CPU Clocks and Clock Interrupts, and Their Effects on Schedulers (2015)

https://accu.org/index.php/journals/2185

by TN1ck • 7 years ago

120 36 7 years ago

accu.org

ChuckMcM• 7 years ago

Something that anyone who writes embedded code should be acutely aware of!

Clocks -> threads -> processes -> schedulers -> storage -> interrupts -> networks. An OS curriculum in a nutshell.

sgillen• 7 years ago

Do you have any good/favorite textbooks (or other resources) to learn those from?

jepler• 7 years ago

If your interest is in waking at a certain time, you should use APIs like `clock_nanosleep(..., TIMER_ABSTIME)` (POSIX.1-2001) which let you say at what time you would prefer to be awakened, instead of APIs which just say for how long you would like to sleep. (looks like the C++ spelling is std::this_thread::sleep_until)

ianhowson• 7 years ago

... and you'll be woken up at some point after the time you requested, at which point you'll be referred back to the article to find out why.

ananonymoususer2• 7 years ago

"For the purposes of this discussion I’m going to assume a generic version of sleep() that accepts a parameter representing time in milliseconds. The concepts scale up or down with the scale of the parameter." Nice write-up, but no version of sleep() accepts a parameter representing time in milliseconds. Many embedded platforms have no sleep() function. Most recently, I had to implement it (and usleep) on MSP-430.

oralty3• 7 years ago

Oh dear, you sound quite confused.

https://linux.die.net/man/3/usleep https://pubs.opengroup.org/onlinepubs/9699919799/functions/n... Which is POSIX, which is supported by a number of OSes, such as QNX, Linux, BSDs, and countless others.

https://docs.microsoft.com/en-us/windows/desktop/api/synchap...

http://www.ertl.jp/ITRON/SPEC/FILE/mitron-400e.pdf See page 43 on "Standard Profile"

https://www.ee.ryerson.ca/~courses/ee8205/Data-Sheets/Tornad...

MSP-430 is not an OS, it is a microcontroller, so your comment makes absolutely no sense. Ironically one of the most popular open source MSP-430 OSes is FreeRTOS.. which of course has vTaskDelay.

Like name an OS (RTOS or otherwise) that does not have an API to sleep for millisecond periods? Not saying none exist, but, it's probably easier to name the exceptions.

com2kid3• 7 years ago

Like name an OS (RTOS or otherwise) that does not have an API to sleep for millisecond periods?

ooh! I worked on one.

Cooperative multithreading, you could schedule a task (callback function) to be called in 'n' hundreds of microseconds.

There was a scheduler that would drop the entire CPU to a super low power state if no tasks were scheduled to happen anytime soon (and set a HW timer to wake the CPU when more tasks were going to come due), but there was no "user land" API to sleep the entire system.

Sleeping the entire system when battery life is important (better to do as much work during each CPU wake cycle as possible, then drop back down to super low sleep for extended periods of time), and when other code needs to run, is rather rude, so we didn't allow it.

ananonymoususer2• 7 years ago

I am not confused, but perhaps you did not understand my meaning. There is no version of "sleep()" that accepts an argument in anything other than seconds. Of course Microsoft has "Sleep()" which does, and of course "usleep()" and "nanosleep()" do. Is this unclear?

zepearl6• 7 years ago

Reading the article made me think again about realtime software/hardware/OS, which I absolutely never understood.

One one hand Wikipedia says "Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines"." ( https://en.wikipedia.org/wiki/Real-time_computing )

On the other hand when I read Wikipedia's article "Real-time operating system" ( https://en.wikipedia.org/wiki/Real-time_operating_system ) my overall understanding is that there is no feature allowing a program/thread/function to enforce e.g. "I have to be executed within 3 milliseconds" nor that that would anyway be enforced by the OS at any cost (scheduler will prioritize higher-priority tasks).

Am I missing something?

Or maybe the statement "Real-time programs must guarantee response within specified time constraints" must be taken literally and the program/thread/function must be written in a way that when some timer expires (and a "terminate"-event is sent to it), it will stop doing whatever it's doing (and it will potentially send back some kind of (partial) result)?

As you can see, this theme confuses me :)

[EDIT: thank you guys! :) ]

marcosdumay2• 7 years ago

Real time OSes have an all or nothing response to real time tasks. They either take it to run on the specified time, or they fail with some error.

