kvpress 0.0.1

Efficiently compress the KV cache of any pretrained transformer

Deploying long-context LLMs is costly due to the linear growth of the key-value (KV) cache in transformer models. For example, handling 1M tokens with Llama 3.1-70B in float16 requires up to 330GB of memory. This repository implements multiple KV cache pruning methods and benchmarks using 🤗 transformers, aiming to simplify the development of new methods for researchers and developers in this field.

Installation

pip install kvpress

We recommend using flash attention if possible:

pip install flash-attn --no-build-isolation

Usage

This repository provides a set of "presses" that compress the KV cache by pruning the least important key-value pairs in each attention head. A press is only applied during the pre-filling phase and is associated with a compression_ratio parameter that controls the amount of pruning. The easiest way to use a press is through our custom KVPressTextGenerationPipeline that is automatically registered as a transformers pipeline with the name "kv-press-text-generation" when kvpress is imported. It handles chat templates and tokenization for you:

from kvpress import ExpectedAttentionPress
from transformers import pipeline

device = "cuda:0"
model= "microsoft/Phi-3.5-mini-instruct"
pipe = pipeline("kv-press-text-generation", model=model, device=device, torch_dtype="auto", model_kwargs={"attn_implementation":"flash_attention_2"})

context = "A very long text you want to compress once and for all"
question = "\nA question about the compressed context" # optional
    
press = ExpectedAttentionPress(compression_ratio=0.4)
answer = pipe(context, question=question, press=press)["answer"]

In the snippet above, the compression is only applied on the context tokens so that you can evaluate the compression for different questions. Check the Wikipedia notebook demo for a more detailed example.

[!IMPORTANT]
We focus on pruning during the pre-filling phase as the KV cache becomes a bottleneck for long-context sequence (100k - 1M tokens) which are essentially long context prompts. This would typically apply to improving prompt caching systems.

[!NOTE]
To use the ObservedAttentionPress, use model_kwargs={"attn_implementation":"eager"} in order to materialize the attention weights (this method is not compatible with flash attention).

Contributing with a new press

We welcome contributions! If you want to implement a new press, open an issue or a pull request. Refer to the FAQ for more information on how presses work and how to create new ones or check the new_press.ipynb notebook for a step-by-step guide.

Available presses

We provide the following presses associated with the following scores:

• RandomPress: random score
• KnormPress: inverse norm of the key (paper)
• ObservedAttentionPress: average attention weight observed during in pre-filling phase (similar to H2O or TOVA)
• SnapKVPress: average attention weight of the last 64 queries (paper)
• ExpectedAttentionPress (ours): expected attention weight during the generation phase (see this notebook)
• StreamingLLMPress: keep only the first and last tokens (paper)

Evaluation

We provide a simple CLI to evaluate the performance of the different presses on several long-context datasets.

Average performance on the RULER dataset with 4k context length and Loogle Short Dependency QA task for 3 models and 7 presses

Please refer to the evaluation directory for more details and results.

FAQ

Which models are supported ?

Some presses depend on the model architecture (e.g. ExpectedAttentionPress and SnapKVPress) hence they might not work with all models. We tested support for LlamaForCausalLM, MistralForCausalLM, Phi3ForCausalLM and Qwen2ForCausalLM but many other models might be supported out of the box because their implementation is often similar in transformers.

What are the memory and throughput gains ?

Memory usage should be reduced by around compression_ratio * kv_cache_size. As the KV cache is smaller, decoding should also be faster. You can measure peak memory usage gain and total time gain using this notebook.

How does a press work ?

A press registers a forward hook to each attention layer during the pre-filling phase:

1. Immediately after the forward pass, the hook is called, and it computes a score for each key-value pair using the press.score method
2. The key-value pairs with the lowest scores are then removed based on the compression_ratio parameter

import torch
from transformers import AutoModelForCausalLM
from kvpress import KnormPress

device = "cuda:0"
ckpt = "meta-llama/Meta-Llama-3.1-8B-Instruct"
model = AutoModelForCausalLM.from_pretrained(ckpt).to(device)
press = KnormPress(compression_ratio=0.4)

inputs = model.dummy_inputs["input_ids"].to(device)

with torch.no_grad():
    print(model(inputs).past_key_values[0][0].shape)
    # torch.Size([3, 8, 5, 128])
    
with torch.no_grad(), press(model):
    print(model(inputs).past_key_values[0][0].shape)
    # torch.Size([3, 8, 3, 128])

Why not using model.generate ?

In fact you can use model.generate with a press by using the press as a context manager:

with press(model):
    outputs = model.generate(inputs)

However, the generate method does not allow to exclude the question from the compression, which would artificially favors methods such as SnapKV. Ideally, we want a compression method that works whatever comes after the context (e.g. for use cases such as chat or document question answering). Finally the generate method does not allow to provide generation for multiple questions at once.

How to create a new press ?

All presses are stored in the presses directory. The easiest way to create a new press is to create a class that inherits from BasePress and implement a score method that computes the score for each key-value pair (see knorm_press.py for a simple example). Check the notebook new_press.ipynb for a step-by-step guide.

Before opening a pull request with a new press, make sure to register it in the __init__.py file of repository and to add it in test_presses.py.

Can I change the compression ratio from one layer to another ?

We provide an experimental feature, which only works with flash attention:

from kvpress import apply_per_layer_compression
# compression_ratios should have the same length as the number of layers
press = apply_per_layer_compression(press, compression_ratios=[...])

Check the demo notebook for more details.

Is quantization supported ?

We don't support quantization of the KV cache yet. Quantization can achieve up to 4x compression moving from (b)float16 to int4 and we believe it is orthogonal to the KV cache pruning strategies proposed in this repository.