Книга: Distributed operating systems

9.7.3. Processes

9.7.3. Processes

All three systems support processes with multiple threads per process. Again in all three cases, the threads are managed and scheduled by the kernel, although a user-level threads packages can be built on top of them. Amoeba does not provide user control over thread scheduling, whereas Mach and Chorus allows processes to set the priorities and policies of their threads in software. Mach provides more elaborate multiprocessor support than the other two.

In Amoeba and Chorus, synchronization between threads is done by mutexes and semaphores. In Mach it is done by mutexes and condition variables. Amoeba and Mach support some form of glocal variables. Chorus uses software registers to define each thread's private context.

Amoeba, Mach, and Chorus all work on multiprocessors, but they differ on how they deal with threads on these machines. In Amoeba, all the threads of a process run on the same CPU, in pseudoparallel, timeshared by the kernel. In Mach processes have fine-grained control over which threads run on which CPUs (using the processor set concept). Consequently, the threads of the same process can run in parallel on different CPUs.

In Amoeba, a similar effect can be achieved by having several single-threaded processes run on different CPUs and share a common address space. Nevertheless, it is clear that the Mach designers have devoted more attention to multiprocessors than have the Amoeba designers.

Chorus supports having multiple threads within a single process running on different CPUs at the same time, but this is handled entirely by the operating system. There are no user-visible primitives for managing thread-to-processor allocation, but there will be in the future.

However, the Amoeba designers have put more work into supporting load balancing and heterogeneity. When the Amoeba shell starts a process, it asks the run server to find it the CPU with the lightest workload. Unless the user has specified a specific architecture, the process may be started on any architecture for which a binary is available, with the user not even being aware which kind has been selected. This scheme is designed to spread the workload over as many machines as possible all the time.

In Mach, processes are normally started on the user's home machine. Only when explicitly requested to do so by the user are processes run remotely on idle workstations, and even then they have to be evicted quickly if the workstation's owner touches the keyboard. This difference relates to the fundamental difference between the processor pool model and the workstation model.

Chorus allows a process to be started on any machine. The UNIX emulation provides a way to set the default site.

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