Docs on prompt scaling (#963)

* docs on prompt scaling

* remove unused var

* nits

llms/README.md

@@ -38,7 +38,9 @@ To see a description of all the arguments you can do:
 >>> help(generate)
 ```
 
-Check out the [generation example](https://github.com/ml-explore/mlx-examples/tree/main/llms/mlx_lm/examples/generate_response.py) to see how to use the API in more detail.
+Check out the [generation
+example](https://github.com/ml-explore/mlx-examples/tree/main/llms/mlx_lm/examples/generate_response.py)
+to see how to use the API in more detail.
 
 The `mlx-lm` package also comes with functionality to quantize and optionally
 upload models to the Hugging Face Hub.
@@ -122,10 +124,44 @@ mlx_lm.convert \
     --upload-repo mlx-community/my-4bit-mistral
 ```
 
+### Long Prompts and Generations 
+
+MLX LM has some tools to scale efficiently to long prompts and generations:
+
+- A rotating fixed-size key-value cache.
+- Prompt caching
+
+To use the rotating key-value cache pass the argument `--max-kv-size n` where
+`n` can be any integer. Smaller values like `512` will use very little RAM but
+result in worse quality. Larger values like `4096` or higher will use more RAM
+but have better quality.
+
+Caching prompts can substantially speedup reusing the same long context with
+different queries. To cache a prompt use `mlx_lm.cache_prompt`. For example:
+
+```bash
+cat prompt.txt | mlx_lm.cache_prompt \
+  --model mistralai/Mistral-7B-Instruct-v0.3 \
+  --prompt - \
+  --kv-cache-file mistral_prompt.safetensors
+``` 
+
+Then use the cached prompt with `mlx_lm.generate`:
+
+```
+mlx_lm.generate \
+    --kv-cache-file mistral_prompt.safetensors \
+    --prompt "\nSummarize the above text."
+```
+
+The cached prompt is treated as a prefix to the supplied prompt. Also notice
+when using a cached prompt, the model to use is read from the cache and need
+not be supplied explicitly.
+
 ### Supported Models
 
-The example supports Hugging Face format Mistral, Llama, and Phi-2 style
-models.  If the model you want to run is not supported, file an
+MLX LM supports thousands of Hugging Face format LLMs. If the model you want to
+run is not supported, file an
 [issue](https://github.com/ml-explore/mlx-examples/issues/new) or better yet,
 submit a pull request.
 

llms/mlx_lm/cache_prompt.py

@@ -56,7 +56,7 @@ def setup_arg_parser():
     parser.add_argument(
         "--max-kv-size",
         type=int,
-        default=1024,
+        default=None,
         help="Set the maximum key-value cache size",
     )
     parser.add_argument(
@@ -147,3 +147,7 @@ def main():
     metadata["tokenizer_config"] = json.dumps(tokenizer_config)
     metadata["max_kv_size"] = str(args.max_kv_size)
     mx.save_safetensors(args.kv_cache_file, cache_dict, metadata)
+
+
+if __name__ == "__main__":
+    main()

llms/mlx_lm/generate.py

@@ -12,7 +12,6 @@ DEFAULT_MAX_TOKENS = 100
 DEFAULT_TEMP = 0.6
 DEFAULT_TOP_P = 1.0
 DEFAULT_SEED = 0
-DEFAULT_MAX_KV_SIZE = 1024
 
 
 def setup_arg_parser():
@@ -81,6 +80,7 @@ def setup_arg_parser():
         "--max-kv-size",
         type=int,
         help="Set the maximum key-value cache size",
+        default=None,
     )
     parser.add_argument(
         "--kv-cache-file",
@@ -199,12 +199,9 @@ def main():
 
     # Determine the max kv size from the kv cache or passed arguments
     max_kv_size = args.max_kv_size
-    if max_kv_size is None:
-        max_kv_size = (
-            int(metadata["max_kv_size"])
-            if cache_history is not None
-            else DEFAULT_MAX_KV_SIZE
-        )
+    if cache_history is not None:
+        max_kv_size = metadata["max_kv_size"]
+        max_kv_size = int(max_kv_size) if max_kv_size.isdigit() else None
 
     generate(
         model,

llms/mlx_lm/version.py

@@ -1,3 +1,3 @@
 # Copyright © 2023-2024 Apple Inc.
 
-__version__ = "0.17.1"
+__version__ = "0.18.0"