Understanding Slice Internals and Memory Leaks

Understanding Slice Internals and Memory Leaks | DevPrep

// Runtime representation (reflect.SliceHeader)
type SliceHeader struct {
    Data uintptr  // Pointer to underlying array
    Len  int      // Number of elements
    Cap  int      // Capacity of underlying array
}

package main

import (
    "fmt"
    "runtime"
)

func processLargeData() []byte {
    // Allocate 100MB
    bigData := make([]byte, 100*1024*1024)
    
    // Fill with data...
    for i := range bigData {
        bigData[i] = byte(i % 256)
    }
    
    // ❌ BUG: Return small slice of big array
    // The entire 100MB stays in memory!
    return bigData[:100]
}

func main() {
    result := processLargeData()
    
    runtime.GC()
    
    var m runtime.MemStats
    runtime.ReadMemStats(&m)
    
    fmt.Printf("Result len: %d\n", len(result))
    fmt.Printf("Heap in use: %d MB\n", m.HeapInuse/1024/1024)
    // Output: Heap in use: 100 MB (leaked!)
}

func processLargeDataFixed() []byte {
    bigData := make([]byte, 100*1024*1024)
    
    for i := range bigData {
        bigData[i] = byte(i % 256)
    }
    
    // ✅ FIX: Copy to new slice
    result := make([]byte, 100)
    copy(result, bigData[:100])
    
    return result
    // bigData is now eligible for GC
}

func main() {
    // len=5, cap=5
    s := []int{1, 2, 3, 4, 5}
    
    // Reslicing: len=3, cap=5 (shares underlying array!)
    s2 := s[:3]
    
    // Modify s2 affects s
    s2[0] = 99
    fmt.Println(s[0]) // 99
    
    // Append within capacity: still shares array
    s2 = append(s2, 10)
    fmt.Println(s[3]) // 10 (!)
    
    // Append beyond capacity: new array allocated
    s2 = append(s2, 20, 30, 40)
    s2[0] = 0
    fmt.Println(s[0]) // 99 (s unaffected)
}

func demonstrateGrowth() {
    var s []int
    prevCap := 0
    
    for i := 0; i < 10000; i++ {
        s = append(s, i)
        if cap(s) != prevCap {
            fmt.Printf("len=%5d cap=%5d growth=%.2fx\n", 
                len(s), cap(s), float64(cap(s))/float64(max(prevCap, 1)))
            prevCap = cap(s)
        }
    }
}

// Output:
// len=    1 cap=    1 growth=1.00x
// len=    2 cap=    2 growth=2.00x
// len=    3 cap=    4 growth=2.00x
// len=    5 cap=    8 growth=2.00x
// ...
// len=  513 cap= 1024 growth=2.00x
// len= 1025 cap= 1280 growth=1.25x  ← Growth slows after 1024

// Force copy when returning subset
func safeSubset(data []byte, start, end int) []byte {
    result := make([]byte, end-start)
    copy(result, data[start:end])
    return result
}

// Clear slice without reallocating
func clearSlice(s []int) []int {
    return s[:0] // len=0, keeps capacity
}

// Full slice expression to limit capacity
func limitedSlice(s []int) []int {
    // s[low:high:max] limits cap to max-low
    return s[0:3:3] // cap is now 3, not len(s)
}

Understanding Slice Internals and Memory Leaks

Problem Statement

Slice Header Structure

The Memory Leak Scenario

The Fix: Copy to New Slice

Slice Capacity Gotchas

Growth Strategy

Safe Slice Operations

Follow-up Questions

Problem Statement

Slice Header Structure

The Memory Leak Scenario

The Fix: Copy to New Slice

Slice Capacity Gotchas

Growth Strategy

Safe Slice Operations

Follow-up Questions