Como usar
- Copie o comando e verifique o pacote antes de instalar no seu ambiente de agente.
- Consulte no original as ferramentas, credenciais e ambientes necessários.
- Descreva os dados de entrada e a entrega esperada; confira a saída com as fontes.
npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill memory-hierarchy-and-cachesAntes de começar
- A instalação não inclui assinatura de modelo, franquia API ou acesso aos seus arquivos.
- Estrelas e instalações indicam atividade, não um teste independente de qualidade ou segurança.
Documento original do Skill
Renderizado do repositório de origem, preservando títulos, exemplos, código, tabelas, links e imagens.
Memory Hierarchy and Caches
Purpose
Teach CPU memory hierarchy: cache levels, associativity, line size, coherence protocols, false sharing, and prefetch behavior — architectural depth complementing optimization practice in skills/low-level-programming/cpu-cache-opt.
When to Use
- Explaining why padding fixes scalability
- Choosing struct layout for multicore
- Interpreting cache miss counters
- Understanding DMA vs CPU cache on embedded SoCs
Workflow
1. Hierarchy (typical desktop/ server)
Registers
├── L1d / L1i (per core, ~32 KiB, ~4 cycles)
├── L2 (per core, ~256 KiB – 1 MiB)
├── L3 (shared last-level, MiB – tens of MiB)
├── DRAM (hundreds of cycles)
└── Storage / NUMA remote (much slower)Embedded MCUs may have only tightly-coupled memory (no L2/L3).
2. Cache line and associativity
- Line size: commonly 64 bytes on x86/ARM64 (verify with
getconf LEVEL1_DCACHE_LINESIZE) - Set-associative: line maps to one set, competes within ways
- Conflict misses: many aliases same set
3. False sharing
c
/* Bad — two atomics on same cache line */
struct {
atomic_int counter_a;
atomic_int counter_b;
} stats;
/* Good — pad to cache line */
struct alignas(64) {
atomic_int counter_a;
char pad[64 - sizeof(atomic_int)];
atomic_int counter_b;
} stats;4. Coherence (multicore)
MESI states: Modified, Exclusive, Shared, Invalid. Writes invalidate other cores' copies of the line — why atomics and locks ping cache lines.
5. Prefetching
c
#ifdef __builtin_prefetch
for (int i = 0; i < n; i++) {
__builtin_prefetch(&data[i + 8], 0, 3);
process(data[i]);
}
#endifHardware stride prefetchers detect sequential access; random access misses.
6. Measurement
bash
perf stat -e cache-references,cache-misses,L1-dcache-load-misses ./app7. Agent usage
/memory-hierarchy-and-caches Diagnose false sharing in this per-thread stats arrayCommon Problems
| Symptom | Cause | Fix |
|---|---|---|
| Scaling collapses | False sharing | Line-align per-thread data |
| High LLC misses | Working set > cache | Block algorithms; NUMA pin |
| DMA incoherence | CPU cache vs device | Flush/invalidate on MCU; dma_sync on Linux |
| Prefetch hurt | irregular access | Remove manual prefetch |
| Huge struct copies | AoS cold lines | SoA layout — see cpu-cache-opt |
Related Skills
skills/low-level-programming/cpu-cache-opt— practical optimizationskills/computer-architecture/cpu-pipelines-and-hazards— load-use stallsskills/computer-architecture/virtual-memory-paging-and-tlb— TLB missesskills/allocators/numa-programming— remote memory latencyskills/profilers/hardware-counters— perf counter events
