FAQ: Threading and JVM

[admin]

What "operating systems" are used with the aJile processors?

aJile processors have a microprogrammed real-time kernel that is internal to the processor. The microprogrammed kernel performs the traditional operating system functions (scheduling, context switching, interrupt preprocessing, error preprocessing, object synchronization). Java threads are native threads on the aJile processor. In addition, extended bytecodes are used to implement Java threading primitives such as sleep, wait, notify, notifyall, yield, monitor enter, monitor exit, and interrupt to provide extremely fast and atomic (non-interruptable) executive operations. These primitives are incorporated in the aJile Java runtime system such that the traditional operating system layer is not required.

[admin]

What type of scheduling is implemented in the aJile processors?

The microprogrammed kernel implements priority preemptive scheduling. The highest priority "ready" thread is always selected to run. Thread priorities can be manipulated via the getPriority() and setPriority() methods from the Java threading API.

[admin]

Do aJile processors support deterministic thread scheduling?

The aJile runtime system includes a periodic thread class to provide deterministic activation of threads. The periodic thread class is used to setup the priority and period of the thread. A periodic thread must programmed with an outer "infinite" loop containing an invoke to the cycle() method. The cycle() method simply suspends the periodic thread until the next activation time. A cyclic hardware timer drives the activation of periodic threads.

[admin]

How many priority levels do aJile processors support?

aJile processors support 32 priority levels. The (10) "Java" priorities are mapped to the even priorities from 2 to 20. The odd priorities from 3 to 21 are used for periodic threads and the priorities above 21 are used for high priority system threads such as the garbage collector.

[admin]

Does aJile provide any "deadlock" prevention protocols?

aJile processors support a priority ceiling protocol for synchronized objects. This is accomplished via assigning a synchronized object a "ceiling" priority equal to the priority of the highest priority thread that may access it (via the setCeiling() method). Any thread accessing the synchronized object will have its priority temporarily raised to the "ceiling" priority and returned to its original priority upon releasing it.

[admin]

Is there a limit on the number of threads?

The number of threads that can be created is determined by the available RAM memory for the thread object and its stack. The thread object is about 160 bytes and the stack size is 1K bytes by default. The stack size can be changed in the JEM builder tool.

[admin]

Is there a limit on the number of threads in threading queues?

There are no limits on the threading queues but there are caveats. Interrupts are disabled when threading queues are being manipulated by the kernel or threading instructions. Most threading queue operations are fast (microseconds or less) but inserting a thread in a large sleep queue (10s of threads) requires examining thread delay times starting from the shortest to the insertion point.

[admin]

How are threads queued for synchronized blocks

When multiple threads are waiting for a synchronized block, the highest priority thread is granted access when it becomes available.

[admin]

What type of garbage collector is available?

The aJile Java runtime system currently supports a mark and sweep garbage collector.

[admin]

Does aJile's implement the Real-Time Specification for Java (RTSJ)?

aJile's microprogrammed RTOS has most of the functionality specified in the RTSJ including a priority preemptive schedular, 32 priority levels, ceiling semaphore protocol, periodic threads, non collected heaps, etc. However, compliance to the RTSJ will require business initiatives to pursue.

[admin]

How is the JVM implemented in aJile processors?

aJile processors are an optimal implementation of the JVM since the Java bytecodes are the native instruction set. The native JVM bytecode implementation eliminates the typical interpreter or JIT software layers and provides the most efficient Java performance in both memory requirements and execution time.

[admin]

Which JVM bytecodes are implemented?

aJile processors implement the entire JVM bytecode instruction set. Additional memory access and threading instructions are defined using the extended bytecode mechanism.

[admin]

How are extended bytecode instructions generated from Java code?

The aJile runtime system defines "signatured" methods (rawJEM class) corresponding to the extended bytecodes. Utilizing the extended bytecode is simply a matter of using the corresponding method and its parameters. During build time, the aJile tool chain replaces invokes to signatured methods with the corresponding extended bytecode for the target load image.
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[admin]

How are peripherals accessed in Java?

The rawJEM class includes methods corresponding to "peek" and "poke" extended bytecodes to access memory mapped peripherals. The memory address is an integer parameter to these instructions and various data widths are available.

[admin]

I run my program, and interrupts are no longer serviced. What is going on?

Some thread is running at priority level 31 (interrupt level priority) and is hogging the processor. This has the effect of disabling all interrupts.