What are the options for real-time scheduling?
One of the saddest things I hear is, 'Well I used to play a lot of games but I just don't have time for them anymore.' In the list of priorities, video games didn't make the cut. In NT6+ (Vista+), use of the Vista Multimedia Class Scheduler is the proper way to achieve real-time operations in what is not a real-time OS. It works, for the most part, though is not perfect since the OS isn't designed for real-time operations. Microsoft considers this priority very dangerous, rightly so. Aug 21, 2020 The governor makes it clear fans at Seahawks games is not a state priority right now. Real-time updates and all local stories you want right in the palm of your hand. It means writing programs that need to perform under hard time constraints, often with extreme resource constraints such as battery life and limited embedded memory. Feb 27, 2018 Base process priority class. Individual thread priorities, offsets of the base priority class. An option to reset the class, some games automatically reset their state back to normal. Vista or newer Windows systems, use of the Multimedia Class Scheduler which is the proper way to achieve real-time operations in what is not a real-time OS. It works, for the most part, though is not perfect since the OS.
A number of scheduling concepts have been developed for implementation in a real-time operating system (RTOS). The most commonly encountered is the pre-emptive scheduler even though it is not inherently a real-time algorithm in contrast to, for example, deadline scheduling, which aims to ensure that critical threads are executed within a given timeframe.
Desktop operating systems are designed around the concept of fairness – that no application should be starved of processing cycles by another. These systems tend to use round-robin scheduling, in which each task will run for a set period of time before being forced to yield access to the processor so that execution can switch to a different task that is ready to run. Once all tasks that are not blocked from running have been allotted a timeslice, execution resumes with the first task and the cycle continues.
In a real-time system, it is generally acceptable to starve less important tasks of processor cycles if there are critical tasks with work to do – although determining how ‘unimportant’ a task really is can be problematic for guaranteeing overall system stability.
How does the typical scheduler operate?
The simplest possible scheduler conceptually is the
main()
loop – it simply cycles through a series of functions. As long as the critical functions execute within the maximum allowable processing latency of the system, the loop will provide satisfactory performance. However, every logical task within the system is provided with the same execution priority and will consume processor cycles even if they have no work to do. It becomes very difficult to guarantee that the loop will finish execution within the maximum allowable latency for all situations. Applications also become difficult to maintain beyond a certain size. At this point, it makes sense to break the application down into discrete tasks and use an RTOS scheduler to control their execution.A pre-emptive RTOS works on the basis that the task with the highest priority and which is ready to run will be the one that is scheduled for execution. Typically, the RTOS will examine the list of tasks after any change of task status – usually after a system call or an interrupt. For example, a task may relinquish control of a mutual-exclusion semaphore (mutex) on which a higher-priority task is blocked. The RTOS will note that the high-priority task is now ready to run and pick it for scheduling. That task will continue execution until it is replaced by a higher-priority task, yields the processor, or becomes blocked again. Because the task can remain running, it is possible that it could starve other tasks of execution time – a risk that system designers need to take into account. Conversely, the RTOS guarantees that the most critical thread that is ready to run will be able to access the processor as soon as it requires it.
What are the common pitfalls in scheduling?
In principle, it is possible to analyze a system for potential scheduling problems and to ensure that the system will meet its deadlines. However, the analysis is greatly complicated by any interprocessor communication. Basic rate-monotonic analysis, one of the earlier theories used for determining schedulability – and the subject of one of the 20 most commonly cited papers in computer science – can only guarantee schedulability for tasks that do not share resources. In practice, most systems demand shared access to memory objects and peripherals that make schedulability, as well as the tendency to deadlock, difficult to predict.
One problem encountered with conventional pre-emptive RTOS schedulers is that of priority inversion. In this situation, a low-priority task obtains access to a shared resource but is pre-empted by a higher priority task, blocking all other tasks that need that resource. If a critical task requires that resource, it cannot run until the low-priority task has released the mutex. But until activity has subsided far enough to allow the low-priority task to run, it will be unable to continue far enough to release the mutex. During this time, the effective priority of the critical task is reduced to that of the low-priority thread: hence priority inversion.
One workaround, although it can introduce other schedulability problems if implemented without safeguards, is to use the priority-inheritance protocol. This mode provides any thread that owns a mutex with the same priority as a more important task that is blocked on it until the semaphore is released.
