Adaptive KV Streaming for llama.cpp

This branch adds an experimental, block-granular KV cache streaming path to the CUDA llama-server. It is intended for running long contexts when model weights leave too little VRAM for the complete KV cache.

With --kv-stream-stage-mib N, the authoritative KV tensors are stored in pinned host memory while a bounded CUDA pool is shared by resident KV pages and a transfer ring. The runtime adapts that split as the context grows: it keeps as many pages resident as the budget allows, reclaims resident space for staging when more streaming is required, and prefetches later layers while the current layer computes. This avoids relying on uncontrolled Unified Memory page thrashing and preserves exact attention over the full context.

Detailed project story, design, implementation, and benchmark results are in Running Qwen 27B on 16G VRAM with Full Context Length: Building Adaptive KV Cache Streaming for llama.cpp.

[!WARNING] This is research code optimized and production-validated primarily for an RTX 5070 Ti with 16 GB VRAM, unsloth/Qwen3.8-27B-GGUF UD-Q3_K_XL, a 262144-token context, Flash Attention, a Q8_0 K cache, a Q4_0 V cache, and one server slot. CUDA correctness tests cover every KV type currently accepted by the CLI, including native and F16-conversion fallback paths. Production performance for other models, KV combinations, parallel slots, and non-CUDA backends is not yet broadly characterized.

Build the modified server

Install a C++ compiler, CMake, and the CUDA toolkit, then run this command from the repository root:

cmake -S . -B build -DGGML_CUDA=ON -DGGML_CUDA_FA_ALL_QUANTS=ON -DCMAKE_BUILD_TYPE=Release && cmake --build build --config Release --target llama-server -j

The executable is created at build/bin/llama-server.

Example using the tested cache configuration:

./build/bin/llama-server \
  --model /path/to/model.gguf \
  --ctx-size 262144 \
  -fa on \
  -ctk q8_0 \
  -ctv q4_0 \
  -ngl all \
  -b 512 \
  -ub 512 \
  -np 1 \
  --kv-stream-stage-mib 2304

The best value for --kv-stream-stage-mib depends on the model, context capacity, GPU, and other VRAM consumers. Start conservatively and increase it while checking startup and peak VRAM use.

Batch and micro-batch sizes

-b sets the logical prompt batch size and -ub sets the largest physical batch submitted to one graph. This branch no longer requires 256/256; -ub may be any positive value no larger than -b.

The Qwen3.8 MMA prefill path processes the full physical batch and allocates only the partial workspace that kernel actually emits. Generic vector and F16-conversion fallback paths use a bounded 256-query workspace: each staged KV span is consumed by all query tiles before its ring slot is released, so wider micro-batches do not multiply KV host-to-device transfers.

The Q8_0/Q4_0 Qwen configuration has been exercised with b/ub values 256/256, 512/512, 768/512, and 1024/1024, including non-divisible final micro-batches. A 122880-token production-shaped run at 512/512 completed with adaptive streaming active. Wider values can require more graph and accumulator memory, so validate them on the target GPU.

Optional Unified Memory for model weights

Adaptive KV streaming works with or without Unified Memory. Leave GGML_CUDA_ENABLE_UNIFIED_MEMORY unset for ordinary CUDA device allocations. To make GPU-offloaded model buffers CUDA managed allocations, launch the same server with the environment variable enabled:

GGML_CUDA_ENABLE_UNIFIED_MEMORY=1 \
./build/bin/llama-server \
  --model /path/to/model.gguf \
  --ctx-size 262144 \
  -fa on \
  -ctk q8_0 \
  -ctv q4_0 \
  -ngl all \
  -b 512 \
  -ub 512 \
  -np 1 \
  --kv-stream-stage-mib 2304

With this flag, CUDA-backed model buffers, including GPU-offloaded weights, are allocated with cudaMallocManaged and their pages can migrate between VRAM and host memory. The adaptive resident-page and transfer-ring pool is intentionally different: it is still allocated with cudaMalloc, so that fixed-size pool remains physically allocated in VRAM instead of becoming managed memory. UVM is therefore optional for this branch and does not change the KV streaming pool into pageable storage.

Recreate the benchmark graph

The benchmark driver automatically selects the largest practical adaptive KV pool for each configured context capacity, sweeps from 8K through the requested maximum, and generates the CSV, PNG, and SVG results:

python3 -m pip install matplotlib

python3 benchmarks/benchmark_kv_stream.py \
  --model /path/to/model.gguf \
  --max-context 192K \
  --batch-size 512 \
  --ubatch-size 512

The only required arguments are the model GGUF and maximum context. See benchmarks/README.md for the pool-probing algorithm, generated files, optional settings, and resumable output directories.

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Upstream llama.cpp README

llama.cpp

Quick start

A few options to get llama.cpp installed on your machine:

• Visit https://llama.app and follow the instructions
• Run with Docker - see our Docker documentation
• Download pre-built binaries from the releases page
• Build from source by cloning this repository - check out our build guide

Once installed:

# Download and run a model directly from Hugging Face
llama cli -hf ggml-org/Qwen3.5-0.8B-GGUF

# Launch OpenAI-compatible API server
llama serve -hf ggml-org/Qwen3.5-0.8B-GGUF

Description

The main goal of llama.cpp is to enable LLM (and VLM) inference with minimal setup and state-of-the-art performance on a wide range of hardware - locally and in the cloud.

• Plain C/C++ implementation without any dependencies
• Apple silicon is a first-class citizen - optimized via ARM NEON, Accelerate and Metal frameworks
• AVX, AVX2, AVX512 and AMX support for x86 architectures
• RVV, ZVFH, ZFH, ZICBOP and ZIHINTPAUSE support for RISC-V architectures
• 1.5-bit, 2-bit, 3-bit, 4-bit, 5-bit, 6-bit, and 8-bit integer quantization for faster inference and reduced memory use
• Custom CUDA kernels for running LLMs on NVIDIA GPUs (support for AMD GPUs via HIP and Moore Threads GPUs via MUSA)
• Vulkan and SYCL backend support
• CPU+GPU hybrid inference to partially accelerate models larger than the total VRAM capacity

The llama.cpp project is build on top of the ggml library.

Supported backends

Documentation

Tools

• cli
• completion
• server
• GBNF grammars

Development

• How to build
• Running on Docker
• Build on Android
• Multi-GPU usage
• Performance troubleshooting
• GGML tips & tricks
• XCFramework
• Completions
• Models
• Release process

Contributing

• Contributors can open PRs
• Collaborators will be invited based on contributions
• Maintainers can push to branches in the llama.cpp repo and merge PRs into the master branch
• Any help with managing issues, PRs and projects is very appreciated!
• Read the CONTRIBUTING.md for more information

Acknowledgements

• yhirose/cpp-httplib - Single-header HTTP server, used by llama-server - MIT license
• stb-image - Single-header image format decoder, used by multimodal subsystem - Public domain
• nlohmann/json - Single-header JSON library, used by various tools/examples - MIT License
• miniaudio.h - Single-header audio format decoder, used by multimodal subsystem - Public domain
• subprocess.h - Single-header process launching solution for C and C++ - Public domain