I work in the automotive embedded C programming domain and someone asked me what's the OS tasks system of my project (AUTOSAR). I am working in static analysis using Astree tool chain. I didn't have to create OS configuration for my project as it's generated by the other teams. So what is tasks in our embedded system? And how it work with thousands of C and H files in large Software? Is it referring to how we build the project?
For example:
void task_OS_10ms_Task(void) {
Rte_task_Cont_BswStart_10ms();
FiM_MainFunction();
DDRC_Proc_ArDebMainFunction_10ms();
DFES_Proc_Data();
ComCIL_Adapt_Proc_10ms();
ComCIL_Co_Proc();
}
void task_OS_1ms_Task(void) {
Rte_task_Cont_BswStart_1ms();
Rte_task_Cont_BswEnd_1ms();
} // End of OS_1ms_Task
With above example, its mean 6 functions will be executed in total 10 ms or each one in 10 ms (total 60 ms)?
A Task is the smallest schedulable unit managed by the OS. It's equivalent to "Thread".
A Task is the object which executes (user) code and which is managed by the
OS. E.g. the OS switches between different Tasks (“schedules”). There are 2
types of Tasks; for more details see [15].
Basic Task: A Task which can not block by itself. This means that it can not
wait for (OS) event(s).
Extended Task: A Task which can block by itself and wait for (OS) event(s).
(Source: AUTOSAR_SWS_OS.pdf ver 4.2)
What you have inside a task are runnable entities or Main Processing Functions (Scheduled Functions) of BSW modules.
The 10ms that you have in the name of the task does not mean that this task will be executed every 10ms. The Period 10ms is part of Schedule Table which has to be defined in OS module. Keep in mind that AUTOSAR OS does not offer deadline monitoring for timing protection. This missing feature can be handled by WD stack.
In your example, All the functions has to be executed every 10ms. But sometimes we can map a runnanble (e.g 100ms) to a task 10ms, in this case the RTE will generate events/counters to be able to execute your runnable every 100ms.
The example you have here is simply talks about task enclosing some functions called cyclically. your task_os_10ms_task() is scheduled every 10ms by OS scheduler, and every function inside it also. Total execution time for the entire task would depend on what is your CPU frequency. It could be few ms or usecs but definitely not 60ms. Similarly, Your task_os_1ms_task() is going to be scheduled by os scheduler every 1ms. And, so are your RTE start and end function in it. Inside the task body it is simply a round robin execution. If your task is preemptive and a higher priority task is pending at then, os scheduler will. preempt your task and start the higher priority task. If not then, entire task is completed and control goes back to OS scheduler.
Related
I've started using a database at work that is based off SQL and Unix.
I am surprised to learn, that if someone requests for a change to be made to their details at around 5PM or a certain date, then the person who is allocated the incident then has to WAIT until 5pm and make the changes manually.
I'm surprised a button that says 'Apply changes later' does not exist, there is only a 'Save' button.
I have seen complicated solutions using Java on stackoverflow, but I am not familiar with UNIX or SQL, and googling brings no results.
Would it be a simple fix?
It wouldn't have to account for any time differences, and I'm assuming would just work off System clock; and I know Java has a calendar function that I assume works off the PC clock.
Java
Java does indeed have a sophisticated facility for scheduling a future task to be executed. See the ScheduledExecutorService class.
Rather than specify a date-time, you pass the schedule method a number of nanoseconds, or milliseconds, or seconds, or minutes, or hours, or days. You also pass a TimeUnit enum instance to indicate which granularity.
And, yes, Java depends on the host operating system for its clock to track the date-time.
Task Master
I suggest using your database to track the jobs to be run, in conjunction with Java. If using only Java, the scheduled jobs would exist only in memory and would disappear if the Java app exits or crashes.
Instead, the Java app on launch should check the database for any pending jobs, and schedule them with an executor. Each job on completion should mark the database "task master" table row as finished.
