[compile] Fix extra cache save on warm start. (#35921)

Signed-off-by: zhxchen17 <zhxchen17@fb.com>

tests/compile/test_startup.py

@@ -61,11 +61,11 @@ def test_moe_startup(monkeypatch, vllm_runner, fresh_vllm_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,
+        num_compiled_artifacts_saved=0,
     ):
         _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
+    assert (
+        counters["aot_autograd"]["autograd_cache_hit"] == 0
+    )  # No miss at aot_autograd level causing disk I/O.

vllm/compilation/backends.py

@@ -221,10 +221,28 @@ class CompilerManager:
     ) -> Callable[..., Any] | None:
         if (compile_range, graph_index, self.compiler.name) not in self.cache:
             return None
-        handle = self.cache[(compile_range, graph_index, self.compiler.name)]
+
+        def parse_value(value: Any) -> tuple[tuple[str, str], str]:
+            assert isinstance(value, dict)
+            handle = value["graph_handle"]
+            assert isinstance(handle[0], str)
+            assert isinstance(handle[1], str)
+            cache_key = value["cache_key"]
+            return handle, cache_key
+
+        try:
+            handle, cache_key = parse_value(
+                self.cache[(compile_range, graph_index, self.compiler.name)]
+            )
+        except Exception:
+            # When the cache is outdated, we should ignore the existing file.
+            # This should cause the correct cache to be generated again.
+            return None
+
         compiled_graph = self.compiler.load(
             handle, graph, example_inputs, graph_index, compile_range
         )
+        self.loaded_artifacts[cache_key] = compiled_graph
         logger.debug(
             "Directly load the %s-th graph for compile range %sfrom %s via handle %s",
             graph_index,
@@ -341,7 +359,10 @@ class CompilerManager:
 
         # store the artifact in the cache
         if is_compile_cache_enabled(additional_inductor_config) and handle is not None:
-            self.cache[(compile_range, graph_index, self.compiler.name)] = handle
+            self.cache[(compile_range, graph_index, self.compiler.name)] = {
+                "graph_handle": handle,
+                "cache_key": cache_key,
+            }
             compilation_counter.num_cache_entries_updated += 1
             self.is_cache_updated = True
             if graph_index == 0: