LCE: Realtime, present and future

Posted Nov 13, 2012 2:37 UTC (Tue) by bronson (subscriber, #4806)
Parent article: LCE: Realtime, present and future

What a journey! It's really good to hear that realtime is getting ready for primetime. (makes me feel old...)

Now I need to hunt around for a project that requires realtime.

Real time for what?

Posted Nov 13, 2012 11:13 UTC (Tue) by man_ls (guest, #15091) [Link] (35 responses)

Speaking of which, what kind of applications require real time these days? Linux audio used to be mentioned often, but these days it appears that CONFIG_PREEMPT is enough; is it?

Apart from that, I suppose that automation of critical functions is an obvious field of application, but is it really? Our favorite kernel might be used in cars but the real stuff, not IVI: driving the motor and such. And also avionics would require real time. But in a world where JavaScript is used to drive copters I am not so sure...

Real time for what?

Posted Nov 13, 2012 13:56 UTC (Tue) by KSteffensen (guest, #68295) [Link] (11 responses)

Stock markets would be my guess. Considering how much automated trading is taking place and how much money is involved I'm guessing this is a big market.

I think there's too much safety hysteria in the automotive business to actually let the cars be driven by a computer.

Real time for what?

Posted Nov 13, 2012 14:07 UTC (Tue) by hummassa (subscriber, #307) [Link] (10 responses)

Seeing as Google self-driving car is now street-legal in Nevada, I wouldn't bet on it.

http://abcnews.go.com/blogs/technology/2012/05/google-sel...

Real time for what?

Posted Nov 13, 2012 14:37 UTC (Tue) by KSteffensen (guest, #68295) [Link] (9 responses)

Cool! I did not know that.

Next big hurdle will be the debate the first time one of these is involved in an accident.

Real time for what?

Posted Nov 13, 2012 14:45 UTC (Tue) by hummassa (subscriber, #307) [Link] (8 responses)

Where have you been the last year? ;-)

They already ran about 300,000 miles, with only two accidents: one, the autonomous driving system was offline (i.e., the car was being driven by the human driver) and two, the car was rear-ended. Apparently, they are safer than me (I usually go some 40,000 miles between minor crashes and scrapes, and I have some 400,000 miles total in twenty years of driving, with ten incidents).

Real time for what?

Posted Nov 13, 2012 15:02 UTC (Tue) by KSteffensen (guest, #68295) [Link] (7 responses)

Let me qualify that. The first accident involving grave human injury and/or death.

I'm quite willing to believe that on the average these things are far more safe than human drivers since they so rarely have to text their girlfriends or fiddle with the radio or whatever. I do think they will have to prove their safety far more rigorously than the average human driver, though.

Real time for what?

Posted Nov 13, 2012 15:20 UTC (Tue) by rvfh (guest, #31018) [Link] (2 responses)

Real test for machine is in difficult situations, where humans may 'feel' that something is not quite right...
Also, I am unsure these cars would work in Europe with crazy drivers and difficult-to-compute timings at some crossings.

(disclaimer: I was born and have lived most of my life in Europe.)

Real time for what?

Posted Nov 13, 2012 15:23 UTC (Tue) by niner (subscriber, #26151) [Link]

Well they could hardly do worse than human drivers... speaking as a cyclist who has to endure and survive them every day.

Real time for what?

Posted Nov 13, 2012 15:26 UTC (Tue) by mpr22 (subscriber, #60784) [Link]

Heck, it'd probably have trouble in Boston, MA (which admittedly has one of the most European street layouts of any major city in the USA).

Real time for what?

Posted Nov 14, 2012 11:58 UTC (Wed) by anselm (subscriber, #2796) [Link] (3 responses)

I do think they will have to prove their safety far more rigorously than the average human driver, though.

I think a Turing-like test should suffice: If during a driving test, a driving license examiner cannot tell whether a computer or a person is driving the car, and it looks as if the entity in question ought to pass, then – if it was actually the computer driving – the setup is OK.

Real time for what?

Posted Nov 21, 2012 19:02 UTC (Wed) by ceswiedler (guest, #24638) [Link] (2 responses)

Or the driving test is wildly insufficient to distinguish bad drivers.

Real time for what?

Posted Nov 22, 2012 11:17 UTC (Thu) by Otus (subscriber, #67685) [Link]

> Or the driving test is wildly insufficient to distinguish bad drivers.

That's not the purpose of most driving tests. Usually the purpose is to see if someone is good enough that they'll learn the rest on their own without being too much of a danger to others.

Realistically most people who pass a driving test are going to be bad drivers for a long while. (Unless they are testing for a license in another state/country and have already driven a lot.)

Real time for what?

Posted Nov 22, 2012 11:24 UTC (Thu) by mpr22 (subscriber, #60784) [Link]

Naturally. There's a driving examiner in the front passenger seat; getting an accurate picture of someone's everyday driving in such conditions is more or less impossible.

Real time for what?

Posted Nov 13, 2012 14:29 UTC (Tue) by Chris_Lesiak (subscriber, #4179) [Link] (8 responses)

Here's an example: Linux CNC

Real time for what?