I have a business critical application which needs to run 24*7. Right now its scheduled using Windows Task Scheduler. The problem with current implementation is whenever the application stops it has to wait for 1 minute to run again.(Since one minute is the minimum time to repeat task in Windows Task Scheduler) So I am building my own task scheduler which will start the process(application) within 5 seconds of terminating the process. How should my task scheduler know if the process has terminated. Do I need to keep polling the process every second to check whether its running or not?
You should write your application as a Windows Service, not a standard application.
Among their other advantages, Windows Services give you the ability to define what happens in the event of a failure (e.g. restart application).
They are also very easy to create in C#.
The short version is I am looking for a way to prioritize certain tasks in SSIS 2005 control flows. That is I want to be able to set it up so that Task B does not start until Task A has started but Task B does not need to wait for Task A to complete. The goal is to reduce the amount of time where I have idle threads hanging around waiting for Task A to complete so that they can move onto Tasks C, D & E.
The issue I am dealing with is converting a data warehouse load from a linear job that calls a bunch of SPs to an SSIS package calling the same SPs but running multiple threads in parallel. So basically I have a bunch of Execute SQL Task and Sequence Container objects with Precedent Constraints mapping out the dependencies. So far no problems, things are working great and it cut our load time a bunch.
However I noticed that tasks with no downstream dependencies are commonly being sequenced before those that do have dependencies. This is causing a lot of idle time in certain spots that I would like to minimize.
For example: I have about 60 procs involved with this load, ~10 of them have no dependencies at all and can run at any time. Then I have another one with no upstream dependencies but almost every other task in the job is dependent on it. I would like to make sure that the task with the dependencies is running before I pick up any of the tasks with no dependencies. This is just one example, there are similar situations in other spots as well.
Any ideas?
I am late in updating over here but I also raised this issue over on the MSDN forums and we were able to devise a partial work around. See here for the full thread, or here for the feature request asking microsoft to give us a way to do this cleanly...
The short version is that you use a series of Boolean variables to control loops that act like roadblocks and prevent the flow from reaching the lower priority tasks until the higher priority items have started.
The steps involved are:
Declare a bool variable for each of the high priority tasks and default the values to false.
Create a pre-execute event for each of the high priority tasks.
In the pre-execute event create a script task which sets the appropriate bool to true.
At each choke point insert a for each loop that will loop while the appropriate bool(s) are false. (I have a script with a 1 second sleep inside each loop but it also works with empty loops.)
If done properly this gives you a tool where at each choke point the package has some number of high priority tasks ready to run and a blocking loop that keeps it from proceeding down the lower priority branches until said high priority items are running. Once all of the high priority tasks have been started the loop clears and allows any remaining threads to move on to lower priority tasks. Worst case is one thread sits in the loop while waiting for other threads to come along and pick up the high priority tasks.
The major drawback to this approach is the risk of deadlocking the package if you have too many blocking loops get queued up at the same time, or misread your dependencies and have loops waiting for tasks that never start. Careful analysis is needed to decide which items deserved higher priority and where exactly to insert the blocks.
I don't know any elegant ways to do this but my first shot would be something like this..
Sequence Container with the proc that has to run first. In that same sequence container put a script task that just waits 5-10 seconds or so before each of the 10 independent steps can run. Then chain the rest of the procs below that sequence container.
Would like to know how the scheduler gets called so that it can switch tasks. As in even if its preemptive scheduling or round robin scheduling - the scheduler should come in to picture to do any kind of task switching. Supposing a low priority task has an infinite loop - when does the scheduler intervene and switch to a higher priority task?
Query is:
1. Who calls the scheduler? [in VxWorks]
2. If it gets called at regular intervals - how is that mechanism implemented?
Thanks in advance.
--Ashwin
The simple answer is that vxWorks takes control through a hardware interrupt from the system timer that occurs continually at fixed intervals while the system is running.
