[Misc] Human-readable `max-model-len` cli arg (#16181)

Signed-off-by: NickLucche <nlucches@redhat.com>
Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

tests/engine/test_arg_utils.py

 # SPDX-License-Identifier: Apache-2.0
 
-from argparse import ArgumentTypeError
+from argparse import ArgumentError, ArgumentTypeError
 
 import pytest
 
@@ -142,3 +142,39 @@ def test_composite_arg_parser(arg, expected, option):
     else:
         args = parser.parse_args([f"--{option}", arg])
     assert getattr(args, option.replace("-", "_")) == expected
+
+
+def test_human_readable_model_len():
+    # `exit_on_error` disabled to test invalid values below
+    parser = EngineArgs.add_cli_args(
+        FlexibleArgumentParser(exit_on_error=False))
+
+    args = parser.parse_args([])
+    assert args.max_model_len is None
+
+    args = parser.parse_args(["--max-model-len", "1024"])
+    assert args.max_model_len == 1024
+
+    # Lower
+    args = parser.parse_args(["--max-model-len", "1m"])
+    assert args.max_model_len == 1_000_000
+    args = parser.parse_args(["--max-model-len", "10k"])
+    assert args.max_model_len == 10_000
+
+    # Capital
+    args = parser.parse_args(["--max-model-len", "3K"])
+    assert args.max_model_len == 1024 * 3
+    args = parser.parse_args(["--max-model-len", "10M"])
+    assert args.max_model_len == 2**20 * 10
+
+    # Decimal values
+    args = parser.parse_args(["--max-model-len", "10.2k"])
+    assert args.max_model_len == 10200
+    # ..truncated to the nearest int
+    args = parser.parse_args(["--max-model-len", "10.212345k"])
+    assert args.max_model_len == 10212
+
+    # Invalid (do not allow decimals with binary multipliers)
+    for invalid in ["1a", "pwd", "10.24", "1.23M"]:
+        with pytest.raises(ArgumentError):
+            args = parser.parse_args(["--max-model-len", invalid])

vllm/engine/arg_utils.py

@@ -3,6 +3,7 @@
 import argparse
 import dataclasses
 import json
+import re
 import threading
 from dataclasses import dataclass
 from typing import (TYPE_CHECKING, Any, Dict, List, Literal, Mapping, Optional,
@@ -368,10 +369,14 @@ class EngineArgs:
             'data type. CUDA 11.8+ supports fp8 (=fp8_e4m3) and fp8_e5m2. '
             'ROCm (AMD GPU) supports fp8 (=fp8_e4m3)')
         parser.add_argument('--max-model-len',
-                            type=int,
+                            type=human_readable_int,
                             default=EngineArgs.max_model_len,
                             help='Model context length. If unspecified, will '
-                            'be automatically derived from the model config.')
+                            'be automatically derived from the model config. '
+                            'Supports k/m/g/K/M/G in human-readable format.\n'
+                            'Examples:\n'
+                            '- 1k → 1000\n'
+                            '- 1K → 1024\n')
         parser.add_argument(
             '--guided-decoding-backend',
             type=str,
@@ -1740,6 +1745,47 @@ def _warn_or_fallback(feature_name: str) -> bool:
     return should_exit
 
 
+def human_readable_int(value):
+    """Parse human-readable integers like '1k', '2M', etc.
+    Including decimal values with decimal multipliers.
+    
+    Examples:
+    - '1k' -> 1,000
+    - '1K' -> 1,024
+    - '25.6k' -> 25,600
+    """
+    value = value.strip()
+    match = re.fullmatch(r'(\d+(?:\.\d+)?)([kKmMgGtT])', value)
+    if match:
+        decimal_multiplier = {
+            'k': 10**3,
+            'm': 10**6,
+            'g': 10**9,
+        }
+        binary_multiplier = {
+            'K': 2**10,
+            'M': 2**20,
+            'G': 2**30,
+        }
+
+        number, suffix = match.groups()
+        if suffix in decimal_multiplier:
+            mult = decimal_multiplier[suffix]
+            return int(float(number) * mult)
+        elif suffix in binary_multiplier:
+            mult = binary_multiplier[suffix]
+            # Do not allow decimals with binary multipliers
+            try:
+                return int(number) * mult
+            except ValueError as e:
+                raise argparse.ArgumentTypeError("Decimals are not allowed " \
+                f"with binary suffixes like {suffix}. Did you mean to use " \
+                f"{number}{suffix.lower()} instead?") from e
+
+    # Regular plain number.
+    return int(value)
+
+
 # These functions are used by sphinx to build the documentation
 def _engine_args_parser():
     return EngineArgs.add_cli_args(FlexibleArgumentParser())