Asymmetric multiprocessing (AMP)

Asymmetric multiprocessing provides an execution environment that's similar to conventional uniprocessor systems. It offers a relatively straightforward path for porting legacy code and provides a direct mechanism for controlling how the CPUs are used. In most cases, it lets you work with standard debugging tools and techniques.

AMP can be:

QNX Neutrino's distributed programming model lets you make the best use of the multiple CPUs in a homogeneous environment. Applications running on one CPU can communicate transparently with applications and system services (e.g., device drivers, protocol stacks) on other CPUs, without the high CPU utilization imposed by traditional forms of interprocessor communication.

In heterogeneous systems, you must either implement a proprietary communications scheme or choose two OSs that share a common infrastructure (likely IP based) for interprocessor communications. To help avoid resource conflicts, the OSs should also provide standardized mechanisms for accessing shared hardware components.

With AMP, you decide how the shared hardware resources used by applications are divided up between the CPUs. Normally, this resource allocation occurs statically during boot time and includes physical memory allocation, peripheral usage, and interrupt handling. While the system could allocate the resources dynamically, doing so would entail complex coordination between the CPUs.

In an AMP system, a process always runs on the same CPU, even when other CPUs run idle. As a result, one CPU can end up being under- or overutilized. To address the problem, the system could allow applications to migrate dynamically from CPU to another. Doing so, however, can involve complex checkpointing of state information or a possible service interruption as the application is stopped on one CPU and restarted on another. Also, such migration is difficult, if not impossible, if the CPUs run different OSs.