unity-performance-and-gc

Unity Performance and GC

Overview

Unity's managed heap is garbage-collected; every allocation in Update is a future GC spike, and the incremental collector only spreads the pain. Most frame drops in Unity projects come from a short list: allocations per frame, per-frame lookups, unpooled spawning, physics queries that allocate, UI rebuilds, and draw calls. Find which one with the Profiler, fix it, and prove the fix with numbers (game-performance-budget).

Core principle: Zero managed allocations per frame in gameplay, measured — not assumed — with the Profiler's GC Alloc column or an allocation test.

Profile first

• Profiler: CPU Usage (Hierarchy, sort by Time ms and GC Alloc), Rendering, Physics, Memory modules. Profile a Development Build with Autoconnect; the Editor adds overhead.
• Deep Profile only to locate a call, never for timing.
• Profile Analyzer compares a before and an after capture; Memory Profiler finds leaks across scene loads.
• Mark your own code: static readonly ProfilerMarker s_Marker = new("Enemy.Think"); using (s_Marker.Auto()) { ... }.

The allocation list

// ❌ allocates or searches every frame
void Update() {
    var enemies = FindObjectsByType<Enemy>(FindObjectsSortMode.None);       // array + scene search
    var close = enemies.Where(e => (e.transform.position - transform.position).sqrMagnitude < r2).ToList(); // LINQ + closure
    hpText.text = "HP: " + hp;                                              // new string every frame
    if (other.tag == "Enemy") { }                                           // allocates the tag string
    var hits = Physics.OverlapSphere(transform.position, 5f);              // new array per call
    StartCoroutine(Flash(new WaitForSeconds(0.1f)));                        // allocation per call
}

// ✅ cached, reused, non-allocating
readonly Collider[] _hits = new Collider[32];
static readonly WaitForSeconds FlashDelay = new(0.1f);
int _shownHp = -1;

void Update() {
    int count = Physics.OverlapSphereNonAlloc(transform.position, 5f, _hits, enemyMask);
    for (int i = 0; i < count; i++) { /* use _hits[i] */ }
    if (hp != _shownHp) { _shownHp = hp; hpText.SetText("HP: {0}", hp); }   // TextMeshPro, no string alloc
    if (other.CompareTag("Enemy")) { }
}