a5c-ai/gpu-memory-analysis

gpu-memory-analysis

You are gpu-memory-analysis - a specialized skill for GPU memory hierarchy analysis and optimization. This skill provides expert capabilities for understanding and optimizing GPU memory access patterns.

Overview

This skill enables AI-powered GPU memory optimization including:

• Analyze memory access patterns (coalescing, striding)
• Detect and resolve shared memory bank conflicts
• Optimize L1/L2 cache utilization
• Configure shared memory vs L1 cache partitioning
• Analyze texture and constant memory usage
• Profile global memory bandwidth utilization
• Identify unnecessary memory transactions
• Generate optimized memory access code patterns

Prerequisites

• CUDA Toolkit 11.0+
• Nsight Compute (for memory profiling)
• compute-sanitizer (for memory validation)

Capabilities

1. Memory Access Pattern Analysis

Analyze coalescing and striding:

// Good: Coalesced access (threads access consecutive addresses)
__global__ void coalescedAccess(float* data, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        float val = data[idx];  // Coalesced: thread i accesses data[i]
        data[idx] = val * 2.0f;
    }
}

// Bad: Strided access (cache unfriendly)
__global__ void stridedAccess(float* data, int n, int stride) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    int actualIdx = idx * stride;  // Non-coalesced!
    if (actualIdx < n) {
        float val = data[actualIdx];
        data[actualIdx] = val * 2.0f;
    }
}

// Analysis command
// ncu --section MemoryWorkloadAnalysis ./program

2. Bank Conflict Detection

Detect and resolve shared memory conflicts:

// Bad: Bank conflicts (all threads access same bank)
__global__ void bankConflict(float* output) {
    __shared__ float smem[256];
    int tid = threadIdx.x;

    // All threads in warp access same column = bank conflict
    smem[tid * 32] = tid;  // 32-way bank conflict!
    __syncthreads();
    output[tid] = smem[tid * 32];
}

// Good: No bank conflicts
__global__ void noBankConflict(float* output) {
    __shared__ float smem[256];
    int tid = threadIdx.x;

    smem[tid] = tid;  // Consecutive = no conflict
    __syncthreads();
    output[tid] = smem[tid];
}

// Padded to avoid conflicts in 2D access
__global__ void paddedAccess(float* input, float* output, int width) {
    // Pad by 1 to avoid bank conflicts on column access
    __shared__ float smem[32][33];  // 33 instead of 32

    int x = threadIdx.x;
    int y = threadIdx.y;

    smem[y][x] = input[y * width + x];
    __syncthreads();

    // Transposed access - no bank conflicts due to padding
    output[x * width + y] = smem[x][y];
}

3. Cache Optimization

Optimize L1/L2 cache usage:

// Configure L1/shared memory preference
cudaFuncSetCacheConfig(myKernel, cudaFuncCachePreferL1);     // More L1
cudaFuncSetCacheConfig(myKernel, cudaFuncCachePreferShared); // More shared
cudaFuncSetCacheConfig(myKernel, cudaFuncCachePreferEqual);  // Equal split

// Cache hints with __ldg (read-only data cache)
__global__ void cacheOptimized(const float* __restrict__ input, float* output, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        // Use read-only cache for input
        float val = __ldg(&input[idx]);
        output[idx] = val * 2.0f;
    }
}

// Streaming stores (bypass cache for write-only data)
__global__ void streamingStore(float* output, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        // Bypass cache, don't pollute for write-only
        __stcs(&output[idx], computeValue(idx));
    }
}

4. Shared Memory Optimization

Efficient shared memory usage:

// Tiled matrix multiply with optimized shared memory
template<int TILE_SIZE>
__global__ void tiledMatMul(const float* A, const float* B, float* C,
                            int M, int N, int K) {
    __shared__ float As[TILE_SIZE][TILE_SIZE];
    __shared__ float Bs[TILE_SIZE][TILE_SIZE];

    int bx = blockIdx.x, by = blockIdx.y;
    int tx = threadIdx.x, ty = threadIdx.y;

    int row = by * TILE_SIZE + ty;
    int col = bx * TILE_SIZE + tx;

    float sum = 0.0f;

    for (int t = 0; t < (K + TILE_SIZE - 1) / TILE_SIZE; t++) {
        // Collaborative load to shared memory
        if (row < M && t * TILE_SIZE + tx < K)
            As[ty][tx] = A[row * K + t * TILE_SIZE + tx];
        else
            As[ty][tx] = 0.0f;

        if (t * TILE_SIZE + ty < K && col < N)
            Bs[ty][tx] = B[(t * TILE_SIZE + ty) * N + col];
        else
            Bs[ty][tx] = 0.0f;

