llama.cpp Introduces Dynamic VBR KV Cache

old-mike · reddit · 2026-07-14

This post introduces the VBR (Variable Bit Rate) KV cache, a new feature in spiritbuun's llama.cpp fork:

• Instead of a fixed KV quantization level for the entire context, it sets a floor, and the cache dynamically steps down quantization levels as the context grows.
• The -ct vbr command automatically calculates the available KV budget based on remaining VRAM and dynamically reduces precision during the decode process.
• Tested on an RTX 3060 12GB compared to fixed quantization:
• Decode TPS is only 4% slower
• TTFT is roughly halved
• Lower VRAM usage
• Automatically adapts to a context of 216k tokens
• The post also mentions a fused kernel bug, for which the author has already reverted the relevant commit.

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