alt Hacker News> The prompt cache caches KV Cache states
Yes. The cache that caches KV cache states is called the KV cache. "Prompt cache" is just index from string prefixes into KV cache. It's tiny and has no computational impact. The parent was correct to question you.
The cost of using it comes from the blend of the fact that you need more compute to calculate later tokens and the fact that you have to keep KV cache entries between requests of the same user somewhere while the system processes requests of other users.
Saying that it is just in index from string prefixes into KV Cache misses all the fun, interesting, and complicated parts of it. While technically the size of the prompt-pointers is tiny compared with the data it points into, the massive scale of managing this over all users and requests and routing inside the compute cluster makes it an expensive thing to implement and tune. Also, keeping the prompt cache sufficiently responsive and storing the large KV Caches somewhere costs a lot as well in resources.
I think that the OpenAI docs are pretty useful for the API level understanding of how it can work (https://developers.openai.com/api/docs/guides/prompt-caching...). The vLLM docs (https://docs.vllm.ai/en/stable/design/prefix_caching/) and SGLang radix hashing (https://lmsys.org/blog/2024-01-17-sglang/) are useful for insights into how to implement it locally for one computer ode.