RuntimeError
vllm.RuntimeError: KV cache allocation failed
Stack trace
Traceback (most recent call last):
File "app.py", line 42, in <module>
outputs = llm.generate(prompts)
File "/usr/local/lib/python3.9/site-packages/vllm/llm.py", line 210, in generate
raise RuntimeError("KV cache allocation failed")
RuntimeError: KV cache allocation failed Why it happens
This error occurs because vLLM cannot allocate sufficient GPU or CPU memory for the key-value cache needed to store intermediate transformer states during generation. It often happens when the batch size, sequence length, or model size exceeds available hardware memory.
Detection
Monitor GPU/CPU memory usage before generation and catch RuntimeError exceptions from vLLM calls to detect cache allocation failures early.
Causes & fixes
Batch size or sequence length is too large for available GPU memory
Reduce the batch size or maximum sequence length in your generation parameters to fit within hardware memory limits.
Model size exceeds available device memory
Use a smaller model variant or switch to a device with more memory (e.g., a GPU with higher VRAM).
Multiple processes or models competing for the same GPU memory
Ensure exclusive GPU access or reduce concurrent workloads to free up memory for vLLM.
Code: broken vs fixed
from vllm import LLM
llm = LLM(model="llama-3.3-70b", max_seq_len=4096)
outputs = llm.generate(["Hello world"] * 8) # RuntimeError: KV cache allocation failed
print(outputs)import os
from vllm import LLM
os.environ["VLLM_MAX_BATCH_SIZE"] = "4" # Reduce batch size to fit memory
llm = LLM(model="llama-3.3-70b", max_seq_len=2048) # Reduce max sequence length
outputs = llm.generate(["Hello world"] * 4) # Fixed: no RuntimeError
print(outputs)Workaround
Catch RuntimeError around the generate call, then retry with smaller batch size or shorter sequences programmatically to avoid crashing.
Prevention
Design your system to monitor memory usage and dynamically adjust batch size and sequence length before calling vLLM to guarantee successful KV cache allocation.