How Does the Go Runtime Detect When a Blocked Goroutine\'s I/O Operation Completes?

How the Go Scheduler Determines When a Goroutine Unblocks from I/O

Unlike standard operating systems, Go's goroutines are not scheduled by the kernel but by the Go runtime. This enables the runtime to manage goroutine execution and perform optimizations, such as identifying when a goroutine is blocking on I/O.

When a goroutine makes an I/O system call, the runtime does not directly invoke the syscall. Instead, it intercepts the request, allowing it to use non-blocking syscalls where possible. This means the runtime continues executing other goroutines while the I/O operation is in progress.

The key to detecting when a goroutine has stopped blocking on I/O lies in the way syscalls are implemented in Go. All I/O syscalls are wrapped by the runtime in such a way that they are essentially delegated to the runtime rather than being directly executed.

For example, consider an HTTP GET request in a goroutine. When the goroutine makes the request, the runtime intercepts it and issues a non-blocking syscall to the operating system. The syscall returns immediately, and the goroutine proceeds to execute other code.

Meanwhile, the runtime maintains a list of all pending I/O requests. For the HTTP GET, the runtime will continuously query the operating system for the request's status until it receives a notification that the response is ready.

When the response arrives, the runtime will wake up the goroutine waiting for it. This is achieved through a mechanism called "polling," where the runtime will periodically check for any I/O requests that have completed. If any request has completed, the corresponding goroutine is notified and scheduled for execution to process the result.

Therefore, the Go runtime uses non-blocking syscalls and polling to determine when a goroutine has finished blocking on I/O and can resume execution. This allows the scheduler to efficiently switch between goroutines and optimize thread utilization even in I/O-intensive scenarios.

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