learn_concurrency.md

what is goroutines

• user-space threads
• managed entirely by the go runtime
• lighter-weight and cheaper
• faster creation, destruction, context switches

GOROUTINE	THREAD
Goroutines are managed by the go runtime.	Operating system threads are managed by os.
stack = 2kb	thread stack = 8kb
Goroutine are not hardware dependent.	Threads are hardware dependent.
Goroutines have easy communication medium known as channel.	Thread doesnot have easy communication medium.
Due to the presence of channel one goroutine can communicate with other goroutine with low latency.	Due to lack of easy communication medium inter-threads communicate takes place with high latency.
Goroutine doesnot have ID because go doesnot have Thread Local Storage.	Threads have their own unique ID because they have Thread Local Storage.
Goroutines are cheaper than threads.	The cost of threads are higher than goroutine.
They are cooperatively scheduled.	They are preemptively scheduled.
They have fasted startup time than threads. They have slow startup time than goroutines.

| | Goroutine has growable segmented stacks. |Threads doesnot have growable segmented stacks.

什么是阻塞

• asynchronous network IO
• sleeping
• channel operation
• system calls
• blocking on primitives in the sync package

Go Runtime

• Go runtime manage how many OS threads to create
• Go runtime manage how goroutines are mapped to and executed on OS threads.
• Go runtime manage assign P with a M
• Go runtime Create threads when needed, keey them around for reuse
• An OS thread is typically assigned with multiple goroutines
• My Mac: 6 Cores, 12 virtual cores and 12 logical processor, I can execute 12 OS threads in parallel., Hyper-Threading

Goroutine states

• Waiting
• Runnable
• Executing

Go Scheduler

• Global Run Queue (GRQ) , FIFO
• Local Run Queue (LRQ), FIFO
• Limit threads accessing runqueue
  • the number of cpu cores
  • too many , too much contention
  • too few, won't use all the cpu cores

communicate channel

• traditional for, loop read channel, need break loop， // 遍历chan
• for range, loop read channel, will break when chan is close, // 遍历chan
• select, (case, timeout, default, ): once read channel, // Quit Channel, 如读的时候需要timeout终止， 写的时候需要quit bool手动终止
• WaitGroup, read all channel, WaitGroup, wg.add, wg.done, wg.wait, 等带多chan
• worker pool, 一般两chan， 一个tasks, 一个results，producer写tasks, worker读tasks写results, consumers读results， 多chan遍历读写

go_concurrency_visualize

ping_pong, 一写一读

• ping_pong

select, 读多chan中的一个

• select

waitGroup， 读多chan

• waitgroup

multiplexing, 读多chan，写入一个chan

• multiplexing

mutex , 多chan 加锁写全局遍历

• mutex