Ue Profiling And Optimization

Profiling & optimization

Optimize by measurement, not guesswork: capture data, find the actual hotspot, fix it, re-measure. The three tool layers are Unreal Insights (deep trace-based profiling), the in-game stat commands (quick triage), and instrumentation macros for your own code. Start with stat commands to identify which thread owns the frame budget, then open Insights for a precise call-by-call view.

When to use this skill

• Frame rate is low or hitching; determining whether the game thread, render thread, or GPU is the bottleneck.
• Adding per-system timing so it appears by name in Insights and stat output.
• Memory growth, out-of-memory events, or finding which system owns an allocation.
• Deciding which optimization lever to pull after identifying the hotspot.

Triage first: stat commands

In the console (PIE or standalone game):

Read stat unit first:

• Game dominates → optimize C++ / Blueprint / tick overhead.
• Draw dominates → reduce draw calls, draw-thread CPU cost.
• GPU dominates → rendering budget, resolution, overdraw, shadows, post-process.
• RHIT (RHI Thread) high → command-list heavy; reduce batching or RHI overhead.

See references/stat-commands.md for the full command table and per-command interpretation guidance.

Unreal Insights — the full profiler

Trace-based, low overhead, works in PIE and packaged builds. Produces .utrace files opened in Engine\Binaries\Win64\UnrealInsights.exe.

Starting a trace

From the Editor bottom toolbar — click Trace to open the Trace widget; select channels and click Start Trace. From the command line:

MyGame.exe -trace=cpu,gpu,frame,bookmark,log -tracehost=127.0.0.1

Channels of interest:

Timing Insights window

Opens from the Session Browser. Key views:

• Frames panel — bar chart of per-frame cost; click a spike to zoom in.
• Timing panel — per-thread timeline of named CPU/GPU scopes, nested call hierarchy.
• Timers tab — aggregated total/avg/max per named scope for the selected time range; sort by Inclusive Time to find the heaviest leaf callers.
• Callers/Callees — trace who calls a selected scope and what it calls.

Memory Insights window

Opened from Menu > Memory Insights inside a trace that has memalloc or memtag active.

• Timeline shows total allocated bytes, live allocation count, and LLM-tagged graphs.
• Investigation panel runs queries: Active Alloc, Growth (A→B), Memory Leaks (A→B→C), Short Living, Long Living.
• Results break down by LLM tag, callstack, asset name, or class name.

Full workflow: references/unreal-insights-and-trace.md.

Instrumentation — stat groups and cycle counters

The stat system shows named timers in stat <group> output and in Insights.

// In a .cpp (file scope — single translation unit):
DECLARE_STATS_GROUP(TEXT("MySystem"), STATGROUP_MySystem, STATCAT_Advanced);
DECLARE_CYCLE_STAT(TEXT("MySystem Update"), STAT_MySystemUpdate, STATGROUP_MySystem);

// In the function:
void UMySystem::Update()
{
    SCOPE_CYCLE_COUNTER(STAT_MySystemUpdate);
    // ... work ...
}

For a stat accessible across multiple files, declare with DECLARE_CYCLE_STAT_EXTERN in the header and DEFINE_STAT in the .cpp.

QUICK_SCOPE_CYCLE_COUNTER is a one-liner for temporary instrumentation — it creates and uses a cycle counter in STATGROUP_Quick with no prior declaration:

void AMyActor::Tick(float DeltaTime)
{
    QUICK_SCOPE_CYCLE_COUNTER(STAT_MyActor_Tick);
    // ... work ...
}

Instrumentation — CPU profiler trace scopes

TRACE_CPUPROFILER_EVENT_SCOPE writes directly to the Trace system (cpu channel) and is visible in the Timing Insights timeline. Lower overhead than the stat system; preferred for high-frequency scopes.

#include "ProfilingDebugging/CpuProfilerTrace.h"

void AMySystem::Step()
{
    TRACE_CPUPROFILER_EVENT_SCOPE(AMySystem::Step);
    // ... work ...
}

For a dynamic (runtime-determined) scope name use TRACE_CPUPROFILER_EVENT_SCOPE_TEXT:

TRACE_CPUPROFILER_EVENT_SCOPE_TEXT(*DynamicName);

Scope variants (all in CpuProfilerTrace.h:453):

• TRACE_CPUPROFILER_EVENT_SCOPE(Name) — literal token, lowest overhead.
• TRACE_CPUPROFILER_EVENT_SCOPE_STR(NameStr) — const string pointer.
• TRACE_CPUPROFILER_EVENT_SCOPE_TEXT(Name) — dynamic TCHAR*/FName.
• TRACE_CPUPROFILER_EVENT_SCOPE_ON_CHANNEL(Name, Channel) — gated on a custom channel.

CPUPROFILERTRACE_ENABLED is 1 in Development/Test builds and 0 in Shipping.

Instrumentation — CSV profiler

CSV_SCOPED_TIMING_STAT records per-frame timings to a CSV file, usable in Shipping and Test builds. Useful for automated performance regression tests.

#include "ProfilingDebugging/CsvProfiler.h"

CSV_DEFINE_CATEGORY(MySystem, true);

void UMySubsystem::Tick(float DeltaTime)
{
    CSV_SCOPED_TIMING_STAT(MySystem, MySubsystem_Tick);
    // ... work ...
}

Full reference: references/instrumenting-cpp.md.

Memory profiling

Quick snapshot

memreport -full

Dumps a detailed memory breakdown to Saved/Profiling/MemReports/. Covers pool allocators, UObject counts, asset memory, texture streaming, etc.

