Mention and explain types of multiprocessor operating systemmention and explain types of multiprocessor operating system
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8.1.2 Multiprocessor Operating System Types
Let us now turn from multiprocessor hardware to multiprocessor software, in particular, multiprocessor operating systems. Various organizations are possible. Below we will study three of them.
Each CPU Has Its Own Operating System
The simplest possible way to organize a multiprocessor operating system is to statically divide memory into as many partitions as there are CPUs and give each CPU its own private memory and its own private copy of the operating system. In effect, the n CPUs then operate as n independent computers. One obvious optimization is to allow all the CPUs to share the operating system code and make private copies of only the data, as shown in Fig. 8-6.
Figure 8-6 Partitioning multiprocessor memory among four CPUs, but sharing a single copy of the operating system code. The boxes marked Data are the operating system's private data for each CPU.
This scheme is still better than having n separate computers since it allows all the machines to share a set of disks and other I/O devices, and it also allows the memory to be shared flexibly. For example, if one day an unusually large program has to be run, one of the CPUs can be allocated an extra large portion of memory for the duration of that program. In addition, processes can efficiently communicate with one another by having, say a producer be able to write data into memory and have a consumer fetch it from the place the producer wrote it.
Still, from an operating systems' perspective, having each CPU have its own operating system is as primitive as it gets.
It is worth explicitly mentioning four aspects of this design that may not be obvious. First, when a process makes a system call, the system call is caught and handled on its own CPU using the data structures in that operating system's tables.
Second, since each operating system has its own tables, it also has its own set of processes that it schedules by itself. There is no sharing of processes. If a user logs into CPU 1, all of his processes run on CPU 1. As a consequence, it can happen that CPU 1 is idle while CPU 2 is loaded with work.
Third, there is no sharing of pages. It can happen that CPU 1 has pages to spare while CPU 2 is paging continuously. There is no way for CPU 2 to borrow some pages from CPU 1 since the memory allocation is fixed.
Fourth, and worst, if the operating system maintains a buffer cache of recently used disk blocks, each operating system does this independently of the other ones. Thus it can happen that a certain disk block is present and dirty in multiple buffer caches at the same time, leading to inconsistent results. The only way to avoid this problem is to eliminate the buffer caches. Doing so is not hard, but it hurts performance considerably.