Add MSE vs MinMax observer comparison tests

- Add comprehensive test suite comparing MSE and MinMax observers
- Test on random tensors with various distributions
- Test on real model weights from transformers
- Add 'slow' pytest marker to pyproject.toml for long-running tests

Author: Avishek Goswami

pyproject.toml

@@ -23,5 +23,6 @@ markers = [
     "unit: tests to ensure code correctness and regression test functionality",
     "example: tests for content in the 'examples' folder",
     "multi_gpu: tests that require multiple GPUs",
+    "slow: tests that take a long time to run (e.g., downloading models)",
 ]
 tmp_path_retention_policy = "failed"

tests/llmcompressor/observers/test_mse_vs_minmax.py

@@ -0,0 +1,309 @@
+"""
+Test to verify that MSE observer performs equal to or better than MinMax observer
+on various tensor distributions, including normal distributions (similar to real weights)
+and actual model weights.
+
+This test checks that the quantization error (MSE) from using MSE observer
+is less than or equal to the error from using MinMax observer.
+"""
+
+import pytest
+import torch
+from compressed_tensors.quantization import fake_quantize
+from compressed_tensors.quantization.quant_args import QuantizationArgs
+
+from llmcompressor.observers import Observer
+
+
+def _create_base_quantization_args(num_bits, strategy, symmetric, group_size):
+    """Helper to create base QuantizationArgs without observer field."""
+    return QuantizationArgs(
+        num_bits=num_bits,
+        strategy=strategy,
+        symmetric=symmetric,
+        group_size=group_size,
+    )
+
+
+def _run_observer_test(tensor, observer_name, strategy, symmetric, num_bits, group_size, module=None):
+    """
+    Helper function to run observer and compute quantization error.
+    
+    Returns: (scale, zero_point, quantized_tensor, mse, global_scale)
+    """
+    weights = _create_base_quantization_args(num_bits, strategy, symmetric, group_size)
+    weights.observer = observer_name
+    
+    observer = Observer.load_from_registry(
+        observer_name, base_name="weight", args=weights, module=module
+    )
+    
+    global_scale = None
+    if strategy == "tensor_group" and module is not None:
+        global_scale = observer.get_global_scale(tensor)
+        module.weight_global_scale = global_scale
+    
+    scale, zero_point = observer(tensor)
+    
+    # Sanity check: scales should be non-negative
+    assert (scale >= 0).all(), "Scale values should be non-negative"
+    
+    weights_clean = _create_base_quantization_args(num_bits, strategy, symmetric, group_size)
+    quantized = fake_quantize(
+        tensor, scale, zero_point, weights_clean,
+        global_scale=global_scale if strategy == "tensor_group" else None
+    )
+    mse = torch.nn.functional.mse_loss(quantized, tensor)
+    
+    return scale, zero_point, quantized, mse, global_scale
+
+
+def _assert_mse_comparison(mse_mse, minmax_mse, strategy, symmetric, is_real_weights=False):
+    """
+    Assert MSE observer performance with appropriate slack.
+    
+    For tensor+symmetric: strict assertion (MSE should be better)
+    For others: allow slack (10% for synthetic, 20% for real weights)
+    Also add epsilon to handle cases where minmax_mse is near 0.
+    """
+    epsilon = 1e-8
+    slack = 1.20 if is_real_weights else 1.10
+    
+    if strategy == "tensor" and symmetric:
+        # Cases where MSE SHOULD be better
+        assert mse_mse <= minmax_mse + epsilon, (
+            f"MSE observer performed worse than MinMax observer!\n"
+            f"Strategy: {strategy}, Symmetric: {symmetric}\n"
+            f"MinMax MSE: {minmax_mse.item():.6e}\n"
+            f"MSE Observer MSE: {mse_mse.item():.6e}\n"
+            f"Difference: {(mse_mse - minmax_mse).item():.6e}"
+        )
+    else:
+        # Not guaranteed, but ensure not catastrophically worse
+        assert mse_mse <= minmax_mse * slack + epsilon, (
+            f"MSE observer performed significantly worse than MinMax observer!\n"
+            f"Strategy: {strategy}, Symmetric: {symmetric}\n"
+            f"MinMax MSE: {minmax_mse.item():.6e}\n"