LLM (Low Level Memory Tracker)

LLM instruments every allocation with a tag. Enable with -llm on the command line; view with stat llm, stat llmfull, or via Insights MemTag channel.

LLM tags are defined in LowLevelMemTracker.h (Runtime/Core/Public/HAL/LowLevelMemTracker.h). Tags relevant to games: UObject, RenderTargets, Shaders, Meshes, Audio, Animation.

Insights Memory Insights

Run with -trace=memalloc,memtag,callstack,module (Development build). Open Memory Insights from the Menu inside a loaded trace. Use "Memory Leaks" query to find allocations alive across a level transition, or "Growth" to find what grew between two moments.

Full workflow: references/memory-profiling.md.

GPU profiling

• stat gpu — per-pass totals in the console.
• ProfileGPU (console command) / GPU Visualizer (Editor menu) — one-frame GPU breakdown, shows pass tree with ms costs.
• Insights GPU track — timeline view of GPU passes alongside CPU work; requires -trace=gpu.

Common GPU costs to investigate: shadow passes (stat shadowrendering), translucency overdraw, post-process stack, screen percentage, too many dynamic lights.

Common optimization levers (after measuring)

Gotchas

• Profiling in Development/Editor misleads on absolute timing; use Shipping-config packaged builds for final numbers. Use Development PIE for iteration.
• Confusing CPU- and GPU-bound — fix the wrong side. Always read stat unit first.
• TRACE_CPUPROFILER_EVENT_SCOPE with a string literal — use _STR variant instead; passing a quoted string to the plain macro adds extra quotation marks in the name.
• DECLARE_CYCLE_STAT in a header — the static DEFINE_STAT it emits causes a linker duplicate if included in multiple TUs. Use the _EXTERN + DEFINE_STAT pair.
• memalloc tracing at scale — capturing every allocation produces enormous traces; prefer memtag for long sessions and use memalloc only for targeted leak hunts.
• Optimizing without measuring — always profile first; fixing a non-bottleneck wastes time and can obscure the real hotspot.
• Stats stripped in Shipping — STATS is 0 in Shipping; use TRACE_CPUPROFILER_EVENT_SCOPE (disabled by CPUPROFILERTRACE_ENABLED) or CSV_SCOPED_TIMING_STAT (available in Test/Shipping) for production-visible metrics.

Version notes

• Stats2.h is deprecated in 5.6 and redirects to Stats.h; include Stats/Stats.h.
• CPUPROFILERTRACE_ENABLED evaluates to 0 in Shipping builds (5.8: CpuProfilerTrace.h:21).
• Memory Insights Android callstack support was added in 5.4; available in 5.8.
• Timing Insights gained a Verse Sampling track in 5.7.

References & source material

Engine source (UE 5.8, under Engine/Source/Runtime/Core/Public/):

• Stats/Stats.h — DECLARE_STATS_GROUP:209, DECLARE_CYCLE_STAT:130, SCOPE_CYCLE_COUNTER:229, QUICK_SCOPE_CYCLE_COUNTER:226, DECLARE_SCOPE_CYCLE_COUNTER:221, CONDITIONAL_SCOPE_CYCLE_COUNTER:235.
• Stats/Stats2.h — deprecated header; redirects to Stats/Stats.h since 5.6.
• ProfilingDebugging/CpuProfilerTrace.h — TRACE_CPUPROFILER_EVENT_SCOPE:453, TRACE_CPUPROFILER_EVENT_SCOPE_STR:408, TRACE_CPUPROFILER_EVENT_SCOPE_TEXT:489, CPUPROFILERTRACE_ENABLED:21, FCpuProfilerTrace::FEventScope:185.
• ProfilingDebugging/CsvProfiler.h — CSV_SCOPED_TIMING_STAT:120, CSV_DEFINE_CATEGORY:50, CSV_CUSTOM_STAT:155.
• ProfilingDebugging/ScopedTimers.h — FDurationTimer:31, FScopedDurationTimer:65, FScopedDurationTimeLogger:193.
• HAL/LowLevelMemTracker.h — ENABLE_LOW_LEVEL_MEM_TRACKER:19, LLM tag scopes.

Official docs (UE 5.8, fetched and confirmed live):

• Unreal Insights — https://dev.epicgames.com/documentation/unreal-engine/unreal-insights-in-unreal-engine
• Trace — https://dev.epicgames.com/documentation/unreal-engine/trace-in-unreal-engine-5
• Timing Insights — https://dev.epicgames.com/documentation/unreal-engine/timing-insights-in-unreal-engine-5
• Memory Insights — https://dev.epicgames.com/documentation/unreal-engine/memory-insights-in-unreal-engine
• Stat Commands — https://dev.epicgames.com/documentation/unreal-engine/stat-commands-in-unreal-engine
• Stats System Overview — https://dev.epicgames.com/documentation/unreal-engine/unreal-engine-stats-system-overview
• Testing and Optimizing — https://dev.epicgames.com/documentation/unreal-engine/testing-and-optimizing-your-content

Deep-dive references in this skill:

• references/unreal-insights-and-trace.md — trace channels, session workflow, Timing Insights anatomy, GPU track.
• references/stat-commands.md — full stat command table, reading stat unit, stat groups for custom subsystems.
• references/instrumenting-cpp.md — all instrumentation macros, CSV profiler, cross-file stat patterns, DEFINE_STAT pitfalls.
• references/memory-profiling.md — LLM, memreport, Insights Memory Insights workflow, query rules, leak hunting.

Related skills: ue-nanite-and-rendering, ue-materials-and-shaders, ue-timers-and-async, ue-memory-and-gc, ue-debugging-techniques.