Pi vs BeagleBone-Black

Industrial minimodule using these AM335x PRUs described in "TQMa335: The Cortex A8 module with PRU support", and more detail of same.

Morgaine Dinova wrote:

 

Gary Stewart wrote:

 

You really need to get better acquainted with real time Linux.

 

And you really need to stop attacking other forum members with personal remarks like this that don't advance the argument. Focus on the message, not the person delivering it.

And you really need to stop attacking other forum members with personal remarks like this that don't advance the argument. Focus on the message, not the person delivering it.

Original message:

The Unix model of operating systems just isn't designed for realtime programming, and no matter what realtime patches you apply or which realtime scheduler you enable (I've played with both), you're never going to get anything like the low response latency and absence of jitter that you can get from the cheapest of microcontrollers.  A far better approach is therefore to combine a bare-metal micro with a *nix system, and let the latter control the former with high-level commands which the micro then executes with very tight timing constraints.

That was an attack on the message. I can't help if you take it personally. As far as that and your far better approach goes see the remaining responses.

I agree, 100us is pretty poor response latency for most realtime requirements, and the jitter isn't even quantified.

It is still, and forever will be dependent on the application (of course) and there are a lot of applications where 100 uS works just fine whether you want to admit it or not. As far as

jitter qualification goes we are not doing a tutorial on real time we are discussing whether real time Linux works (depending on the applications of course). There are many companies

that find real time Linux perfect for their needs. There are a few big time RTOS companies, QNX and LynuxWorks are two that come to mind, that offer a full feature RTOS

approach and not your "far better approach" when is not better (depending on the application of course). There are quite a few full feature free RTOS's that do to. Perhaps you

should go tell all of them that they are wrong. I'm sure they would value your opinion on that subject.

Here's an interesting quantitive study comparing the performance of ordinary SuSE with realtime SuSE.

The study is on a real-time Linux for enterprise servers using 8 core Opterons, not a typical real time application (and with different needs than the typical real time applications),

not your typical real time hardware (boy is that an understatement), and certainly not typical of real time applications discussed here. Enterprise servers anyone ?

Hard realtime response doesn't just seek to reduce response latency, but also its jitter.  What's more, a common goal is to reduce jitter to effectively zero ("effectively zero" in this case typically meaning sub-clock period) by using edge detection on input and running a fixed set of instructions to deliver a fixed-latency response.

This was a general discussion of using real time Linux. Hard real time was mentioned but it is not the specific subject. The specific subject is people like you and others saying real time Linux doesn't work

despite lots of evidence that says otherwise (and taking it personally when somebody points this out, see I told you I'd get back to that). The statement about jitter in hard real time is correct. While the goal

is to reduce jitter to effective zero, like most goals in the real world it can not always be achieved. For the record there are commercial products that use hard real time Linux (depending on the application of

course) so it seems that they have tamed the jitter monster enough to make it work anyway. That's called engineering. I'm sure they would like to hear from you to set them strait on why it doesn't really work.

Sorry about the massive redundancy but that simple statement just keeps flying over some peoples heads.

Gary Stewart wrote:

 

Original message:

 

The Unix model of operating systems just isn't designed for realtime programming, and no matter what realtime patches you apply or which realtime scheduler you enable (I've played with both), you're never going to get anything like the low response latency and absence of jitter that you can get from the cheapest of microcontrollers.  A far better approach is therefore to combine a bare-metal micro with a *nix system, and let the latter control the former with high-level commands which the micro then executes with very tight timing constraints.

 

That was an attack on the message. I can't help if you take it personally. As far as that and your far better approach goes see the remaining responses

You are deliberately misreferencing.  Your personal attack was not the comment on the above, but the comment below:

Gary Stewart wrote:

 

You really need to get better acquainted with real time Linux.

That has nothing to do with the subject matter, and everything to do with attacking the messenger.  Don't do it.

And then because your earlier personal attack wasn't enough, you did it again:

Gary Stewart wrote:

 

Perhaps you should go tell all of them that they are wrong. I'm sure they would value your opinion on that subject.

and again

Gary Stewart wrote:

 

I'm sure they would like to hear from you to set them strait on why it doesn't really work.

Just stop it.  Talk about the technical topic, don't criticise fellow forum members when you reply to them, it adds nothing to the discussion.

Your technical comments are welcome, and interesting.  If you disagree with an observation, just say you disagree and provide contrary evidence.  You don't need to attack the messenger as part of your rebuttal.  It's not a valid form of engineering discussion.

