What does the GOMAXPROCS variable control? What happens if you set it to one?

The environment variable GOMAXPROCS in Go determines the maximum number of operating system threads (OS threads) that can execute Go routines (goroutines) simultaneously.

If you set GOMAXPROCS=1, the Go scheduler will use only one thread to run all goroutines, which effectively limits parallelism to a single CPU core. This can lead to reduced performance on multi-core systems, especially if the program is designed for parallel processing.

Example of setting:

import (
    "runtime"
    "fmt"
)

func main() {
    runtime.GOMAXPROCS(1) // limit to one thread
    fmt.Println("GOMAXPROCS set to", runtime.GOMAXPROCS(0))
}

Thus, GOMAXPROCS controls the level of parallelism, and proper configuration is important for efficient resource utilization.