Go
Summary - GC
Combine tri-color and GOGC to write a memory analysis.
GC summary
Go's GC is a concurrent, tri-color, mark-and-sweep collector. The program keeps running between two short STWs, and the write barrier keeps things correct. The tuning knobs are GOGC (how often) and GOMEMLIMIT (how much); the measurement tools are gctrace, ReadMemStats and the heap profile.
But the biggest win comes from changing the code: fewer and shorter-lived heap allocations.
Measure, then tune
A typical investigation: use gctrace=1 to see what percentage of CPU the GC takes; if it's high, find the lines that allocate most with the alloc_space heap profile; fix them; fine-tune with GOGC/GOMEMLIMIT only as a last step. Changing the settings first hides the problem instead of solving it.
Exercise Allocation comparison
Compare two functions with testing.AllocsPerRun: one starts each call with make([]int, 0) and appends 100 items, the other starts with capacity via make([]int, 0, 100). Print the capacity version's allocation count (1) and whether it is less than the other.
Show example solution
package main
import (
"fmt"
"testing"
)
var sink []int
func main() {
a := testing.AllocsPerRun(10, func() {
s := make([]int, 0)
for i := 0; i < 100; i++ {
s = append(s, i)
}
sink = s
})
b := testing.AllocsPerRun(10, func() {
s := make([]int, 0, 100)
for i := 0; i < 100; i++ {
s = append(s, i)
}
sink = s
})
fmt.Println(b, b < a)
}
Test yourself
-
Does Go's GC move objects in memory (compaction)?
- No; Go's GC doesn't move objects and limits fragmentation with size classes
- Yes, every cycle
- Only large objects
- Only with GOGC=off
Show answer
No; Go's GC doesn't move objects and limits fragmentation with size classes — Go uses a non-moving GC; the allocator keeps fragmentation low by placing objects in pages by size class.
-
Memory limit
debug.____(900 << 20) // ~900 MiB
Show answer
debug.SetMemoryLimit(900 << 20) // ~900 MiB
-
Group summary
- Tri-color
- Marking algorithm
- Write barrier
- Concurrent correctness
- GOGC
- GC frequency
- sync.Pool
- Reduces allocation
Next lesson: The Go compiler
