CUP中待執行的程式碼，thread的容器(實際執行任務的是thread)
一個process可擁有多個thread
有new、ready、running、waiting、terminated、suspended6種狀態
每個process間是獨立的，較難互相溝通
- The process has its own Process Control Block, Stack, and Address Space.
在多功作業系統(Multitasking Operating System)中，可以同時執行多個process，這就是multiprocessing。但一個CPU一次只能執行一個process，所以才有多核心CPU的誕生
process中的thread可以並行(concurrency)或平行(parallelism)的方式運作

thread(執行緒/線程)

組成process的部分
多個thread可以被同個process擁有
有Running、Ready、Blocked3種狀態
每個thread間並非獨立的。它們共享定址空間、資源，所以當main thread有任何的變動、或者前面的thread crash會都影響到其他thread
- threads of the same process share address space and other resources.
執行程式碼時，thread會將變數存在stack(記憶體空間)中。stack只有該thread可以使用，並不共享，但heap可以被同個process的thread共享
- Thread has Parents’ PCB, its own Thread Control Block, and Stack and common Address space.

thread pool

線程池。粗略地說的可以把thread pool視為存放預先建立(實例化)好的thread的地方，當thread執行完了任務，就去queue依序拿一個來執行。先實例化thread，是因為這樣就不用重複建立，可以提高效能

In computer programming, a thread pool is a software design pattern for achieving concurrency of execution in a computer program. Often also called a replicated workers or worker-crew model, a thread pool maintains multiple threads waiting for tasks to be allocated for concurrent execution by the supervising program.

By maintaining a pool of threads, the model increases performance and avoids latency in execution due to frequent creation and destruction of threads for short-lived tasks.

The number of available threads is tuned to the computing resources available to the program, such as a parallel task queue after completion of execution.

以下是上文翻譯

在程式的領域，thread pool是一種用於達到並行執行程式的設計模式。它也常被稱作replicated workers 或 worker-crew model。

thread pool會有多個thread在等被發配到任務。thread pool以不重複建立又拆掉thread，來提升效能、避免運行時的延遲。

可以同時被維持的thread的數量取決於程式可以取得的計算資源(CPU之意)。例如平行執行時可取得的資源會比一個完成後再執行下一個少

🖊名詞解釋

名詞	解釋	其他
process control block	process control block是一種資料結構，裡面存了關於process的資料(ex: state, number, `program counter`, `cpu register`, scheduling information, memory management...)
program counter	program counter會指出process下一個要執行的指令存放的路徑
cpu register	register是一種暫存器，可存放指令、儲存路徑、資料...。	Along with the program counter this state information must be saved when an interrupt occurs and to allow the process to be continued correctly afterward.
context switching	CPU執行一個process時，另一個事件發生了。它去處理新事件之前會把執行到一半的任務的進度存到PCB，之後再讀進度繼續執行	a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point.
multiprocessing	系統同時用多個CUP(processors)同時執行多個process	useful for CPU-bound processes, such as computationally heavy tasks
multithreading	單個CUP(processors)同時執行多個thread	useful for IO-bound processes, such as reading files from a network or database
concurrency	快速context switching產生的假象	`interleaved` operation
parallelism	同時執行多個process，不context switching	using multiple CUP