GOMAXPROCS

Learn to set the processor count with GOMAXPROCS.

GOMAXPROCS

GOMAXPROCS is the number of Ps (processors) that can run Go code at once. Its default is the available CPU count. It is set with an environment variable or in code:

GOMAXPROCS=4 ./app
runtime.GOMAXPROCS(4)  // returns the old value

Changing it at run time needs a brief stop-the-world; it is usually done once at program start.

When to lower it

When several Go services share CPUs on one machine, or a container's CPU quota is below the core count, setting GOMAXPROCS to the quota reduces CPU throttling and latency spikes. Starting with Go 1.25 the runtime reads the cgroup quota itself and keeps updating if the quota changes.

Exercise Worker count

For a CPU-bound worker pool, take the worker count from runtime.GOMAXPROCS(0). Write isciSayisi() int; if the value is below 1 return 1. Print whether the result is 1 or more.

Show example solution

package main

import (
    "fmt"
    "runtime"
)

func isciSayisi() int {
    n := runtime.GOMAXPROCS(0)
    if n < 1 {
        return 1
    }
    return n
}

func main() {
    fmt.Println(isciSayisi() >= 1)
}

Test yourself

In an I/O-heavy service (mostly waiting on the network), what does raising GOMAXPROCS usually give?
- Not much; waiting goroutines don't use a P
- Linear speed-up
- Less memory
- A faster network
Show answer

Not much; waiting goroutines don't use a P — Network waits happen in the netpoller and don't hold a P; if CPU isn't the bottleneck, adding Ps doesn't help.

Read without changing

p := runtime.GOMAXPROCS(____)

Show answer

p := runtime.GOMAXPROCS(0)

Situation and advice

CPU-bound work

Equal to cores
I/O-bound work

Default is enough
Container with a 2-CPU quota

2
Go 1.25+

Reads the quota itself

Next lesson: runtime.Gosched

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