The OSes also take into their hands to enforce that the tasks will finish at the time they asked for. What means that they either terminate by themselves, or the OS will terminate them, again with some explicit error.

zepearl6• 7 years ago

Thx but well, this kind of goes frontally against the other 2 answers I got (I understood that nothing ever gets explicitly terminated by the OS and I felt happy with that approach as it kind of made sense to me, but some now strong doubts are coming back again...).

adrianratnapala2• 7 years ago

OSes and applications that get called real-time vary a lot.

But yes, when it is meant rigorously, you can definitely schedule a thing to run on exactly at particular times, preempting whatever lower priority thing was running before.

In my last embedded project that meant we had thing that got called every 64 microseconds -- and it was our job to ensure it finished in time for the next tick and also left enough CPU for all the other threads.

zepearl6• 7 years ago

Thx >>and it was our job to ensure it finished in time for the next tick and also left enough CPU for all the other threads

Then, for the realtime program to run correctly, it would involve as well an understanding of all other processes/programs that are running at potentially the same time (e.g. "competitors" and "collaborators") and/or with the same or higher priorities, etc.., right?

Therefore, candidate summarized definition:

realtime (HW?/)OSs allow you to execute SW which will run with no interruption, as long as no other SW having the same or higher level of priority interferes?

Does it sound right?

gizmo6863• 7 years ago

Generally real-time systems are realtime at every level. The idea is that you don't have a situation where you need to send a terminate event, because the program would have already terminated.

The point of a realtime OS is not to make your programs run in real-time, but to make it possible for your programs to run in real-time. This is simply not possible on a general OS like Linux, because the kernel can always to do something else while your program is running.

Consider the software that drives the printhead in your printer. The timing here is critical. If a command gets sent too early or too late, the print will not come out right.

You probably don't want the OS to be what tells you to send a command every hundred millisecond. Getting this timing right is a core feature of your application, all you need from the OS is to not get in the way of your getting the timing right.

If you are in a state where you need to send a partial result then the program computing the result is likely not really realtime.

zepearl6• 7 years ago

Thx >>The point of a realtime OS is not to make your programs run in real-time, but to make it possible for your programs to run in real-time.

So basically, if I program something and somehow set it to be "realtime" then I can be sure that it won't be suspended before it finishes whatever it is doing?

Such a condition cannot be kept indefinitely - soon or later there will be activities that will need the OS' attention (therefore my program will have to be suspended), no? (If yes then I have problems understanding the threshold...).

loeg• 7 years ago

nor that that would anyway be enforced by the OS at any cost (scheduler will prioritize higher-priority tasks).

When sorted against ordinary time-sharing program priorities, any real-time task is a higher priority.

pdkl95• 7 years ago

A practical example I once had to solve:

I was writing the BIOS/flash-loader/debugger code for a Z80 clone[1] we made. The compiler send instruction packets over the RS-232 serial port at 115200 kBd (kbaud). Using 8N1, this is 12800 B/s which means we have a deadline to read the current byte out of the serial port register ever 78us or data will be lost as the new incoming byte on the serial port is written to the port's IO register.

This sounded like a lot of time, until I was informed some of the boards we sold ran at (iirc) 128kHz. After accounting for memory access time and the runtime of relevant instructions, it worked out that we had at most a budget of 60-70 instructions/byte, which wasn't anywhere close to enough to process the byte (including a simple checksum). Even using a normal ISR wasn't possible: the usual register push/pop would have a major portion of our instruction budget. I ended up dedicating a set of registers to receiving the bytes in realtime into as buffer, while processing the data non-realtime using the remaining registers.

Depending on how strict your deadlines are, "realtime" can mean accounting for every clock cycle the CPU is running. An OS (realtime or otherwise) is for people with softer deadlines and CPU time to spare.

[1] https://en.wikipedia.org/wiki/Rabbit_2000

StillBored• 7 years ago

I must have missed any mention of tickless schedulers which seem to dominate modern systems because its more efficient both in power costs (allowing the cpu to go into deeper sleeps) and generally for performance/throughput tasks in a system with low CPU contention (because the scheduler never runs).

CPU Clocks and Clock Interrupts, and Their Effects on Schedulers (2015)

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