4.3 Context SwitchingThe kernel switches among threads in an effort to share the CPU effectively; this activity is called context switching. When a thread executes for the duration of its time slice or when it blocks because it requires a resource that is currently unavailable, the kernel finds another thread to run and context switches to it. The system can also interrupt the currently executin..

The Process StructureIn addition to the references to the substructures, the process entry shown in Figure 4.1 contains the following categories of information: • Process identification: the PID and the parent PID • Signal state: signals pending delivery and summary of signal actions• Tracing: process tracing information• Timers: real-time timer and CPU-utilization counters  The process substruc..

4.2 Process StateEvery process in the system is assigned a unique identifier termed the process identifier (PID). PIDs are the common mechanism used by applications and by the kernel to reference processes. PIDs are used by applications when the latter send a signal to a process and when receiving the exit status from a deceased process. Two PIDs are of special importance to each process: the PI..

Part II: ProcessesChapter 4.1 Introduction to Process ManagementA process is a program in execution. (프로세스의 정의. 정확하게 따지자면, 프로그램과는 구분된다.)A process has an address space containing a mapping of its program’s object code and global variables. It also has a set of kernel resources that it can name and on which it can operate using system calls. These resources include its credentials, signal state, a..

3.8 Kernel Tracing FacilitiesOperating-system kernels are large and complex pieces of software, encompassing thousands of lines of mainly C code, organized into dozens of subsystems and including hundreds of device drivers. Understanding what is happening within the operating system while it is running is important not only to developers of the code but also to the much larger group of people wh..

3.7 Resource ServicesAll systems have limits imposed by their hardware architecture and configuration to ensure reasonable operation and to keep users from accidentally (or maliciously) creating resource shortages. (오오.. 모든 시스템은 한계를 가진다 by 안정성)At a minimum, the hardware limits must be imposed on processes that run on the system. It is usually desirable to limit processes further, below these har..

3.6 Timing ServicesThe kernel provides several different timing services to processes. These services include timers that run in real time and timers that run only while a process is executing.   Real TimeThe system’s time offset since January 1, 1970, Universal Coordinated Time (UTC), also known as the Epoch, is returned by the system call gettimeofday. (협정 세계시) Most modern processors (includin..

3.5 Memory-Management ServicesThe memory organization and layout associated with a FreeBSD process is shown in Figure 3.3. Each process begins execution with three memory segments: text, data, and stack. The data segment is divided into initialized data and uninitialized data (also known as bss). The text is read-only and is normally shared by all processes executing the file, whereas the data a..

3.4 Clock InterruptsThe system is driven by a clock that interrupts at regular intervals. Each interrupt is referred to as a tick. On the PC, the clock ticks 1000 times per second. At each tick, the system updates the current time of day as well as user-process and system timers.  Handling 1000 interrupts per second can be time consuming. To reduce the interrupt load, the kernel computes the num..

3.3 Traps and InterruptsTraps, like system calls, occur synchronously for a process. Traps normally occur because of unintentional errors, such as division by zero or indirection through an invalid pointer. The process becomes aware of the problem either by catching a signal or by being terminated. Traps can also occur because of a page fault, in which case the system makes the page available an..