The difference between php threads and processes

1. What is a process

A process is an instance of program execution, and a process can allocate Provide resources such as CPU and memory. A process generally includes an instruction set and system resources, where the instruction set is your code, and system resources include CPU, memory, I/O, etc. (Recommended learning: PHP Programming from entry to proficiency)

A process is a dynamic execution process of a program in a data set. It can be simply understood as "the executing program", which is An independent unit for CPU resource allocation and scheduling.

The process generally consists of three parts: program, data set, and process control block. The program we write is used to describe what functions the process needs to complete and how to complete it; the data set is the resources that the program needs to use during execution; the process control block is used to record the external characteristics of the process, describe the execution change process of the process, and the system It can be used to control and manage processes. It is the only sign that the system senses the existence of a process.

The limitation of the process is that the overhead of creation, cancellation and switching is relatively high.

2. What is a thread?

A thread is an execution stream of a process. A thread cannot allocate system resources. It is part of the process and runs independently smaller than the process. The unit.

Explain: The process has two characteristics: one is the ownership of the resource, and the other is the scheduling execution (instruction set). The thread is part of the scheduling execution, which refers to the path of the process execution process, also called program execution. flow. Threads are sometimes called lightweight processes.

Threads are a concept developed after processes. A thread is also called a lightweight process. It is a basic CPU execution unit and the smallest unit in the program execution process. It is composed of a thread ID, a program counter, a register set and a stack. A process can contain multiple threads.

The advantage of threads is that it reduces the overhead of concurrent execution of programs and improves the concurrency performance of the operating system. The disadvantage is that threads do not have their own system resources and only have resources that are essential during runtime, but the same Each thread of a process can share the system resources owned by the process. If the process is compared to a workshop, then the threads are like workers in the workshop. However, there is a lock mechanism for some exclusive resources, and improper handling may cause "deadlock".

The difference between process and thread

(1) Process is an independent unit for resource allocation and scheduling, while thread is the basic unit of CPU scheduling

(2) The same process can include multiple threads, and the threads share the resources of the entire process (registers, stacks, context). A thread can only belong to one process, and a process can have multiple threads. A process includes at least one thread

(3) The creation of a process calls fork or vfork, and the creation of a thread calls pthread_create. After the process ends All threads it owns will be destroyed, and the end of the thread will not affect the end of other threads in the same process

(4) Threads are lightweight processes, and their creation and destruction require The time is much smaller than that of the process. The CPU is allocated to threads, that is, the threads are actually running on the CPU.

(5) Synchronization and mutual exclusion are generally required when executing in threads, because they share all resources of the same process

(6) Threads have their own private attributes TCB, thread id, Registers, hardware context, and the process also has its own private attributes process control block PCB. These private attributes are not shared and are used to identify a process or a thread

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