GOGC and GOMEMLIMIT

Learn the GOGC and GOMEMLIMIT environment variables.

GOGC and GOMEMLIMIT

GOGC (default 100) sets by what percentage the live heap may grow before GC starts: with a 100 MB live heap the next GC starts around 200 MB. A larger GOGC means less GC but more memory; a smaller one the opposite.

GOGC=200 ./app        # less frequent GC
GOGC=off ./app        # GC disabled (careful!)
GOMEMLIMIT=1GiB ./app # soft memory limit

GOMEMLIMIT (Go 1.19) puts a soft upper bound on total memory; near the limit GC runs more often.

A good setting in containers

A common recipe in a container limited to 512 MB: set GOMEMLIMIT to ~90% of the limit. Go then runs GC more often before memory fills up and avoids being OOM-killed. Under very high traffic, GOGC=off + GOMEMLIMIT runs GC only when needed; but if the live heap exceeds the limit GC runs constantly (a death spiral), so it must be monitored carefully.

Exercise Change the GC percent

Call eski := debug.SetGCPercent(50) with runtime/debug, restore with debug.SetGCPercent(eski) and print the returned value (50). Also read the current limit with debug.SetMemoryLimit(-1) and print whether it is greater than 0.

Show example solution

package main

import (
    "fmt"
    "runtime/debug"
)

func main() {
    eski := debug.SetGCPercent(50)
    fmt.Println(debug.SetGCPercent(eski))
    fmt.Println(debug.SetMemoryLimit(-1) > 0)
}

Test yourself

With a 50 MB live heap and GOGC=100, roughly when does the next GC start?
- When the heap reaches ~150 MB
- Every 2 minutes
- When the heap reaches ~100 MB
- When the heap reaches ~50 MB
Show answer

When the heap reaches ~100 MB — Target = live heap × (1 + GOGC/100) = 50 × 2 = 100 MB.

Set it from code

eski := debug.____(200)

Show answer

eski := debug.SetGCPercent(200)

Setting and effect

GOGC=50

Frequent GC, less memory
GOGC=400

Rare GC, more memory
GOGC=off

GC off
GOMEMLIMIT

Soft memory limit

Next lesson: GC and memory

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