This is in reply to the technical subject matter, which is interesting.

Gary Stewart wrote:

 

      Morgaine wrote:

 

      I agree, 100us is pretty poor response latency for most realtime requirements, and the jitter isn't even quantified.

 

It is still, and forever will be dependent on the application (of course) and there are a lot of applications where 100 uS works just fine whether you want to admit it or not. As far as

jitter qualification goes we are not doing a tutorial on real time we are discussing whether real time Linux works (depending on the applications of course). There are many companies

that find real time Linux perfect for their needs. There are a few big time RTOS companies, QNX and LynuxWorks are two that come to mind, that offer a full feature RTOS

approach and not your "far better approach" when is not better (depending on the application of course). There are quite a few full feature free RTOS's that do to.

I agree, it is always dependent on the realtime requirements of an application, and a poorly performing realtime system may well be good enough for an application that has only weak realtime requirements.  There are undoubtedly many applications for which the 100us or so latency and unspecified jitter of realtime Linux is perfectly adequate, or at least acceptable.  I even use it myself for MIDI work (I use Linux for everything, realtime for MIDI musician apps), and it is on the verge of being acceptable if one isn't very demanding.  One could never call it great though, as the hand-ear-brain system easily notices the poor latency.

We're not doing a tutorial, but we are discussing the technical merits of different approaches to achieving realtime performance in an embedded Linux system, and there is not the slightest doubt that the realtime performance of realtime Linux is very poor compared to that of any old bare metal embedded microcontroller.

That's not in any doubt because it's not a matter of opinion but a matter of measurement.  You seem to be disputing the quantitive study that I linked because it used high-end machinery, but high-end machinery will almost always yield better performance in a realtime operating system simply because everything runs faster, so I can't agree with your dismissal of that study.  On a low-end Linux system, naturally the performance would have been even worse.

Morgaine.

I wonder why I gave up on the Pi forums.

It used to be so much more peaceful here.....

Mark

I noticed your interesting blog post about evaluating the Freescale Freedom-KL25z board. 

That's a very nice little Cortex-M0+ microcontroller and a well-featured board, especially for frugal low power standalone applications.  Have you tried linking it to the Pi for I/O expansion and better realtime response?  The very low price would seem to make it quite a good companion for the Pi, and your "easy to use" comments were encouraging, inasmuch as the barrier for budding engineers to get to grips with microcontroller programming would be quite low.

Morgaine Dinova wrote:

 

Gary Stewart wrote:

 

Original message:

 

The Unix model of operating systems just isn't designed for realtime programming, and no matter what realtime patches you apply or which realtime scheduler you enable (I've played with both), you're never going to get anything like the low response latency and absence of jitter that you can get from the cheapest of microcontrollers.  A far better approach is therefore to combine a bare-metal micro with a *nix system, and let the latter control the former with high-level commands which the micro then executes with very tight timing constraints.

 

That was an attack on the message. I can't help if you take it personally. As far as that and your far better approach goes see the remaining responses

 

You are deliberately misreferencing.  Your personal attack was not the comment on the above, but the comment below:

 

Gary Stewart wrote:

 

You really need to get better acquainted with real time Linux.

 

That has nothing to do with the subject matter, and everything to do with attacking the messenger.  Don't do it.

 

And then because your earlier personal attack wasn't enough, you did it again:

 

Gary Stewart wrote:

 

Perhaps you should go tell all of them that they are wrong. I'm sure they would value your opinion on that subject.

 

and again

 

Gary Stewart wrote:

 

I'm sure they would like to hear from you to set them strait on why it doesn't really work.

 

Just stop it.  Talk about the technical topic, don't criticise fellow forum members when you reply to them, it adds nothing to the discussion.

 

Your technical comments are welcome, and interesting.  If you disagree with an observation, just say you disagree and provide contrary evidence.  You don't need to attack the messenger as part of your rebuttal.  It's not a valid form of engineering discussion.

  You are deliberately misreferencing.

No, you are. The statement:

  The Unix model of operating systems just isn't designed for realtime programming, ...

the "attack":

You really need to get better acquainted with real time Linux.

Pretty sure that is a response to the message not an attack on the messenger and directly related to that statement, subject matter or not.

  Just stop it.  Talk about the technical topic, don't criticise fellow forum members when you reply to them, it adds nothing to the discussion.

Does your regular use of fanbois or Raspberry Pi fanbois in completely non-technical posts of yours count as criticizing fellow forum members ?

