[torch.compile] Improve cold and warm start compile tests (#35709)

Signed-off-by: Richard Zou <zou3519@gmail.com>

tests/compile/test_startup.py

@@ -0,0 +1,71 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Cold start and warm start tests for vLLM-compile.
+
+Cold start runs in a forked child (must fork before CUDA init) which
+populates on-disk caches and asserts cold-start counters.  Warm start
+then runs in the parent with clean in-memory state but populated caches.
+"""
+
+import multiprocessing as mp
+
+from torch._dynamo.utils import counters
+
+from vllm.compilation.counter import compilation_counter
+from vllm.config import CompilationConfig, CompilationMode, CUDAGraphMode
+
+MODEL = "microsoft/Phi-tiny-MoE-instruct"
+
+
+def _run_vllm(vllm_runner):
+    with vllm_runner(
+        MODEL,
+        trust_remote_code=False,
+        max_model_len=256,
+        max_num_batched_tokens=1024,
+        load_format="dummy",
+        compilation_config=CompilationConfig(
+            mode=CompilationMode.VLLM_COMPILE,
+            cudagraph_mode=CUDAGraphMode.NONE,
+        ),
+        num_gpu_blocks_override=8,
+    ):
+        pass
+
+
+def _cold_start(vllm_runner):
+    counters.clear()
+    with compilation_counter.expect(
+        num_compiled_artifacts_saved=3,
+        num_compiled_artifacts_loaded=0,
+    ):
+        _run_vllm(vllm_runner)
+    assert counters["aot_autograd"]["total"] == 33
+    assert counters["aot_autograd"]["autograd_cache_miss"] == 3
+    assert counters["aot_autograd"]["autograd_cache_hit"] == 0
+
+
+def test_moe_startup(monkeypatch, vllm_runner, fresh_vllm_cache):
+    monkeypatch.setenv("VLLM_ENABLE_V1_MULTIPROCESSING", "0")
+
+    # Cold start in a forked child (must fork before CUDA init).
+    # This model has 32 identical transformer layers which produce
+    # 33 subgraphs after splitting on attention — only 3 are unique.
+    ctx = mp.get_context("fork")
+    p = ctx.Process(target=_cold_start, args=(vllm_runner,))
+    p.start()
+    p.join()
+    assert p.exitcode == 0, "Cold-start child failed"
+
+    # Warm start — compiled artifacts loaded from disk cache.
+    counters.clear()
+    with compilation_counter.expect(
+        num_compiled_artifacts_loaded=3,
+        # TODO: warm start should not save any artifacts
+        # https://github.com/vllm-project/vllm/issues/35708
+        num_compiled_artifacts_saved=1,
+    ):
+        _run_vllm(vllm_runner)
+    assert counters["aot_autograd"]["total"] == 30
+    assert counters["aot_autograd"]["autograd_cache_miss"] == 0
+    assert counters["aot_autograd"]["autograd_cache_hit"] == 1