Golang collaboration scheduling (I): Collaboration Status

When explaining the part of operating system process scheduling , Almost all books will list a process state transition diagram first , Through the state diagram , It can clearly connect each link of process scheduling , Easy to understand .

Go The runtime scheduler can actually be seen as OS A somewhat simplified version of the scheduler Ben , One goroutine In its life cycle , It also includes the transformation of various states . The principles of these States and the switching between states are clarified , Yes, find out the whole Go The scheduler can be very helpful .

The following is my summary goroutine State transition diagram of , The circular box indicates the status , Arrows and text messages indicate the direction and conditions of switching ：

Status details

Here's a brief analysis , And the State Gidle stay Go The scheduler code is not really used , So ignore . in fact , once runtime I've created a new one goroutine structure , Will set its state to Grunnable And add it to the task queue , So let's start with this state .

Grunnable

Golang in , A coroutine will be set to Grunnable state ：

establish

Go In language , Including user entry functions main·main Implementation goroutine All tasks including , It's all through runtime·newproc -> runtime·newproc1 These two functions create , The former is actually a layer of encapsulation for the latter , Provide variable parameter support ,Go Linguistic go Keywords will eventually be mapped by the compiler to runtime·newproc Call to . When runtime·newproc1 After resource allocation and initialization , The status of the new task will be set to Grunnable, Then it is added to the current P In the private task queue of , Waiting for the schedule to execute .

Blocking task wakeup

When a blocking task （ Status as Gwaiting） When you are awakened when the waiting conditions are met — Like a task G#1 To someone channel Writing data will wake up before waiting to read the channel The task of data G#2——G#1 By calling runtime·ready take G#2 Reset the status to Grunnable And added to the task queue . About task blocking , It will be introduced in detail later .

other

Another path is from Grunning and Gsyscall The state changes to Grunnable, We have also merged them into the following introduction . All in all , be in Grunnable Your task must be in a task queue , Waiting for the scheduled execution at any time .

Grunning

All States are Grunnable All tasks can pass findrunnable Functions are scheduled （P&M） obtain , And then pass execute The function switches its state to Grunning, Last call runtime·gogo Load its context and execute .

I talked about it before. Go In essence, a cooperative scheduling scheme is adopted , A running task , You need to call yield To explicitly give up the processor ; stay Go1.2 after , The runtime also begins to support a certain degree of task preemption —— When the system thread sysmon It is found that a task takes too long to execute or runtime When judging that garbage collection is required , Will set the task to ” Can be preempted “ Of , When the task makes the next function call , Will give up the processor and switch back to Grunnable state . About Go1.2 Implementation details of preemption mechanism in , I will have another opportunity to introduce myself later .

Gsyscall

Go In order to ensure high concurrency performance at runtime , When the mission is performed OS Before system call , First call runtime·entersyscall The function sets its state to Gsyscall—— If the system call is blocked or takes too long , Will be the current M And P Separate —— When the system call returns , Execute thread call runtime·exitsyscall Try to retrieve P, If it succeeds and the current task is not preempted , Switch the status back to Grunning And continue ; Otherwise, set the status to Grunnable, Waiting to be scheduled again .

Gwaiting

When the resources or operating conditions required by a task cannot be met , Need to call runtime·park The function enters this state , After that, unless the conditions are met , Otherwise, the task will be in a waiting state and cannot be executed . Except for the ones mentioned before channel Except for the example of ,Go The timer of language 、 The Internet IO Operations may cause blocking of tasks .

untime·park Function contains 3 Parameters , The first is to unlock the function pointer , The second is a Lock The pointer , Finally, a string is used to describe the cause of the blocking . Obviously , The first two parameters are paired structures —— Because the task may get some before blocking Lock, these Lock You must save the task status before releasing , To avoid data competition . We know channel Must pass Lock Ensure mutually exclusive access , A blocked task G#1 You need to put yourself in channel Waiting in the queue , If it is released before the context is saved Lock, May lead to G#2 The unknown state of G#1 Set as Grunnable, So release Lock Must be in runtime·park Finish in . Due to blocking when the task holds Lock Different types —— Such as Select The lock of the operation is actually a set of Lock Set —— Therefore, it is necessary to point out Unlock In a specific way . The last parameter is mainly in gdb It is convenient to find the cause of task blocking during debugging . By the way , When all tasks are in Gwaiting In the state of , This means that the current program has entered a deadlock state , It is impossible to continue , that runtime This situation will be detected , And output all Gwaiting Mission backtrace Information .

Gdead

Last , When a task is finished , Would call runtime·goexit End your life —— I'm going to set the state to zero Gdead, And chain the structure to a part that belongs to the current P The free G In the list , For future use .

Go The concurrency model of the language basically follows CSP Model ,goroutine Completely depend on channel signal communication , Didn't like Unix Process wait or waitpid Waiting mechanism , There is no such thing “POSIX Thread” Medium pthread_join The convergence mechanism of , More like kill or signal This kind of interrupt mechanism . Every goroutine Exit and destroy by yourself after completion , Without a trace .