Chapter 2 Process & Thread
- The average operating system will be running many processes, even when it has only just booted
- Processes are programs in execution
- The difference is subtle, but absolutely crucial.
- a computer scientist wants to bake a birthday cake for his daughter
- He has a birthday cake recipe and all the input: flour, eggs, sugar, extract of vanilla, and so on.
- The recipe is the program, the computer scientist is the processor (CPU),and the cake ingredients are the input data.
- The process is the activity consisting of our baker reading the recipe, fetching the ingredients, and baking the cake.
- processes are a logical resource
- The operating system ties together several physical resources and presents them as a new resource referred to a processes
- The concept of a process is supported by hardware however
- A process is the activity of a program on a CPU, also called Task
- Physical Concurrency = parallelism
- need Multiple CPU
- Logical Concurrency = Time-shared CPU
- Dynamic
- Concurrency
- Independent
- Asynchronous
- Programs
- Data
- PCB
- One CPU/core can run one process at a time
- The CPU/core is then shared between all processes to give the appearance they are all running simultaneously
- This is done many times per second
- This is known as a context switch
- A hardware concept that defines all of the memory that can be accessed/used by program code
- Switching from one process to another means changing what memory can be accessed
- One process can’t access the memory of another process
- Hence the term context switch when giving the CPU to another process
- Preempts（抢占） (breaks) the currently executing process and saves its state
- Changes the hardware context to match the next process to be run
- Restores the state of the next process and resume its execution
- The operating system executes in kernel mode
- When a process is given the CPU, an instruction is issued to the CPU to switch it to user mode
- There is no instruction to switch the CPU back to kernel mode!
- Only interrupts cause the CPU to return to kernel mode
- Upon receiving an interrupt, the CPU suspends the current execution, switches to kernel mode, and invokes a pre-configured（预配置的）kernel routine（程序） to handle the interrupt
- Hardware interrupts
- Example: a disk completing a requested read/write operation
- Software interrupts
- Example: “traps” used to implement system calls
- The timer device sends an interrupt to the CPU periodically
- The operating system determines how regular these interrupts occur when initializing the hardware at boot time
- Running
- Ready
- Blocked
- Ready Suspend
- Block Suspend
- New
- Exit
- The operating system kernel tracks which resources belong to which process by storing the information in a data structure known as the Process Control Block (PCB)
- Process identification number (PID)
- Program counter and register values (when process is not running)
- Memory information (information to rebuild the hardware context)
- Related processes (parent PID, child PIDs, etc.)
- Accounting information (owner, run time, etc.)
- Links to other resource data structures (open files, network connections, etc.)
- Clock Interrupt
- I/O Interrupt
- Memory Fault
- An operation between the processes.
- When the dispatcher to recover the current process and dispatch CPU to another ready process, the operation will be referenced.
- An operation in one process.
- When the process image calls a system calls provided by kernel subsystems, the operation will be referenced.
- Submission of a batch job
- User logs on
- Created to provide a service such as printing
- Process creates another process
- Descriptopn of Memory Requirement
- Text Region
- Data Region
- Allocate and initialize a PCB structure, including initial register values
- Allocate at least three memory regions:
- Text: Contains the code loaded from the executable file
- Data/heap
- Stack region: Used for intermediate results, local variables, function call parameters etc. (also for system calls), etc.
- Connect standard input/output streams (console input and output) to the process
- Initialize hardware context to match new process
- Switch to user mode
- Jump” (set program counter) to pre-defined code entry point
- Batch job issues Halt instruction
- User logs off
- Quit an application
- Error and fault conditions
- Program invokes a system call to the operating system for self-termination, indicating a result
- Operating system stores the result in the PCB and frees all memory used by the process
- PCB remains in a ‘terminated’ state until the parent process retrieves the result
- Running
- Ready
- Blocked
- All thread management is done by the application.
- The kernel is not aware of the existence of threads.
- Kernel maintains context information for the process and the threads.
- Scheduling is done on a thread basis.
- Thread creation done in the user space.
- Bulk of scheduling and synchronization of threads done in the user space.
- Takes less time to create a new thread than a process.
- Less time to terminate a thread than a process.
- Less time to switch between two threads within the same process.
- Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel.
- A process can accomplish more than one thing at a time
- An error in one thread can cause the entire program to terminate/crash
- Threads can access/modify the same area of memory at the same time, causing corruptions