Here's more detail:
When vxWorks starts, it configures your hardware to generate a timer interrupt every n milliseconds, where n is often 10 but completely depends on your hardware. The timer interval is generally set up by vxWorks in your Board Support Package (BSP) when it starts.
Every time the timer fires an interrupt, the system starts executing the timer interrupt handler. The timer interrupt handler is part of vxWorks, so now vxWorks has control. The first thing it does is save the CPU state (such as registers) into the Task Control Block (TCB) of the currently running task.
Then eventually vxWorks runs the scheduler to determine who runs next. To run a task, vxWorks copies the state of the task from its TCB into the machine registers, and after it does that the task has control of the CPU.
Bonus info:
vxWorks provides hooks into the task switching logic so you can have a function get called whenever your task gets preempted.
indiv provides a very good answer, but it is only partially accurate.
The actual working of the system is slightly more complex.
The scheduler can be executed as a result of either synchronous or asynchronous operations.
Synchronous refers to operations that are caused as a result of the code in the currently executing task. A prime example of this would be to take a semaphore (semTake).
If the semaphore is not available, the currently executing task will pend and no longer be available to execute. At this point, the scheduler will be invoked and determine the next task that should execute and will perform a context switch.
Asynchronous operations essentially refer to interrupts. Timer interrupts were very well described by indiv. However, a number of different elements could cause an interrupt to execute: network traffic, sensor, serial data, etc...
It is also good to remember that the timer interrupt does not necessarily cause a context switch! Yes, the interrupt will occur, and delayed task and the time slice counters will be decremented. However, if the time slice is not expired, or no higher priority task transitions from the pended to the ready state, then the scheduler will not actually be invoked, and you will return back to the original task, at the exact point where execution was interrupted.
Note that the scheduler does not have its own context; it is not a task. It is simply code that executes in whatever context it is invoked from. Either from the interrupt context (asynchronous) or from the invoking task context (synchronous).
Unless you have a majorily-customized target build, the scheduler is invoked by the Timer interrupt. Details are platform-specific, though.
The scheduler also gets invoked if current task gets completed or blocks.
I wish to develop an application in VB.NET to provide to following functionality and hope you can give me some pointers on which direction to take.
I need some kind of “server” type component which sits in the background monitoring request from users and performing various task. (this component can be install locally or centrally)
The users submit an instruction to the “server” to perform a certain task at a designated date and time. (or perform the task straight away)
The “server” would perform the task at the desired date and time and inform the user the result of the task.
I have thought of using a central database to which the user writes the instructions. The “server” could read from the database to obtain the instructions, and write the result back to the database.
I want a fast reaction to the instructions, so the “server” must poll the database every few seconds; I fear this may be detrimental to performance. Also how do I get the server to perform the task at the desired time?
Again checking all outstanding tasks against the current time is not very efficient, so I thought about utilising the Windows Scheduler, but I am not sure of the best way of integrating this functionality.
I would be grateful for any ideas, pointers or suggestions.
Have you looked at quartz.net? It's a scheduling framework which might be useful to you.
We have a similar system where we work, utilising a webservice to accept requests, run them when required, and notify callers with the results if necessary.
In our case the callers were other applications and not people.
The web service consisted of the following methods: (rough version, not exact)
int AddJob(string jobType, string input, datetime startTime) // schedules job and sets timer to call StartJobs when needed, and then returns job id
void GetResults(int jobId, out string status, out string output) // gets results (status="queued / running / completed / failed")
void StartJobs() //called via a timer as needed to kick off scheduled jobs
We also built in checks to limit how many jobs of could run simultaneously, and whether they could retry if they failed, and emails admins if any jobs fail the last attempt.
Our version is much more comprehensive than this, with the jobs actually being webservices themselves, supporting simultaneous running, built-in workflow so jobs can wait on others, but maybe it will give you some ideas. It's not a trivial project, but was fun to implement!