Friday, October 14, 2011

RTEMS Configuration and Resource Limits

This post started as an answer to a question from an ESA Summer of Code In Space student. She had hit one of the things that every person new to RTEMS hits at one point. She attempted to create a pthread mutex via pthread_mutex_init() during the package's initialization. It failed and returned EAGAIN. This was an especially surprising error since it happened in a C++ global constructor which was executed before the first task was entered. It appeared to her that RTEMS was not initialized or something even weirder was wrong. RTEMS was, in fact, initialized and since C++ global constructors are supposed to run before main(). On RTEMS, we run them in the context of the first user task which executes. Let me start with an obvious and decidedly unhelpful assertion:

RTEMS != GNU/ Linux

I am sure that didn't help at all to understand why she got an out of resources error. But it is a lead-in to try to explain the philosophical difference between RTEMS and GNU/Linux that leads us to this. The error she encountered was is in fact an out of resources error. Most people start programming on operating systems that do not discourage you from using dynamic allocation and do not put restrictive limits on the number of instances of an object class you can create. For example, on a GNU/Linux system, you don't worry about how many instances of an OS object you create. There are often limits but these are so high as to not present problems. But the Linux kernel does have some behavioural and limits configuration parameters available to the end user.  If you are curious about these, see /etc/sysctl.conf and sysctl(8).

In contrast, RTEMS is a member of a class of real-time operating systems that was designed for to target systems with safety and hard real-time requirements. There are often limited computing resources. In this design view, it is better to pre-allocate as much as possible so you don't have to deal with running out of resources at run-time. This makes the resulting system safer and less likely to have a weird failure mode in this situation. It is not uncommon for the entire set of tasks, semaphores, etc. to be well known and listed in the application design documentation.  Configuring the resources required is common in this environment. Moreover, it is not uncommon for malloc() to be forbidden after system initialization. There is nothing inherently right or wrong with either of these contrasting philosophies. They are just different approaches given different system requirements.

In RTEMS you configure the maximum number of each type of object you want. The defaults tend to be 0. Memory is reserved for RTEMS separate from the C Program Heap based upon your cofniguration requests. The sample in testsuites/samples/ticker has the following configuration in the file system.h:

#include /* for device driver prototypes */




The ticker application says it needs a console (used for stdio) and clock tick (time passage) device drivers. It may have a maximum of four concurrently instantiated Classic API tasks. It is using a Classic API style initialization task -- the alternative is a POSIX Threads initialization thread. And each of the tasks it is creating has a stack larger than the minimum required. It is assumed that each task requires only the minimum amount of stack space so we have to tell the RTEMS configuration to reserve some extra memory for those that are larger than minimum. You can look at the calls to rtems_task_create() in init.c for the calling parameters that indicate the desired stack size.

The hello world sample application is simpler. It doesn't need a Clock device driver and would only have one task. Your application will likely have a more complicated configuration than hello world but it doesn't need to specify any configuration parameters unless it requires that class of object to be supported.

Our young programmer simply missed defining CONFIGURE_MAXIMUM_POSIX_MUTEXES to however many is required. This is an RTEMS specific issue that is not done on non-embedded operating systems.

With all that background on the hard limit focus on RTEMS configuration, it is easy to forget that RTEMS also has "unlimited object creation mode" and "unified workspace" options. This is probably more useful for our intrepid programmer at this stage. These configuration options lets you specify that you want a potentially unlimited number of a class of objects and that you want the RTEMS Workspace and the C Program Heap to be the same pool of memory.

    rtems_resource_unlimited( 5 )
    rtems_resource_unlimited( 10 )

The above specifies that POSIX mutexes and condition variables are "unlimited" but the set is extended by five (5) instances of mutexes and ten (10) instances of condition variables at a time. When you create the sixth mutex instance, instances 6-10 will be added to the inactive pool for that object class.

The full set configuration macros are (hopefully) well documented in the Configuring a System chapter of the RTEMS User's Manual. This is a link to the appropriate section for RTEMS 4.10.1:

For normal application development, that's really about all there is to this issue. If you get an out of resources error, you will need to raise the limit. When looking inside the code, any time you see a NULL returned from _Objects_Allocate() fail, it is a maximum objects issue. If you see a task, thread or message queue create fail, then you are not accounting for variable memory like stack space or message buffers which must be allocated when the object instance is created.

Since our intrepid young lady is actually porting a software package, let me throw out another thought  which impacts this situation. There is likely a fixed base set of objects the package creates such as for global mutexes or message queues. A user of package X will create instances of objects that it defines. So if the package X has a macaroni object that requires a mutex, condition variable, and a message queue, then you can let the end user of package X know that for each macaroni instance, they need to reserve A, B, and C. For certain cases, like the tasking in Ada and Go, RTEMS provides higher level configuration helpers like CONFIGURE_MAXIMUM_ADA_TASKS to encapsulate this configuration information.

I know my answer to her was a over the top but I realized that she had run into something that many others hit when using RTEMS for the first time. I really wanted her, and now you, to understand this part of RTEMS. Figuring out how many of each kind of object when developing an application can be tough but figuring that same information out when porting a software package is can really be a challenge. Embedded developers focused on safe, reliable systems don't like surprises and using various techniques to avoid running out of resources at run-time is a big part of it.

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