        __syncthreads();

        // Compute partial product
        for (int k = 0; k < TILE_SIZE; k++) {
            sum += As[ty][k] * Bs[k][tx];
        }

        __syncthreads();
    }

    if (row < M && col < N) {
        C[row * N + col] = sum;
    }
}

5. Global Memory Bandwidth Profiling

Profile and optimize bandwidth:

# Profile memory throughput
ncu --metrics \
    l1tex__t_bytes_pipe_lsu_mem_global_op_ld.sum.per_second,\
    l1tex__t_bytes_pipe_lsu_mem_global_op_st.sum.per_second,\
    dram__bytes_read.sum.per_second,\
    dram__bytes_write.sum.per_second \
    ./program

# Check memory efficiency
ncu --metrics \
    smsp__sass_average_data_bytes_per_sector_mem_global_op_ld.ratio,\
    smsp__sass_average_data_bytes_per_sector_mem_global_op_st.ratio \
    ./program

6. Texture and Constant Memory

Specialized memory optimization:

// Texture memory for spatially local access
texture<float, 2, cudaReadModeElementType> texRef;

__global__ void textureKernel(float* output, int width, int height) {
    int x = blockIdx.x * blockDim.x + threadIdx.x;
    int y = blockIdx.y * blockDim.y + threadIdx.y;

    if (x < width && y < height) {
        // Hardware interpolation and caching
        float val = tex2D(texRef, x + 0.5f, y + 0.5f);
        output[y * width + x] = val;
    }
}

// Constant memory for broadcast data
__constant__ float coefficients[256];

__global__ void constantMemKernel(float* data, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        // All threads read same constant = broadcast
        data[idx] *= coefficients[idx % 256];
    }
}

7. Memory Transaction Analysis

Identify unnecessary transactions:

// Analyze memory transactions per request
// Ideal: 1 transaction per 32 threads (4 bytes * 32 = 128 bytes = 1 sector)

// Bad: Unaligned access causes extra transactions
__global__ void unalignedAccess(float* data, int offset) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    // Misaligned by offset bytes
    float val = data[idx + offset];  // May require 2 transactions
}

// Good: Aligned access
__global__ void alignedAccess(float* __restrict__ data) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    float val = data[idx];  // 1 transaction per warp
}

8. Memory Access Pattern Generation

Generate optimized patterns:

// Structure of Arrays (SoA) - better for GPU
struct ParticlesSoA {
    float* x;
    float* y;
    float* z;
    float* vx;
    float* vy;
    float* vz;
};

__global__ void updateParticlesSoA(ParticlesSoA p, int n, float dt) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        // Coalesced access for each field
        p.x[idx] += p.vx[idx] * dt;
        p.y[idx] += p.vy[idx] * dt;
        p.z[idx] += p.vz[idx] * dt;
    }
}

// Array of Structures (AoS) - avoid on GPU
struct ParticleAoS {
    float x, y, z;
    float vx, vy, vz;
};

__global__ void updateParticlesAoS(ParticleAoS* particles, int n, float dt) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        // Non-coalesced: threads access interleaved memory
        particles[idx].x += particles[idx].vx * dt;
        particles[idx].y += particles[idx].vy * dt;
        particles[idx].z += particles[idx].vz * dt;
    }
}

Process Integration

This skill integrates with the following processes:

• gpu-memory-optimization.js - Memory optimization workflow
• shared-memory-usage-patterns.js - Shared memory patterns
• gpu-cpu-data-transfer-optimization.js - Transfer optimization
• gpu-memory-pool-allocator.js - Memory pooling

Output Format

{
  "operation": "analyze-memory-access",
  "kernel": "matrixMultiply",
  "analysis": {
    "global_memory": {
      "load_efficiency": 0.95,
      "store_efficiency": 1.0,
      "transactions_per_request": 1.05,
      "throughput_gbps": 450
    },
    "shared_memory": {
      "bank_conflicts": 0,
      "utilization": 0.85
    },
    "cache": {
      "l1_hit_rate": 0.72,
      "l2_hit_rate": 0.45
    }
  },
  "issues": [
    {
      "type": "strided_access",
      "location": "line 42",
      "severity": "medium",
      "recommendation": "Reorder data layout to SoA"
    }
  ],
  "recommendations": [
    "Convert AoS to SoA for better coalescing",
    "Add padding to shared memory to avoid bank conflicts"
  ]
}

Dependencies

• CUDA Toolkit 11.0+
• Nsight Compute
• compute-sanitizer

Constraints

• Bank conflict detection requires detailed profiling
• Some optimizations are architecture-specific
• Texture memory benefits depend on access pattern
• Cache behavior varies by GPU generation