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LWN.net Weekly Edition for May 23, 2013
An "enum" for Python 3
An unexpected perf feature
LWN.net Weekly Edition for May 16, 2013
A look at the PyPy 2.0 release
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 (subscriber, #15091)
Posted Nov 13, 2012 13:56 UTC (Tue) by KSteffensen (subscriber, #68295)
I think there's too much safety hysteria in the automotive business to actually let the cars be driven by a computer.
Posted Nov 13, 2012 14:07 UTC (Tue) by hummassa (subscriber, #307)
Posted Nov 13, 2012 14:37 UTC (Tue) by KSteffensen (subscriber, #68295)
Next big hurdle will be the debate the first time one of these is involved in an accident.
Posted Nov 13, 2012 14:45 UTC (Tue) by hummassa (subscriber, #307)
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).
Posted Nov 13, 2012 15:02 UTC (Tue) by KSteffensen (subscriber, #68295)
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.
Posted Nov 13, 2012 15:20 UTC (Tue) by rvfh (subscriber, #31018)
(disclaimer: I was born and have lived most of my life in Europe.)
Posted Nov 13, 2012 15:23 UTC (Tue) by niner (subscriber, #26151)
Posted Nov 13, 2012 15:26 UTC (Tue) by mpr22 (subscriber, #60784)
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).
Posted Nov 14, 2012 11:58 UTC (Wed) by anselm (subscriber, #2796)
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.
Posted Nov 21, 2012 19:02 UTC (Wed) by ceswiedler (subscriber, #24638)
Posted Nov 22, 2012 11:17 UTC (Thu) by Otus (guest, #67685)
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.)
Posted Nov 22, 2012 11:24 UTC (Thu) by mpr22 (subscriber, #60784)
Posted Nov 13, 2012 14:29 UTC (Tue) by Chris_Lesiak (subscriber, #4179)
Posted Nov 13, 2012 14:42 UTC (Tue) by man_ls (subscriber, #15091)
Posted Nov 13, 2012 21:48 UTC (Tue) by dlang (✭ supporter ✭, #313)
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.
Posted Nov 13, 2012 22:36 UTC (Tue) by man_ls (subscriber, #15091)
Posted Nov 13, 2012 22:58 UTC (Tue) by dlang (✭ supporter ✭, #313)
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.
Posted Nov 14, 2012 12:07 UTC (Wed) by anselm (subscriber, #2796)
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 …
Posted Nov 21, 2012 9:32 UTC (Wed) by mb (subscriber, #50428)
Posted Nov 21, 2012 9:40 UTC (Wed) by dlang (✭ supporter ✭, #313)
That page was big on the 'how to' but utterly lacking in the 'why go to this effort'
Posted Nov 22, 2012 13:35 UTC (Thu) by mb (subscriber, #50428)
> 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.
Posted Nov 13, 2012 14:29 UTC (Tue) by epa (subscriber, #39769)
Posted Nov 13, 2012 16:19 UTC (Tue) by gregkh (subscriber, #8)
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.
Posted Nov 15, 2012 11:34 UTC (Thu) by epa (subscriber, #39769)
Posted Nov 16, 2012 18:29 UTC (Fri) by jtc (subscriber, #6246)
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!]
Posted Nov 16, 2012 18:40 UTC (Fri) by dlang (✭ supporter ✭, #313)
LWN doesn't go after individual sponsors, they subscribe to an advertisement service that puts the ads in place based on their own criteria.
Posted Nov 16, 2012 19:12 UTC (Fri) by Trelane (subscriber, #56877)
At the Professional Hacker level or above:
Posted Nov 16, 2012 21:06 UTC (Fri) by jtc (subscriber, #6246)
Damn. I'm a "starving hacker". I'll have to upgrade once I find a job.
Thanks for the info.
Posted Nov 16, 2012 21:07 UTC (Fri) by Trelane (subscriber, #56877)
Posted Nov 13, 2012 16:41 UTC (Tue) by dashesy (subscriber, #74652)
BTW, one more example I can add is DAQs.
Posted Nov 19, 2012 3:20 UTC (Mon) by mathstuf (subscriber, #69389)
Posted Nov 19, 2012 15:36 UTC (Mon) by dashesy (subscriber, #74652)
Posted Nov 13, 2012 19:42 UTC (Tue) by tbird20d (subscriber, #1901)
Posted Nov 13, 2012 23:01 UTC (Tue) by iarenaza (subscriber, #4812)
Posted Nov 16, 2012 18:20 UTC (Fri) by jtc (subscriber, #6246)
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 (subscriber, #50141)
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