Symmetric multiprocessing and Microkernel


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Symmetric multiprocessing and Microkernel

  1. 1. Symmetric multiprocessing Symmetric multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single shared main memory, have full access to all I/O devices, and are controlled by a single OS instance, and in which all processors are treated equally, with none being reserved for special purposes. Most common multiprocessor systems today use an SMP architecture. In the case of multi-core processors, the SMP architecture applies to the cores, treating them as separate processors. In a symmetric multiprocessor (SMP), the kernel can execute on any processor, and typically each processor does self-scheduling from the pool of available processes or threads. The kernel can be constructed as multiple processes or multiple threads, allowing portions of the kernel to execute in parallel. The SMP approach complicates the OS. It must ensure that two processors do not choose the same process and that processes are not somehow lost from the queue. Techniques must be employed to resolve and synchronize claims to resources. An SMP operating system manages processor and other computer resources so that the user may view the system in the same fashion as a multiprogramming uniprocessor system. A user may construct applications that use multiple processes or multiple threads within processes without regard to whether a single processor or multiple processors will be available. Thus a multiprocessor OS must provide all the functionality of a multiprogramming system plus additional features to accommodate multiple processors. The key design issues include the following:
  2. 2. • Simultaneous concurrent processes or threads: Kernel routines need to be re-entrant to allow several processors to execute the same kernel code simultaneously. With multiple processors executing the same or different parts of the kernel, kernel tables and management structures must be managed properly to avoid deadlock or invalid operations. • Scheduling: Scheduling may be performed by any processor, so conflicts must be avoided. If kernel-level multithreading is used, then the opportunity exists to schedule multiple threads from the same process simultaneously on multiple processors. • Synchronization: With multiple active processes having potential access to shared address spaces or shared I/O resources, care must be taken to provide effective synchronization. • Memory management: Memory management on a multiprocessor must deal with all of the issues found on uniprocessor computers. In addition, the OS needs to exploit the available hardware parallelism, such as multiport memories, to achieve the best performance. The paging mechanisms on different processors must be coordinated to enforce consistency when several processors share a page or segment and to decide on page replacement. • Reliability and fault tolerance: The OS should provide graceful degradation in the face of processor failure. The scheduler and other portions of the OS must recognize the loss of a processor and restructure management tables accordingly. The advantages of SMP include a large global memory and better performance per Watt, important for SWaP (size, weight and power) sensitive applications thanks to the use of fewer memory controllers. Instead of splitting memory between multiple CPUs, SMP’s large global memory is accessible to all of the processor cores. Data intensive applications, such as image processing and data acquisition systems, often prefer large global memories that can be accessed at data rates up to 100s of Mbytes/sec. These large memory applications benefit from the single large memory common in most multi-core designs.
  3. 3. SMP also provides simpler node-to-node communication, and SMP applications can be programmed to be independent of node count. SMP especially lends itself to the use of new multi-core processor designs. Lastly, operating systems is that they perform load-balancing for the tasks between all available cores. In Performance, where more than one program executes at the same time, an SMP system will have considerably better performance than a uni-processor because different programs can run on different CPUs simultaneously.In cases where an SMP environment processes many jobs, administrators often experience a loss of hardware efficiency. Software programs have been developed to schedule jobs so that the processor utilization reaches its maximum potential. Good software packages can achieve this maximum potential by scheduling each CPU separately, as well as being able to integrate multiple SMP machines and clusters. Access to RAM is serialized; this and cache coherency issues causes performance to lag slightly behind the number of additional processors in the system. The disadvantages of SMP the fact that the memory latency and bandwidth of a given node can be affected by other nodes, and cache “thrashing” may occur in some applications.
  4. 4. Microkernel A Microkernel OS architecture A microkernel is a small OS core that provides the foundation for modular extensions. The term is somewhat fuzzy, however, and there are a number of questions about microkernels that are answered differently by different OS design teams. The microkernel approach was popularized by its use in the Mach OS, which is now the core of the Macintosh Mac OS X operating system. In theory, this approach provides a high degree of flexibility and modularity. A number of products now boast microkernel implementations, and this general design approach is likely to be seen in most of the personal computer, workstation, and server operating systems developed in the near future.
  5. 5. Less essential services and applications are built on the microkernel and execute in user mode. Although the dividing line between what is in and what is outside the microkernel varies from one design to the, next the common characteristic is that many services that traditionally have been part of the OS are now external subsystems that interact Swith the kernel and with each other; these include device drivers, file systems, virtual memory manager, windowing system, and security services. OS components external to the microkernel are implemented as server processes; these interact with each other on a peer basis, typically by means of messages passed through the microkernel. Thus, the microkernel functions as a message exchange: It validates messages, passes them between components, and grants access to hardware. The microkernel also performs a protection function; it prevents message passing unless exchange is allowed. For example, if an application wishes to open a file, it sends a message to the file system server. If it wishes to create a process or thread, it sends a message to the process server. Each of the servers can send messages to other servers and can invoke the primitive functions in the microkernel. This is a client/server architecture within a single computer. A number of advantages for the use of microkernels have been reported in the literature. These include: • Uniform interfaces – all services are provided by means of message passing. • Extensibility - allowing the addition of new services. • Flexibility – not only can new features be added to the OS, but also existing features can be subtracted to produce a smaller, more efficient implementation • Portability - Intel’s near monopoly of many segments of the computer platform market is unlikely to be sustained indefinitely. Thus, portability becomes an attractive feature of an OS. Changes needed to port the system to a new processor are changed in the microkernel and not in other services. • Adding a new service does not require modifying the kernel. • It is more secure as more operations are done in user mode than in kernel mode. • A simpler kernel design and functionality typically results in a more reliable operating system.
  6. 6. Reference: [1] Stallings, W. (2011), Operating Systems: Internals and Design Principles 7 th Ed., Prentice Hall International, Inc. [2] [3] Lecture Slides in ULearn [4] multiprocessing-649411.html
  7. 7. Reference: [1] Stallings, W. (2011), Operating Systems: Internals and Design Principles 7 th Ed., Prentice Hall International, Inc. [2] [3] Lecture Slides in ULearn [4] multiprocessing-649411.html