hz-simpleperf-debug

Simpleperf Debug Skill

When to Use

Use this skill when you need hardware-level CPU performance insights on Meta Quest devices:

• Classifying whether an app is CPU-bound, memory-bound, or I/O-bound
• Finding CPU hotspot functions consuming the most cycles
• Measuring kernel vs userspace CPU overhead per thread
• Identifying cache-thrashing or branch-prediction issues
• Supplementing Perfetto trace analysis with hardware PMU counter data

This skill complements hz-perfetto-debug. Perfetto shows what your app is doing over time. Simpleperf shows where the CPU is spending hardware cycles — cache misses, branch mispredictions, and instruction throughput that Perfetto can't see.

VR Performance Context

Quest devices run on mobile ARM SoCs with strict thermal and power budgets. CPU-bound apps hit frame drops when:

Refresh Rate	CPU Frame Budget	Notes
120 Hz	8.3 ms	Tight — simpleperf critical for finding hotspots
90 Hz	11.1 ms	Default target for most apps
72 Hz	13.9 ms	Fallback for heavier apps

Simpleperf's hardware counters reveal bottlenecks invisible to software tracing.

hzdb Setup

Simpleperf profiling is powered by the hzdb CLI. Invoke via npx — no install required:

npx -y @meta-quest/hzdb --version

Examples below use the bare hzdb command for brevity — substitute npx -y @meta-quest/hzdb. Connect your Quest via USB with developer mode enabled.

Quick Start Workflow

1. Classify the Workload

Before optimizing, determine the bottleneck type:

# Classify the foreground app's workload (10-second sample)
hzdb perf simpleperf classify

# Target a specific app
hzdb perf simpleperf classify --app com.example.myapp

# Custom duration
hzdb perf simpleperf classify --duration 15

Returns a classification with evidence:

Classification	Indicator	Optimization Strategy
CPU-bound	High IPC, low stall ratio	Optimize algorithms, reduce draw calls, batch work
Memory-bound	High stall ratio (stalled-cycles-backend / cpu-cycles)	Reduce cache misses, improve data locality, shrink working set
I/O-bound	High context switches per second	Reduce blocking I/O, use async, minimize thread contention

2. Record CPU Hotspots

Capture a CPU cycle profile to find the most expensive functions:

# Record CPU hotspots for the foreground app
hzdb perf simpleperf record

# Custom frequency and duration
hzdb perf simpleperf record --frequency 4000 --duration 10

# Target a specific app
hzdb perf simpleperf record --app com.example.myapp

The recording samples CPU cycles at the specified frequency (default 4000 Hz) and generates a profile showing which functions consume the most CPU time.

3. Measure Kernel Overhead

Determine how much CPU time is spent in kernel vs userspace per thread:

# Measure kernel overhead for the foreground app
hzdb perf simpleperf kernel-overhead

# Custom duration
hzdb perf simpleperf kernel-overhead --app com.example.myapp --duration 10

Returns per-thread breakdown of user-mode vs kernel-mode CPU cycles. High kernel overhead (>20%) in a thread suggests:

• Excessive syscalls (file I/O, memory allocation)
• Driver overhead (GPU command submission, sensor access)
• Lock contention in kernel synchronization primitives

Analysis Workflow

Step 1: Classify First

Always start with classification. This prevents wasting time optimizing the wrong thing.

hzdb perf simpleperf classify --app com.example.myapp --duration 10

Decision tree based on results:

• CPU-bound → Record hotspots (Step 2a), look at top functions
• Memory-bound → Record with cache-miss events, check data access patterns
• I/O-bound → Check kernel overhead, look at thread contention in Perfetto

Step 2a: CPU-Bound Apps — Find Hotspots

hzdb perf simpleperf record --app com.example.myapp --duration 10

Review the top functions by CPU cycle consumption. Common VR hotspots:

Function Pattern	Likely Cause	Fix
Physics.* / PhysX	Complex physics simulation	Reduce collider count, simplify meshes, increase fixed timestep
Render* / Draw*	Too many draw calls	Batch materials, use GPU instancing, reduce unique materials
GC_* / gc_alloc	Garbage collection pressure	Pool allocations, avoid per-frame allocations
memcpy / memmove	Large data copies	Use references, reduce buffer sizes, avoid unnecessary copies
LZ4_* / compress	Asset decompression	Pre-decompress, use lighter compression, cache results

Step 2b: Memory-Bound Apps — Check Cache Behavior

If classification shows memory-bound, the issue is likely cache misses or memory bandwidth:

• Large working sets thrashing L1/L2 cache
• Random access patterns defeating prefetcher
• False sharing between threads on adjacent cache lines

Use Perfetto hz-perfetto-debug to correlate memory-bound regions with specific code paths.

Step 3: Measure Kernel Overhead

hzdb perf simpleperf kernel-overhead --app com.example.myapp

Interpreting results by thread:

Thread	Expected Kernel %	High Kernel % Indicates
Main/Game thread	< 5%	Excessive file I/O, logging, or allocations
Render thread	5-15%	Normal (GPU driver overhead). >20% = driver issue
Worker threads	< 5%	Thread synchronization overhead
Audio thread	< 10%	Normal for audio HAL calls

Step 4: Combine with Perfetto

Simpleperf tells you where cycles go. Perfetto tells you when and in what context. Use together:

1. Simpleperf classification reveals the bottleneck type
2. Simpleperf hotspot recording identifies the expensive functions
3. Perfetto trace (hzdb perf capture) shows when those functions run relative to frame boundaries
4. Use hzdb perf query to correlate function timing with frame drops

Common Pitfalls

• Don't profile in thermal throttling. Let the device cool before recording — throttled clocks distort cycle counts. Check thermal state first with hzdb device info.
• Sample duration matters. Short recordings (<5s) may not capture representative behavior. Use at least 10 seconds for classification.
• simpleperf requires shell access. If adb shell simpleperf fails, ensure developer mode is enabled and USB debugging is authorized.
• Frequency vs accuracy tradeoff. Higher sampling frequency (>8000 Hz) can perturb the workload on mobile SoCs. Default 4000 Hz is a good balance.
• Classification is a snapshot. An app can be CPU-bound during gameplay and I/O-bound during scene loads. Profile the specific scenario you're optimizing.

References

For detailed guides on specific topics, see:

• Workload Classification — PMU counter interpretation and bottleneck identification
• CPU Hotspot Analysis — Recording and analyzing CPU cycle profiles
• Kernel Overhead — Measuring and reducing kernel-mode CPU usage