We're not doing a tutorial, but we are discussing the technical merits of different approaches to achieving realtime performance in an embedded Linux system, and there is not the slightest doubt that the realtime performance of realtime Linux is very poor compared to that of any old bare metal embedded microcontroller.

Not sure how discussing hard real time jitter relates to discussing the performance of real time Linux compared to any old bare metal processor. Seems like it would be more appropriate if we were discussing real time Linux performance verses

any other full feature RTOS performance (apples to apples).

Your technical comments are welcome, and interesting.  If you disagree with an observation, just say you disagree and provide contrary evidence.

I do, and you do your best to ignore it. Well until now at least.

I agree, it is always dependent on the realtime requirements of an application, and a poorly performing realtime system may well be good enough for an application that has only weak realtime requirements.  There are undoubtedly many applications for which the 100us or so latency and unspecified jitter of realtime Linux is perfectly adequate, or at least acceptable.

OK, thanks ? But excuse me if I add that the quality of the performance is in the eye of the people building the application and the performance of the application itself not what negative spin you

want to put on it. Your continual use of damming with faint praise, adding "poorly performing", "weak realtime requirements", and "at least acceptable" to your sentences are classic examples,

clearly show bias on your part. You keep using absolute terms (always negative) to describe relative qualities (for the dependent part). The poor performance you speak of is prettymuch in line

with most commercially available full feature RTOS's given equivalent hardware and the weak real time requirements are common to a lot of applications. I'm not sure how, technically speaking of

course this makes them weak and I'm sure that the commercial RTOS people would hotly dispute the description of their RTOS as poorly performing.

We're not doing a tutorial, but we are discussing the technical merits of different approaches to achieving realtime performance in an embedded Linux system, and there is not the slightest doubt that the realtime performance of realtime Linux is very poor compared to that of any old bare metal embedded microcontroller.

Not sure how discussing hard real time jitter in Linux relates to discussing the performance of real time Linux compared to any old bare metal processor. Seems like it would be more appropriate if we were

discussing hard real time Linux performance verses any other full fearture RTOS hard real time performance (apples to apples).

You seem to be disputing the quantitive study that I linked because it used high-end machinery, but high-end machinery will almost always yield better performance in a realtime operating system simply because everything runs faster, so I can't agree with your dismissal of that study.

No, I clearly disputed the study because it was for enterprise class servers which is not even close to a typical real time application, and it did not use hardware that was even close to typical hardware. I now add that it was not a

hard real time test even though you used it as an example for explaining hard real time jitter problems and the test results for one particular version of non-hard real time Linux optimised for the enterprise server environment does

not tell me anything about others that are optimised for more general hard real time and real time applications.

Systems that require hard real time behavour usually also require high reliability. If I instruct my robotarm to move to coordinate x, y, I appreciate it if he actually stops at that position all the time. One way to obtain high reliability is to keep things as simple as possible which is exactly what a separate controller approach is standing for. The more complex a design, the bigger the possibillities that it contains bugs.

I know there are other cases. Nobody is saying embedded linux and posix isn't having a place on the market, but it simply isn't a swiss knife for evey real time embedded design.

If we would be discussing this on a meeting, I would say "it's about time to move to the next point on the agenda. We are diverting way to far from the original topic"

Maybe our real time response is simply 2 slow.

I agree Mark.  This thread has lost focus. 

It WAS about a comparison of the Raspberry Pi to BeagleBone Black and how we'll use each.  Now we have gotten down to Real Time OS and FPGA like abilities.  95% of the market is going to use BOTH boards for hobbies and prototyping.  I was one of the first people to say my preferences (RPi Model A for small stuff because of power consumption and BBB for large projects because it has more of the features I'm looking for like IO.)

I would say lets fork the BBB technical talk to another thread (maybe in the BeagleBone forum) because that seems to all this is now.

Morgaine

Not yet, as I'm waiting for the headers to arrive. Silly me expected jaycar to carry them, but no...

I did order 3 more to go with the ione E14 sent. How could you resist.?

An expansion board is lacking as well, which is a shame as the number of I/O make this better priced than a Mega.

I have a cunning plan, but need to sit down and discuss it with someone for doing the boards and then selling them to make them very affordable.

If you've been following the top members forum, you'll also see that currently mbed.org is my best option for programming it, and someone has kindly done half a library that allows the existing arduino sketch to be used.

I've ordered another Pi interface so eventually it will get connected, or a normal arduino and this used as a remote using the built-in accelerometer and touch slider.

Cheers

Mark