[Experimental] Prefix Caching Support (#1669)

Co-authored-by: DouHappy <2278958187@qq.com>
Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>

tests/prefix_caching/test_prefix_caching.py

@@ -0,0 +1,41 @@
+"""Compare the with and without prefix caching.
+
+Run `pytest tests/prefix_caching/test_prefix_caching.py`.
+"""
+import pytest
+
+from vllm import LLM, SamplingParams
+
+prefix = (
+    "You are an expert school principal, skilled in effectively managing "
+    "faculty and staff. Draft 10-15 questions for a potential first grade "
+    "Head Teacher for my K-12, all-girls', independent school that emphasizes "
+    "community, joyful discovery, and life-long learning. The candidate is "
+    "coming in for a first-round panel interview for a 8th grade Math "
+    "teaching role. They have 5 years of previous teaching experience "
+    "as an assistant teacher at a co-ed, public school with experience "
+    "in middle school math teaching. Based on these information, fulfill "
+    "the following paragraph: ")
+
+
+@pytest.mark.parametrize("model", ["facebook/opt-125m"])
+@pytest.mark.parametrize("max_tokens", [16])
+def test_prefix_caching(
+    example_prompts,
+    model: str,
+    max_tokens: int,
+):
+    llm = LLM(model=model)
+    # -1 since the last token can change when concatenating prompts.
+    prefix_pos = len(llm.llm_engine.tokenizer.encode(prefix)) - 1
+    prompts = [prefix + prompt for prompt in example_prompts]
+    sampling_params = SamplingParams(temperature=0.0, max_tokens=max_tokens)
+    outputs_without_prefix = llm.generate(prompts, sampling_params)
+    outputs_with_prefix = llm.generate(prompts,
+                                       sampling_params,
+                                       prefix_pos=[prefix_pos] * len(prompts))
+    for output_without_prefix, output_with_prefix in zip(
+            outputs_without_prefix, outputs_with_prefix):
+        assert (output_without_prefix.outputs[0].token_ids ==
+                output_with_prefix.outputs[0].token_ids)
+    assert len(llm.llm_engine.scheduler.prefix_pool.prefixes) == 1