RTOS vs GPOS : Priority inversion

All right. Now, let's discuss about the difference between RTOS and GPOS in terms of priority inversion. Let's say you are going out with your car and we will assume you are car as a lower priority vehicle, enters the busy traffic intersection and you may be thinking that you can easily cross the intersection. Now, let's see unfortunately all these vehicles come to a sudden stop and you are struck right in the middle of the intersection of the roads. Suddenly, you here the siren of an ambulance which is heading to a hospital over this line. The ambulance is of course a higher priority vehicle, isn't it? Now, It cannot cross this intersection because unfortunately you cannot move, that is you are struck right now. So, the ambulance now has to wait until you move out of its way. That means, a higher priority vehicle is waiting for lower priority vehicle to exit. That is exactly what priority inversion is the higher priority vehicle behaving like it is a lower priority and a lower priority vehicle is behaving like a higher priority. So, this is the case of priority inversion. Now, let's apply this into computing word. Let's say, your car is lower priority task, and these taxis are medium priority, and this ambulance is higher priority task. And this intersection area is a shared resource. Because this is common for both the roads, isn't it? Now, let's say lower priority task has acquired the key to access this shared area.But it is not allowed to run on the CPU because, lots of medium priority tasks are running on the CPU which won't allow lower priority task to take the CPU. Now, when the higher priority task enters it cannot access this shared area, isn't it? Because, key is taken by the lower priority task. But lower priority task is not allowed to run. It can only give up the key once it is allowed to run, isn't it? So, until the lower priority task runs key will be locked for this shared resource and higher priority task cannot access it. So, the higher priority task has to wait until lower priority task releases the key, causing the in version in priority. In GPOS, this is not at all a major issue.So, but in RTOS, it will definitely lead to issues because, you are blocking the higher priority task to execute. But most of real time kernel a use some techniques to solve this issue. So, RTOS can create paths on the fly which allows the higher priority tasks to by pass a lower priority task, or that we call as rescheduling or there could be other techniques like temporarily making, this as a higher priority task and this as a lower priority task,so that giving chance for the lowest priority task to run and release the key. So, all these techniques RTOS uses to at least to minimize the priority inversion problem. So, priority inversion is not at all a problem in general purpose OS. But this will surely cause a problem in real time operating system. So, what are the features that a RTOS has got but a GPOS Doesn't? The first point I would like to mention is the real time operating system has got the Priority based Preemptive scheduling. That means, it always favors the highest priority task in the system. All right. But a GPOS cannot act like this, if it does than it will kill the throughput of the system. So, GPOS usually go for those scheduling techniques which increase the throughput of the system.And the next point is you will not find a very long Critical section code segments where the interrupt as well as the preemption of the system is disabled, in order to protect the critical section. You learn more about protecting critical section later in this course. Don't worry about that. But, in the case of GPOS this may not be the case. And in RTOS avoiding of priority inversion is a must. Otherwise that may lead to the failure of the system. But in the case of GPOS priority inversion problem doesn't add significant problem. And RTOS always has bounded interrupt latency as well as bounded scheduling latency. But in the case of the GPOS the interrupt latency and task scheduling latency may vary significantly due to complexity of the system as well as due to the increase in System load as time elapses.