Summary - GC
- Go course
- Expert
- Lesson 12/34
- 4 min
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.
Practice 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.
Starter code
package main
import (
"fmt"
"testing"
)
func main() {
// buraya yaz
}Sample 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)
}Expected output
1 true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Does Go's GC move objects in memory (compaction)?
- No; Go's GC doesn't move objects and limits fragmentation with size classes Correct answer
- Yes, every cycle
- Only large objects
- Only with GOGC=off
Explanation: Go uses a non-moving GC; the allocator keeps fragmentation low by placing objects in pages by size class.
2 Memory limit
debug.____(900 << 20) // ~900 MiBdebug.SetMemoryLimit(900 << 20) // ~900 MiBExplanation: SetMemoryLimit is the in-code counterpart of GOMEMLIMIT.
3 Group summary
| Term | Meaning |
|---|---|
Tri-color |
Marking algorithm |
Write barrier |
Concurrent correctness |
GOGC |
GC frequency |
sync.Pool |
Reduces allocation |