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Distributed operating systemsThe code placed around the system call to do the checking is called a jacket. Somewhat analogous to the problem of blocking system calls is the problem of page faults. If a thread causes a page fault, the kernel, not even knowing about the existence of threads, naturally blocks the entire process until the needed page has been fetched, even though other threads might be runnable. Another problem with user-level thread packages is that if a thread starts running, no other thread in that process will ever run unless the first thread voluntarily gives up the CPU. Within a single process, there are no clock interrupts, making round-robin scheduling impossible. Unless a thread enters the runtime system of its own free will, the scheduler will never get a chance. An area in which the absence of clock interrupts is crucial is synchronization. It is common in distributed applications for one thread to initiate an activity to which another thread must respond and then just sit in a tight loop testing whether the response has happened ...» |
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