Many RTOS implementations support priority inheritance or a close relative of the technique, the priority ceiling protocol, which prevents a low-priority task from being elevated to the highest possible priority in the system. There are dangers in using the protocol: designers need to ensure that a normally low-priority task will not simply hog a resource and keep running indefinitely in a state in which it cannot easily be pre-empted.
There also also subtleties in implementation. If an application is moved from a single-core to a dual-core processor that uses the priority-ceiling protocol, it cannot guarantee mutual exclusion. So a distributed priority ceiling protocol has to be used instead.
Because of the problems of analyzing schedulability in asynchronous, interrupt-driven real-time systems, many systems that have to guarantee dependable behaviour resort to some form of strict time-sharing. In this scenario, important tasks are guaranteed a number of cycles within a period of time to run, even though they have nothing to do, just in case they do need to respond to a problem. ARINC 653 avionics systems have used this approach for years and a number of automotive systems have adopted the Flexray architecture, which is based on a similar time-triggered approach.
Each partition in an ARINC 653 system has its own dedicated, protected memory space and each partition can run a multitasking system. Vital functions usually have dedicated partitions. Even with such rigidly enforced partitions, timing problems can still arise through interactions with hardware. One problem that has been identified in a paper by GE Aviation and Wind River Systems lies in the use of direct memory access (DMA). If a partition towards the end of its time-slice decides to initiate a long DMA transfer, the partition that runs immediately afterwards can stall because the DMA hardware has exclusive access to the memory bus – effectively shortening the new partition’s timeslice and creating the potential for it to miss its own deadline.
The recommendation in this case is to transfer the responsibility for setting up DMA transfers to a system-level task that takes into account the amount of time a partition has remaining before it is forced to relinquish the processor.
Similarly, interrupt handling can upset the operation of an otherwise strictly time-sliced system. A number of systems prevent all but the system timer, which is used to help manage scheduling, from being able to assert an interrupt. Others may record the interrupt and then allow the affected system to poll for the associated data when it next runs.
As you may already know, Windows shares your computer's CPU resources between running apps. How many resources will be given to a process is determined by its priority. The higher the priority level, the more resources will be allocated to the process. In this article, we will see how to set or change process priority in Windows 10.
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There are 6 priority levels available to processes in Windows, as follows:
- Low
- Below normal
- Normal
- Above normal
- High
- Realtime
Normal is the default level. Most apps start with this priority level and run without issues. The user can temporarily change process priority to speed up the app or slow it down and make it consume less resources. A new priority level applied to the app will take effect till the app's process terminates. Once you exit it, the next time it will open with the default priority level (Normal) unless the app itself has a setting to change its priority automatically.
Some apps can adjust their priority automatically. The popular WinRAR and 7-Zip archivers are able to raise its priority to 'Above normal' to speed up the archiving process. Or media players like Winamp can raise their process priority during playback.
Before you proceed, you need to know the following. The Realtime priority level is not meant to be set by the user. It can cause system instability. An application running at this priority can consume 100% of CPU and intercept keyboard and mouse input, making the PC unusable.
To change process priority in Windows 10, do the following.
- Open Task Manager.
- Switch it to the More details view if required using the 'More details' link in the bottom right corner.
- Switch to the Details tab.
- Right-click the desired process and select Set priority from the context menu. In the submenu drop down, select the desired priority level, for example, Above Normal.
- The following dialog will open:Confirm the operation and you are done.
There is a way to start a process with the desired priority. This can be done with a console command 'start', available in the classic command prompt (cmd.exe). Here is how.
How to start an app with a specific priority level
- Open a new command prompt instance.
- Type or copy-paste the following command:This will start Notepad with the Above Normal priority.
Replace the value with the desired priority level, for example, High or BelowNormal. Substitute the executable's path with the full path to the app you want to run.
Finally, using the console tool wmic, you can change an already running app's process priority level. This can be useful in various automation scripts.
How to change the app priority level using wmic
- Open a new command prompt instance.
- Type or copy-paste the following command:Replace the 'Process Name' portion with the actual name of the process, for example, 'notepad.exe'.
Replace the 'Priority Level' portion according to the next table:Priority Level Value Priority Level Name 256 Realtime 128 High 32768 Above normal 32 Normal 16384 Below normal 64 Low You can use Value or Name in the command. The following two examples do the same:
That's it.
Real Time Priority For Games Today
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