cpu-cache-opt

Q: How do I install cpu-cache-opt?

Run npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill cpu-cache-opt in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/cpu-cache-opt/ (personal) or .claude/skills/cpu-cache-opt/ (project).

by mohitmishra786Frontend & Design

CPU cache optimization skill for C/C++ and Rust. Use when diagnosing cache misses, improving data layout for cache efficiency, using perf stat cache counters, understanding false sharing, prefetching, or structuring AoS vs SoA data layouts. Activates on queries about cache misses, cache lines, false sharing, perf cache counters, data layout optimization, prefetch, AoS vs SoA, or L1/L2/L3 cache performance.

Installs: 657
Last 24h: —
Trending rank: —
Security: Not audited

Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill cpu-cache-opt

01
Install
Run the command in your project. The CLI asks which agents to install for.
02
Describe the task
Ask your agent for something this skill covers. It loads cpu-cache-opt automatically when relevant.
03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

CPU Cache Optimization

Purpose

Guide agents through cache-aware programming: diagnosing cache misses with perf, data layout transformations (AoS→SoA), false sharing detection and fixes, prefetching, and cache-friendly algorithm design.

Triggers

"My program has high cache miss rates — how do I fix it?"
"What is false sharing and how do I detect it?"
"Should I use AoS or SoA data layout?"
"How do I measure cache performance with perf?"
"How do I use __builtin_prefetch?"
"My multithreaded program is slower than single-threaded due to cache"

Workflow

1. Measure cache performance

# Basic cache counters
perf stat -e cache-references,cache-misses,cycles,instructions ./prog

# L1/L2/L3 miss breakdown
perf stat -e \
    L1-dcache-load-misses,\
    L1-dcache-loads,\
    L2-dcache-load-misses,\
    LLC-load-misses,\
    LLC-loads \
    ./prog

# Cache miss rate = L1-dcache-load-misses / L1-dcache-loads
# > 5% is concerning; > 20% is severe

# False sharing detection
perf stat -e \
    machine_clears.memory_ordering,\
    mem_load_l3_hit_retired.xsnp_hitm \
    ./prog

2. Cache line basics

Cache line size: 64 bytes on x86-64, ARM (most platforms)
L1 cache: 32–64 KB, ~4 cycles latency
L2 cache: 256 KB–1 MB, ~12 cycles latency
L3 cache: 6–64 MB, ~40 cycles latency
Main memory: ~200–300 cycles latency

// Check cache line size
long cache_line = sysconf(_SC_LEVEL1_DCACHE_LINESIZE);

// Align data to cache line
struct alignas(64) HotData {
    int counter;
    // ... 60 bytes of data that fit in one line
};

// C
typedef struct {
    int x;
} __attribute__((aligned(64))) AlignedData;

3. AoS vs SoA data layout

// AoS (Array of Structures) — default layout
struct Particle {
    float x, y, z;     // position (12 bytes)
    float vx, vy, vz;  // velocity (12 bytes)
    float mass;         // (4 bytes)
    int   flags;        // (4 bytes)
};
Particle particles[N];  // Bad for loops that only need position

// Problem: accessing particles[i].x loads x,y,z,vx,vy,vz,mass,flags
// But we only need x,y,z → 75% of loaded data is wasted

// SoA (Structure of Arrays) — cache-friendly for SIMD + sequential access
struct ParticlesSoA {
    float *x, *y, *z;
    float *vx, *vy, *vz;
    float *mass;
    int   *flags;
};

// Accessing x[i] for i=0..N loads 16 consecutive x values → 0% waste
// Also auto-vectorizes better

4. Common cache-unfriendly patterns

// BAD: random access (linked list traversal)
Node *node = head;
while (node) {
    process(node->data);
    node = node->next;  // pointer chasing = cache miss per node
}

// BETTER: pool allocate nodes contiguously
// Or: rewrite as contiguous array with indices

// BAD: stride > cache line in matrix traversal
for (int i = 0; i < N; i++)
    for (int j = 0; j < M; j++)
        sum += matrix[j][i];  // column-major access on row-major array

