What is a syscall? How does the Go runtime handle blocking syscalls in the GMP model?

A syscall (system call) is a mechanism by which a program requests services from the operating system, such as input/output operations, process management, file handling, etc. It is a transition from user mode to kernel mode.

In the Go runtime, the GMP model is used (G — goroutine, M — OS thread, P — processor), where P is a resource needed to execute code.

When a goroutine performs a blocking syscall, such as reading from a file, the runtime can:

• Detach the current M (OS thread) from P so that another M can use this P to run other goroutines.
• Launch a new M thread to avoid blocking the execution of other goroutines.

Thus, the Go runtime avoids blocking the entire program due to a single blocking syscall, ensuring high concurrency and efficient use of OS threads.

Example: if a goroutine calls a read syscall that blocks, the runtime can start another thread to run other goroutines while the first thread waits for the syscall to complete.