The advances made in multi-core technology and associated middleware allow developers to combine the best principles of multi-processing, virtualisation, real-time and hard partitioning to create a highly optimised execution environment for embedded applications. Here we look at the technology impact of multi-core processors on operating system and application software design.

Of late, innovation in processor architecture has been focused on creating multi-core processors. These multi-core processors introduce two or more processing cores in a single chip, thereby giving operating systems and applications access to increased computing power.

One of the significant advantages of these multi-core processors is the additional computing resources without any significant increase in size and weight; previous generations of multiTextprocessing configurations involved two or more physical chips that required additional real estate on processor boards.

The immediate benefits are obvious; applications that were designed around uni-processor configurations can replace uni-processor cores with dual or quad core processors. The computing power of these configurations increases dramatically with no appreciable change in its physical configuration.

The software impact of multi-core processors is fairly immediate on operating systems design. The OS has to adapt to support symmetric multi-processing (SMP) or asymmetric multi-processing (AMP), two major approaches for support of multi-core processors.

The operating system design has to adapt in the areas of scheduling, interrupt handling, synchronisation and load balancing. Application programs can also be affected by multi-core processors based on the ability of the OS to provide fine-grained control of process scheduling to applications.

For example, an application can make a request to execute on a specific processor core only. However, the increase in compute power through multi-core processors can be better harnessed through another recent trend in OS design, namely virtualisation.

Virtualize with binary compatibility layers
Virtualisation is a technique used to create an execution environment for software that is similar to the one it was originally designed for, but on a different hardware or operating system. It can be achieved usually at two levels; OS virtualisation and hardware virtualisation.

Operating System virtualisation is done using binary compatibility layers that run on heterogeneous operating system environments, while presenting an interface similar to the original OS environment. This is most often done to achieve migration and execution of applications across multiple heterogeneous operating system environments. For example, the ability to run Windows applications on Linux uses a virtualisation technique that simulates the behaviour of the Windows operating system on Linux.

Hardware virtualisation involves the emulation of the underlying hardware capabilities to allow operating systems themselves to run in a hardware environment different from its original environment. Software programs that emulate the underlying hardware capabilities are called virtual machines (VM) or virtual machine monitors (VMM).

A VM abstracts the capabilities of hardware and makes it available in environ-ments different from the original hardware. Some of the well known virtual machines are VMware, which emulates a standard Intel x86 PC architecture on a Macintosh environment, and the Java Virtual Machine (JVM), that emulates a specialised byte-code for a pseudo-processor.

Hardware virtualisation can also be extended to allow multiple heterogeneous operating systems to execute on single physical machine. The ample computing resources of modern multi-core processors make this extension possible. However, these multiple instances of heterogeneous operating systems need to execute in a resource isolated environment, with no functional impact to other instances of operating systems. This is essential since they will be sharing computing resources.

Hardware virtualisation for an OS
Enabling multiple instances of heterogeneous operating systems on a single machine involves solving technical challenges in virtualisation and resource isolation, while retaining complete binary compatibility and acceptable level of performance.

Virtualising multiple instances of an operating system can be done using either full virtualisation or partial virtualisation. The virtual machine in either case virtualises the hardware to provide the illusion of real hardware for the operating systems executing on this virtual machine. However, both full and partial virtualisations have some key differences in their overall architecture, leading to a different set of trade-offs.

Full virtualisation of the underlying hardware requires virtualising all the capabilities of the processor and board. This involves complex manipulations of memory management and privilege levels that are computationally intensive on commodity processors.

This leads to performance overheads that are much higher than the non-virtualised versions of the OS. However, the biggest benefit of full virtualisation is to allow operating systems to run unmodified, although at the cost of a significant performance overhead.

Figure 1: Virtualized OS architecture on a multi-core processor

Partial or para-virtualisation is usually a technique where the underlying hardware is not completely simulated in software. This architecture allows commodity operating systems to be easily virtualised on commodity processors, although with the requirement that the virtualised operating system requires code modifications to adhere to the partially virtualised architecture. However, the performance of partially virtualised architectures is much better than the fully virtualised machines, usually within a few percent of the non-virtualised versions.

The other key requirement for running multiple operating systems in the context of a virtual machine is the ability to isolate the physical resources of a computer. This is achieved by time-space partitioning, a concept used extensively in safety-critical and secure systems. In a time-space partitioned system, the virtual machine sub-divides two key computing resources: CPU time and physical memory.

The physical memory is divided into unique, non-overlapping ranges, and assigned to individual heterogeneous virtualised operating systems. The time scheduler allocates periods of CPU time to each virtualised OS that is usually fixed and cyclic. This gives the illusion of exclusive access to computing resources for the virtualised operating systems. The ability of the virtual machine to support time-space partitioning is a basic prerequisite for the execution of multiple virtualised operating systems on a single machine.

Both full and partial virtualisations support 100% binary compatibility with the stand-alone version of the operating system. It also allows the ability to retain the benefits of multiple address spaces within a single operating system instance.

One significant difference between a stand-alone operating system and a virtualised version is that the virtualised OS runs in a less privileged mode (user mode). This is necessary since the virtual machine that provides the virtualised architecture is the sole entity that is running at highest privileged level (supervisor mode). Figure 1 above shows the generic architecture supporting multiple heterogeneous operating systems running on a virtual machine.