Posted Nov 13, 2012 14:42 UTC (Tue) by man_ls (guest, #15091) [Link] (4 responses)

Cool! Those look pretty critical to me: having catastrophic consequences if they fail -- at least causing damage to property, and probably to operators.

Real time for what?

Posted Nov 13, 2012 21:48 UTC (Tue) by dlang (guest, #313) [Link] (2 responses)

> Cool! Those look pretty critical to me: having catastrophic consequences if they fail -- at least causing damage to property, and probably to operators.

I have built a CNC machine, so I have first-hand experience here.

It depends how the system fails.

If they fail by not scheduling for 100ms (or even 1s), there is probably no problem besides delaying the work.

If they fail by sending the wrong commands out to the equipment, things can be worse.

But these do not require real-time scheduling to operate.

Real time for what?

Posted Nov 13, 2012 22:36 UTC (Tue) by man_ls (guest, #15091) [Link] (1 responses)

I was thinking more along the lines of plasma cutters or robots, also mentioned in the link: failing to switch off the plasma may have bad consequences. Or a cutting machine which needs precise synchronization between different pieces. But perhaps most of these machines are designed to be fail-safe.

Real time for what?

Posted Nov 13, 2012 22:58 UTC (Tue) by dlang (guest, #313) [Link]

failing to shut off the plasma cutter has very little effect (other than possibly a small amount of damage to the piece being cut), if you go off the edge, the plasma isn't going to be sustained, if you leave the cutter in place, the plasma will eat a small amount around the location until the circuit can't be sustained any longer and the plasma will cut off.

These things rely far less on precise timing than you think.

Just about all of the DIY devices rely on stepper motors, which move a specific distance when pulsed, not normal motors run for a specific amount of time.

If the pulses are late, things move a little slower. If there is enough momentum in the system, it's possible for that momentum to cause the equivalent of 'jumping a tooth on a gear' and being slightly out of position, the solution to this problem is to slow the machine down a bit.

synchronization between different pieces is a matter of either moving one motor, then a different motor, then the first one again, or in setting up the movement for both motors and sending a 'move now' pulse. In either case, slight delays don't break anything.

Real time for what?

Posted Nov 14, 2012 12:07 UTC (Wed) by anselm (subscriber, #2796) [Link]

Some time ago I consulted for a leading machine tool manufacturer here in Germany for a day or two and, after my actual work was done, was invited to take a guided tour of the shop floor. I was surprised to find out that they had these huge CNC machines attached to Linux-based controllers. I asked the guy who was showing me around whether they were using real-time Linux, and the answer was no, the normal generic Linux kernel worked fine as far as they were concerned. Considering that they are selling these gadgets to clients all over the world, and that any failure would probably cost them dearly, that put things into perspective for me …

Real time for what?

Posted Nov 21, 2012 9:32 UTC (Wed) by mb (subscriber, #50428) [Link] (2 responses)

There is a subproject for LinuxCNC Preempt-RT support:
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Debian_Wheezy_Li...

Real time for what?

Posted Nov 21, 2012 9:40 UTC (Wed) by dlang (guest, #313) [Link] (1 responses)

doing so and posting benchamrk numbers showing latency doesn't matter, what would matter is if there is anything showing that this makes a difference in the part being machined.

That page was big on the 'how to' but utterly lacking in the 'why go to this effort'

Real time for what?

Posted Nov 22, 2012 13:35 UTC (Thu) by mb (subscriber, #50428) [Link]

> doing so and posting benchamrk numbers showing latency doesn't matter

Ehm what?

> what would matter is if there is anything showing that this makes a difference in the part being machined.

If it would make a difference to the machined parts, either the previous RTAI based implementation or the new Linux-RT-preempt based implementation would be seriously broken.

> That page was big on the 'how to' but utterly lacking in the 'why go to this effort'

We do this, because it simplifies the software a lot and gets rid of all those ugly RTAI kernel modules.

Real time for what?

Posted Nov 13, 2012 14:29 UTC (Tue) by epa (subscriber, #39769) [Link] (10 responses)

My guess would be systems which are real-time but not safety-critical. In principle, Linux is too complex for use in systems where a bug can cause deaths. In practice I daresay there will be those using it anyway.

Real time for what?

Posted Nov 13, 2012 16:19 UTC (Tue) by gregkh (subscriber, #8) [Link] (6 responses)

Hm, like MRI machines? Air traffic control systems? Airplane flight navigation? Coal fired power generators? Windmill controls? Yacht stabilizers? Spacecraft control systems? Laser welding robots? High-speed milling machines?

I could go on and on. Linux is used for lots of things like this, and has been for years (something like over 85% of the US power production is controlled directly by Linux machines). A lot of these systems use the real-time patches, and others don't, but to think that Linux isn't being used in these type of situations is wrong.

Real time for what?

