[Frontend] Warn if user `max_model_len` is greater than derived `max_model_len` (#7080)

Signed-off-by: Jefferson Fialho <jfialho@ibm.com>
Co-authored-by: Nick Hill <nickhill@us.ibm.com>

vllm/config.py

@@ -6,6 +6,7 @@ from typing import TYPE_CHECKING, ClassVar, List, Optional, Tuple, Type, Union
 import torch
 from transformers import PretrainedConfig
 
+import vllm.envs as envs
 from vllm.logger import init_logger
 from vllm.model_executor.layers.quantization import QUANTIZATION_METHODS
 from vllm.model_executor.models import ModelRegistry
@@ -1541,15 +1542,21 @@ def _get_and_verify_max_len(
                     "Disabling sliding window is not supported for models "
                     "model_max_length in the config. Please raise an issue "
                     "so we can investigate.")
-            pass
         else:
-            raise ValueError(
+            msg = (
                 f"User-specified max_model_len ({max_model_len}) is greater "
-                "than the derived max_model_len "
-                f"({max_len_key}={derived_max_model_len} or model_max_length="
+                f"than the derived max_model_len ({max_len_key}="
+                f"{derived_max_model_len} or model_max_length="
                 f"{model_max_length} in model's config.json). This may lead "
-                "to incorrect model outputs or CUDA errors. Make sure the "
-                "value is correct and within the model context size.")
+                "to incorrect model outputs or CUDA errors.")
+            if envs.VLLM_ALLOW_LONG_MAX_MODEL_LEN:
+                logger.warning(
+                    "%s Make sure the value is correct and within the "
+                    "model context size.", msg)
+            else:
+                raise ValueError(
+                    f"{msg} To allow overriding this maximum, set "
+                    "the env var VLLM_ALLOW_LONG_MAX_MODEL_LEN=1")
     return int(max_model_len)
 
 

vllm/envs.py

@@ -50,6 +50,7 @@ if TYPE_CHECKING:
     VLLM_NO_DEPRECATION_WARNING: bool = False
     CMAKE_BUILD_TYPE: Optional[str] = None
     VERBOSE: bool = False
+    VLLM_ALLOW_LONG_MAX_MODEL_LEN: bool = False
 
 
 def get_default_cache_root():
@@ -331,6 +332,15 @@ environment_variables: Dict[str, Callable[[], Any]] = {
     # If set, vllm will skip the deprecation warnings.
     "VLLM_NO_DEPRECATION_WARNING":
     lambda: bool(int(os.getenv("VLLM_NO_DEPRECATION_WARNING", "0"))),
+
+    # If the env var VLLM_ALLOW_LONG_MAX_MODEL_LEN is set, it allows
+    # the user to specify a max sequence length greater than
+    # the max length derived from the model's config.json.
+    # To enable this, set VLLM_ALLOW_LONG_MAX_MODEL_LEN=1.
+    "VLLM_ALLOW_LONG_MAX_MODEL_LEN":
+    lambda:
+    (os.environ.get("VLLM_ALLOW_LONG_MAX_MODEL_LEN", "0").strip().lower() in
+     ("1", "true")),
 }
 
 # end-env-vars-definition