RTOS has unfair scheduling i.e scheduling is based on priority. GPOS has fair scheduling i.e it can be adjusted dynamically for optimized. well, in GPOS the code is not modular i.e. developer can not choose Kernel modules selectively. GPOSes are not scalable. but in the case of RTOS Kernel code. The main difference between GPOS and RTOS is that the RTOS should be deterministic. That is, the time consumed by the operating system to.
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Google directs to this page a lot. So I am rewriting this post so that most of you can benefit. I had read a lots of articles to compose this material. There is not short-cut for a good answer.
If you want to really know the topic spend mins to read and understand it. I have tried to go from layman terms to more technical stuff. Are ROTSs really fast? Many Embedded interviewer ask this question. And most naive answer you could come up with is ‘ROTS rtps fast’.
Well, never use these words. More appropriate answer would be ROTS are deterministic. RTOS gauratee you that particular operation would complete at the worst this much time. This is the very basic criteria of being a RTOS.
Discuss to Learn: Difference between RTOS and GPOS (General Purpose Operating Systems)
Its all about money, if you can save even 25 cents on one embedded device hardware, and embedded devices are sold in millions of units say memory card Companies can make millions of dollars. This ensurers the fairness with which programs are executed.
But it gives no gaurntee that the high priroirty thread will be given preference to the lower priority one.
In some cases the OS may decay the priority or dyanamically adjust of the thread in order to achive fairness. It does not have any upper bound. In General, the more the number of threads the more time GPOS takes to schedule and start executing the the thread.
In highly time constraints RTOS system this delay could be devise. In RTOS however if high priority process is ready to run it will start executing ‘very soon’. All it tells is, the Algorithms gppos ROTS kernel should be deterministic and should be able to perform even if no of resources are dfiference. Consequently, a high-priority user gpis can never preempt a kernel call, but must wait for the entier call to complete, even if the call was invoked by the lowerest priority proecess.
In ROTS other hand, kernel opreations are preemptive. It means low priority task will be preemted even if its executing any system call.
What is difference between RTOS and GPOS?
There would be some delays some times, but a carefully designed RTOS will have those xifference very small. And one more important point, even for these delatils the upper bound of delay time would be well defined. To achieve this goal, the RTOS kernel must be simple and as elegant as possible.
Anf services with a short execution path should be included in the kernel itself. Any operations that require significant work for instance, process loading must be assigned to external processes or threads. Such an approach helps ensure that there is an upper bound on the longest nonpreemptible code path through the kernel. Moreover, a preemptive kernel does not address other conditions that can impose unbounded latencies, such as the loss of priority information that occurs when a client invokes a driver or other system service d Priority Inversion Problem This is problem which can arrive in preemptive priority based scheduling.
RTOS must handle this.
Google it, Mars Path finder robot has this problem. How RTOS are deterministic? As doctor in movie ‘I, Robot’ says to Will Smith “Now, that’s the right question” Preemption rtod very important criteria which I explained earlier, Kernel should have enough preemptive points from where it gpis return.
GPOS are usually not preemptive. So from where other Latency Comes? In between a high priority thread comes, If your kernel is non-preemptive until system call finishes your high priority thread would not get to execute.
So preemptive kernel is must. If kernel is preemptive you can determine the worst gps where High priority thread would start execution. This something could be a ‘sensor input’ or a key press.
What’s The Difference Between A RTOS And A GPOS?
Normally this ‘something’ is hardwired to processor with a line. Bettween means, say temperature sensor says, initiate the cooling process.
It dump a signal to the processor. Normally, in OS terminology, we call this kind of signals interrupt. After a interrupt occurs following action are taken by OS a Interrupt intiated b A interrupt handler is found c Interrupt is handled d Makes the task runnable e Task is scheduled f Task, yeah dears actual RTOS task runs here.
How fast is your interrupt handler. In this ypos of interrupt handling all other interrupts are masked. This may cause jitters in RTOS env. Normally you can miss interrupts during this kind. Disable only current interrupt. That’s kind of recommended approach. To sum up interrupt latency comes from 1. Disabling of interrupts spin lock etc 2.
Bad driver using Fast interrupt mode 3. Process the high priority interrupt first. So RTOS interrupt handling mechanism should take of above points. Creation of new address space for each task and managing it takes time.
Its a different topic alltogether. Those posts will tell more detailed concepts of RTOS.
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