Posted Nov 15, 2012 11:34 UTC (Thu) by epa (subscriber, #39769) [Link] (5 responses)

Hmm, you're right. I would draw a distinction between the software running on the air traffic controller's desktop (which has often been Unix for decades now) and the microcontroller which controls the air intake to the engines. The former has fairly relaxed real-time requirements (the display has to update, but once a second might be acceptable) and I would call it 'soft safety-critical'. The software running on your doctor's desktop PC could kill you if it displays the wrong notes and causes the doctor to prescribe the wrong medicine - yet this is not normally an application considered safety-critical where special software methods must be used to guarantee correctness. By contrast, a failure of the jet engine microcontroller (assuming it has one, I am speculating) can cause instant disaster.

Real time for what?

Posted Nov 16, 2012 18:29 UTC (Fri) by jtc (guest, #6246) [Link] (4 responses)

" The software running on your doctor's desktop PC could kill you if it displays the wrong notes and causes the doctor to prescribe the wrong medicine - yet this is not normally an application considered safety-critical where special software methods must be used to guarantee correctness."

It's also requires neither a soft nor hard real-time kernel. :-)

[On a complete tangent: Is anyone else getting completely sick of that constantly changing Perforce ad. that adorns the top and upper right side of almost every page on LWN. I find it quite distracting and have to move another window to cover the right side of the page each time I open a new lwn page, so that I don't have the ad. screaming at me all the time while I'm reading! They've been running this ad. for many weeks - it's about time the found a new sponsor. Argghh!]

Real time for what?

Posted Nov 16, 2012 18:40 UTC (Fri) by dlang (guest, #313) [Link] (3 responses)

I believe that as a subscriber you can disable advertisements.

LWN doesn't go after individual sponsors, they subscribe to an advertisement service that puts the ads in place based on their own criteria.

Real time for what?

Posted Nov 16, 2012 19:12 UTC (Fri) by Trelane (subscriber, #56877) [Link] (2 responses)

> I believe that as a subscriber you can disable advertisements.

At the Professional Hacker level or above:
https://lwn.net/op/FAQ.lwn#subs

Real time for what?

Posted Nov 16, 2012 21:06 UTC (Fri) by jtc (guest, #6246) [Link] (1 responses)

"At the Professional Hacker level or above:"

Damn. I'm a "starving hacker". I'll have to upgrade once I find a job.

Thanks for the info.

Real time for what?

Posted Nov 16, 2012 21:07 UTC (Fri) by Trelane (subscriber, #56877) [Link]

NP. Glad you found it informative. :)

Real time for what?

Posted Nov 13, 2012 16:41 UTC (Tue) by dashesy (guest, #74652) [Link] (2 responses)

Well not if they are audited for a very narrow use case on dedicated long-life hardware, and fixed on a certain Kernel version without updates and with minimal applications (i.e. instruments).

BTW, one more example I can add is DAQs.

Real time for what?

Posted Nov 19, 2012 3:20 UTC (Mon) by mathstuf (subscriber, #69389) [Link] (1 responses)

I assume that "DAQ" is meant to be "DAW" (digital audio workstation)?

Real time for what?

Posted Nov 19, 2012 15:36 UTC (Mon) by dashesy (guest, #74652) [Link]

Actually I meant Data Acquisition (DAQ) with additional output channels.

Real time for what?

Posted Nov 13, 2012 19:42 UTC (Tue) by tbird20d (subscriber, #1901) [Link]

At Sony we're using Linux realtime in digital cameras and camcorders, for control of hardware (things like autofocus).

Real time for what?

Posted Nov 13, 2012 23:01 UTC (Tue) by iarenaza (subscriber, #4812) [Link]

I know of at least one Spanish company building protection, control and measurement equipment for electrical grids that use real time linux.

Real time for what?

Posted Nov 16, 2012 18:20 UTC (Fri) by jtc (guest, #6246) [Link]

"what kind of applications require real time these days?"

If Gleixner, when he quoted those download statistics ("About 30% of those went to European corporations, 25% to American corporations, 20% to Asian corporations, 5% to academic institutions"...), had also disclosed who those corporations were, this probably would give a good real-world answer to your question - or at least to the question: "What kinds of applications is the Linux realtime kernel being used or considered for these days?". Has he released this info? I suspect not - I suppose he's justifiably paranoid about being sued for breach of privacy or something like that.

" Linux audio used to be mentioned often, but these days it appears that CONFIG_PREEMPT is enough; is it?"

I can't answer that, but I will speculate that since music and MIDI recording and playback requires hard realtime capability, any companies considering using Linux for this would tend to favor the realtime kernel over the mainline kernel, since although the mainline kernel, perhaps, does fine x% of the time, the realtime kernel is likely (or certain, perhaps) to do fine y% of the time, where y% is significantly greater than x%.

LCE: Realtime, present and future

Posted Nov 16, 2012 10:41 UTC (Fri) by dmk (guest, #50141) [Link]

Try robotics, where you want to find out where your machine is located out of sensors (gyro, wheelticks). If you have a constant or small variance delay in the measurements, you can estimate it and compensate for it. but if you have high variance in the delay your position estimate will suffer.