// GOOD: row-major access
for (int i = 0; i < N; i++)
    for (int j = 0; j < M; j++)
        sum += matrix[i][j];

// BAD: large struct with hot + cold fields
struct Record {
    int id;           // hot: accessed every iteration
    char name[128];   // cold: accessed rarely
    int value;        // hot
    char desc[256];   // cold
};

// GOOD: separate hot and cold data
struct RecordHot { int id; int value; };
struct RecordCold { char name[128]; char desc[256]; };
RecordHot hot_data[N];
RecordCold cold_data[N];

5. False sharing

False sharing occurs when two threads write to different variables that share a cache line, causing constant cache-line invalidations.

// BAD: counters likely on same cache line (8 bytes each, line = 64 bytes)
int counter_a;  // thread A's counter
int counter_b;  // thread B's counter

// Both on the same cache line → every write invalidates the other thread's cache

// GOOD: pad to separate cache lines
struct alignas(64) PaddedCounter {
    int value;
    char padding[60];  // Ensure next counter is on different cache line
};

PaddedCounter counters[NUM_THREADS];
// Thread i: counters[i].value++

// C++ standard approach
struct alignas(std::hardware_destructive_interference_size) PaddedCounter {
    int value;
};

6. Prefetching

Manual prefetch hints to hide memory latency:

#include <immintrin.h>  // or <xmmintrin.h>

// Prefetch for read (locality 0=non-temporal, 3=high temporal)
__builtin_prefetch(ptr, 0, 3);  // prefetch for read, high locality
__builtin_prefetch(ptr, 1, 3);  // prefetch for write, high locality

// SSE prefetch (x86)
_mm_prefetch((char*)ptr, _MM_HINT_T0);   // L1
_mm_prefetch((char*)ptr, _MM_HINT_T1);   // L2
_mm_prefetch((char*)ptr, _MM_HINT_T2);   // L3
_mm_prefetch((char*)ptr, _MM_HINT_NTA);  // non-temporal (streaming)

// Typical pattern: prefetch N iterations ahead
#define PREFETCH_DIST 8
for (int i = 0; i < N; i++) {
    if (i + PREFETCH_DIST < N)
        __builtin_prefetch(&data[i + PREFETCH_DIST], 0, 3);
    process(data[i]);
}

Prefetching rules:

Prefetch too early = cache evicted before use
Prefetch too late = no benefit
Prefetch distance = memory latency / time per iteration (typically 8–32 elements)

7. Cache-friendly algorithm design

// Loop blocking / tiling for matrix operations
// Process cache-fitting blocks instead of full rows/columns
#define BLOCK 64  // tuned to L1 cache size

void matrix_mult_blocked(float *C, float *A, float *B, int N) {
    for (int i = 0; i < N; i += BLOCK)
    for (int k = 0; k < N; k += BLOCK)
    for (int j = 0; j < N; j += BLOCK)
    // Inner block fits in L1 cache
    for (int ii = i; ii < i + BLOCK && ii < N; ii++)
    for (int kk = k; kk < k + BLOCK && kk < N; kk++)
    for (int jj = j; jj < j + BLOCK && jj < N; jj++)
        C[ii*N+jj] += A[ii*N+kk] * B[kk*N+jj];
}

For perf cache event reference and false sharing detection patterns, see references/cache-counters.md.

Related skills

Use skills/profilers/linux-perf for perf stat and perf record cache measurements
Use skills/profilers/valgrind — cachegrind simulates cache behaviour
Use skills/low-level-programming/simd-intrinsics — SoA layout pairs with SIMD vectorization
Use skills/low-level-programming/memory-model for false sharing in concurrent contexts

FAQ

What does the cpu-cache-opt skill do?

How do I install cpu-cache-opt?

Run npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill cpu-cache-opt in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/cpu-cache-opt/ (personal) or .claude/skills/cpu-cache-opt/ (project).

Which AI agents can use cpu-cache-opt?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is cpu-cache-opt safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains cpu-cache-opt?

It is published by mohitmishra786 in mohitmishra786/low-level-dev-skills. It has 657 recorded installs.