, Stargazer <stargazer3...@gmail.com> wrote:
> Greetings, > > I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-) > > So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms? > > D
ThreadX, from Express Logic, also provides this information. They offer hard numbers on specific processors that they have tested with. There are also a number of compilers that support "OS Awareness" for ThreadX which allow you to perform your own timing analysis on your specific processor. Just remember that these numbers are only valid if you follow the rules for your specific processor and / or operating system.
Stargazer wrote:

> Greetings, > > I am wondering are there RTOSes that document interrupt and task > switching latencies on supported platforms and provide with tools to > validate that. I read some advertising hype from OS vendors on how > small their latencies are, but there were no numbers. There were also > several independent studies that got to numbers, which OS salesmen > probably prayed would have never been published :-) > > So does anybody knows RTOS(es) that do document and validate the > latencies on supported platforms?
I came across this recently http://www.innovasic.com/ Their Fido1100 seems to have a real-time focus, and they quote system jitter times, and MAX's, so you might get into there. -jg
> I receive questionnaires occasionally from researchers, magazines, etc wi=
th
> reference to FreeRTOS.org. My favourite question is "What is the context > switch time?". Last time I got this I wrote back and said, "what exactly=
to
> you want me to measure?" The answer came back "The time to switch from o=
ne
> task to another". I could not resist "On a PIC18 or a Pentium?" - to whi=
ch
> they replied "whatever the average is". Enough said.
"Average" is good, LOL. While the time a task switch takes on a given system might be of interest, this is only minor anyway. What really matters is the IRQ latency (the OP _did_ ask about it which is pretty untypical, looks like he knows what he is after). This is again platform dependent, of course, but if I were to buy a system (which I am not, I am rather selling one) I would like to know whether the scheduler unmasks the IRQ while switching tasks, are there system calls which keep the IRQ masked for longer than others etc. On top of that there is code like device drivers etc. low level stuff which can also mask the interrupts, and while these may or may not be part of an OS just one of the wrong "model" could kill the latency figure beyond usability... Dimiter ------------------------------------------------------ Dimiter Popoff Transgalactic Instruments http://www.tgi-sci.com ------------------------------------------------------ On Sep 10, 6:39 pm, "FreeRTOS.org" <noem...@address.com> wrote:
> "Stargazer" <stargazer3...@gmail.com> wrote in message > > news:1189427264.040957.138540@g4g2000hsf.googlegroups.com... > > > Greetings, > > > I am wondering are there RTOSes that document interrupt and task > > switching latencies on supported platforms and provide with tools to > > validate that. I read some advertising hype from OS vendors on how > > small their latencies are, but there were no numbers. There were also > > several independent studies that got to numbers, which OS salesmen > > probably prayed would have never been published :-) > > > So does anybody knows RTOS(es) that do document and validate the > > latencies on supported platforms? > > Don't believe anything you read - measure it yourself. Be very careful as > to exactly WHAT any measurements are actually measuring, and that the OS > state is consistent across all your tests. > > I receive questionnaires occasionally from researchers, magazines, etc wi=
th
> reference to FreeRTOS.org. My favourite question is "What is the context > switch time?". Last time I got this I wrote back and said, "what exactly=
to
> you want me to measure?" The answer came back "The time to switch from o=
ne
> task to another". I could not resist "On a PIC18 or a Pentium?" - to whi=
ch
> they replied "whatever the average is". Enough said. > > Another of my favourite quotes from an advert "20% faster context switch > time", with no reference to what. LOL. Another, "fastest boot time, onl=
y x
> micro seconds" - without saying what is performed during the boot. Its a=
ll
> bull. > > BTW, FreeRTOS.org does publish some numbers, but these are rather out of > date. SafeRTOS has timing figures for nearly all operations (it has to f=
or
> safety critical use), but this is only available in the purchased > documentation or under NDA. > > -- > Regards, > Richard. > > +http://www.FreeRTOS.org > 13 official architecture ports, 1000 downloads per week. > > +http://www.SafeRTOS.com > Certified by T=DCV as meeting the requirements for safety related systems.