[offloader] v2: Hide weight onloading latency via prefetching (#29941)

Signed-off-by: Ming Yang <minos.future@gmail.com>
Signed-off-by: Michael Goin <mgoin64@gmail.com>
Co-authored-by: Michael Goin <mgoin64@gmail.com>

tests/basic_correctness/test_prefetch_offload.py

@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Test prefetch offloading correctness with Llama model."""
+
+from ..utils import compare_two_settings
+
+
+def test_prefetch_offload_llama():
+    """Test prefetch CPU offloading with Llama-3.2-1B-Instruct.
+
+    Compares outputs between:
+    1. Baseline (no offloading)
+    2. Prefetch offloading (group_size=8, num_in_group=2, prefetch_step=1)
+
+    This tests prefetching-based offloading on a dense model.
+    """
+    compare_two_settings(
+        "meta-llama/Llama-3.2-1B-Instruct",
+        [
+            # Prefetch offloading configuration
+            "--offload-group-size",
+            "8",
+            "--offload-num-in-group",
+            "2",
+            "--offload-prefetch-step",
+            "1",
+            # Selective offloading: only MLP weights
+            "--offload-params",
+            "gate_up_proj",
+            "down_proj",
+        ],
+        [],  # Baseline: no offloading
+    )