Processes and Threads
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| Why do we have processes? | they support the ability to concurrent operations even with only 1 cpu. (modern computers could not exist without them) |
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| What is a process? | A process is a series of steps and decisions involved in the way work is completed. it is also just an instance of an executing program, including the current values of the program counter, registers, and variables. |
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| What is the process model? | is a model used to make it much easier to think about what is going on inside the system. All the runnable software on the computer, sometimes including the operating system, is organized into a number of sequential processes. |
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| What is multiprogramming? | parallel processing in which several programs are run at the same time on a uniprocessor |
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| What is a daemon | Processes that stay in the background to handle some activity such as email, Web pages, news, printing, and so on |
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| What is a multiprocessor | A multiprocessor is a computer system with two or more central processing units (CPUs), with each one sharing the common main memory as well as the peripherals. |
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| Why create a new process? | Creating new processes is particularly useful when the work to be done can easily be formulated in terms of several related, but otherwise independent interacting processes | if a large amount of data is being fetched over a network for subsequent processing, it may be convenient
to create one process to fetch the data and put them in a shared buffer while
a second process removes the data items and processes the
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| What happens after a process is created in unix and windows? | the parent and child have their own distinct address spaces. |
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| what is copy-on-write? | that whenever either of two processes wants to modify part of the memory, that chunk of memory is explicitly copied first to make sure the modification occurs in a private memory area |
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| What is process termination? | Sooner or later the new process will terminate, usually due to one of the following conditions: 1. normal exit (voluntary) 2. error exit (voluntary) 3. fatal error (involuntary) 4. killed by other process (involuntary) |
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| what is normal process exit (voluntary)? | the compiler executes a system call to tell the operating system that it is finished. programs always have an icon or menu item that the user can click to tell the process to remove any temporary files it has open and then terminate |
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| what is process error exit? (voluntary)? | if a program tries to compile and it doensn't exist |
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| what is fatal process error? (involuntary)? | Error caused by the process, often due to a program bug. Examples include executing an illegal instruction, referencing nonexistent memory, or dividing by zero | a process
can tell the operating system that it wishes to handle certain errors itself, in which
case the process is signaled (interrupted) instead of terminated when one of the errors
occurs.
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| killed by another process (involuntary)? | some process is telling the OS to kill some other process |
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| What is a handle? | in windows the parent process is given a special token (handle) that it uses to control the child. since there is no concept of process hierachy in windows |
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| What is process states? | there are three primary states a process can be in: 1. Running (actually using the cpu at that instant) 2. Ready (runnable or temporarily stopped to let another process run) 3. blocked (unable to run until som external event happens) |
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| what transitions is between the states? | 1. process blocks for input 2. scheduler picks up another process 3. scheduler picks this process 4. input becomes available |
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| What triggers transition 1? | when the OS discovers that a process can't continue right now. |
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| what triggers transition 2? | It occurs when the process scheduler decides that the running process has run long enough, and it is time to let another process have some CPU time | the process scheduler decides what runs and for how long.
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| What triggers transition 3? | It occurs when all the other processes have had their fair share and it is time for the first process to get the CPU to run again |
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| What triggers transition 4? | It occurs when the external event for which a process was waiting (such as the arrival of some input) happens. | If no other process is running at that
instant, transition 3 will be triggered and the process will start running. Otherwise
it may have to wait in ready state for a little while until the CPU is available and its
turn comes.
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| What happens when a disk interrupt is going on? | the system makes a decision to stop running the current process and run the disk process, which was blocked waiting for that interrupt |
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| What is process creation? | Four principal events create and terminate processes as needed during operation: 1. System initialization 2. Execution of a process-creation system call by a running process 3. A user request to create a new process 4. initiation of a batch job |
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