+            f"MSE Observer MSE: {mse_mse.item():.6e}\n"
+            f"Difference: {(mse_mse - minmax_mse).item():.6e}\n"
+            f"Ratio: {(mse_mse / (minmax_mse + epsilon)).item():.4f}x"
+        )
+
+
+@pytest.mark.parametrize(
+    "strategy,symmetric,num_bits",
+    [
+        ("tensor", True, 8),
+        ("tensor", False, 8),
+        ("channel", True, 8),
+        ("channel", False, 8),
+        ("tensor_group", True, 4),
+        ("tensor_group", False, 4),
+        ("channel", True, 4),
+        ("channel", False, 4),
+    ],
+)
+@pytest.mark.parametrize(
+    "std",
+    [0.05, 0.2, 1.0],
+    ids=["narrow", "medium", "wide"],
+)
+def test_mse_vs_minmax_on_random_tensor(strategy, symmetric, num_bits, std):
+    """
+    Test that MSE observer produces quantization error <= MinMax observer
+    on random tensors with normal distribution (similar to real model weights).
+    
+    Real model weights typically follow a normal distribution with:
+    - Mean near 0
+    - Standard deviation around 0.02-0.1 for initialized weights
+    - Range roughly [-0.5, 0.5] for most layers
+    
+    Testing with different std values exposes cases where MinMax performs poorly
+    on wide or heavy-tailed distributions, where MSE should shine.
+    """
+    # Generate random tensor with normal distribution similar to real weights
+    torch.manual_seed(42)
+    # Use different std values to test various distribution widths
+    tensor = torch.randn(128, 256) * std  # Normal distribution with specified std
+    
+    group_size = 32 if strategy == "tensor_group" else None
+    
+    # Create separate modules for tensor_group to avoid shared mutable state
+    module_minmax = None
+    module_mse = None
+    if strategy == "tensor_group":
+        module_minmax = torch.nn.Linear(256, 128)
+        module_minmax.weight.data = tensor.T
+        module_mse = torch.nn.Linear(256, 128)
+        module_mse.weight.data = tensor.T
+    
+    # Test with MinMax observer
+    _, _, _, minmax_mse, _ = _run_observer_test(
+        tensor, "memoryless_minmax", strategy, symmetric, num_bits, group_size, module_minmax
+    )
+    
+    # Test with MSE observer
+    _, _, _, mse_mse, _ = _run_observer_test(
+        tensor, "memoryless_mse", strategy, symmetric, num_bits, group_size, module_mse
+    )
+    
+    # Assert with appropriate slack for synthetic data
+    _assert_mse_comparison(mse_mse, minmax_mse, strategy, symmetric, is_real_weights=False)
+
+
+@pytest.mark.parametrize(
+    "tensor_shape",
+    [
+        (64, 128),
+        (128, 256),
+        (256, 512),
+        (32, 64, 128),  # 3D tensor
+    ],
+)
+def test_mse_vs_minmax_various_shapes(tensor_shape):
+    """
+    Test MSE vs MinMax on tensors of various shapes with normal distribution.
+    Uses realistic weight distribution parameters.
+    """
+    torch.manual_seed(42)
+    # Use realistic weight distribution: mean=0, std=0.05
+    tensor = torch.randn(*tensor_shape) * 0.05
+    
+    # MinMax
+    _, _, _, minmax_mse, _ = _run_observer_test(
+        tensor, "memoryless_minmax", "channel", True, 8, None, None
+    )
+    
+    # MSE
+    _, _, _, mse_mse, _ = _run_observer_test(
+        tensor, "memoryless_mse", "channel", True, 8, None, None
+    )
+    
+    # Channel quantization: MSE not guaranteed better, allow 10% slack
+    _assert_mse_comparison(mse_mse, minmax_mse, "channel", True, is_real_weights=False)
+
+
+def test_mse_vs_minmax_extreme_values():
+    """Test MSE vs MinMax on tensors with extreme values."""
+    torch.manual_seed(42)
+    
+    # Test with very small values
+    tensor_small = torch.randn(64, 128) * 0.01
+    # Test with very large values
+    tensor_large = torch.randn(64, 128) * 100.0
+    # Test with skewed distribution
+    tensor_skewed = torch.cat([
+        torch.randn(64, 100) * 0.1,
+        torch.randn(64, 28) * 10.0
+    ], dim=1)
+    
+    for tensor, name in [
+        (tensor_small, "small"),
+        (tensor_large, "large"),
+        (tensor_skewed, "skewed"),
+    ]:
+        weights = QuantizationArgs(
+            num_bits=8,
+            strategy="channel",
+            symmetric=True,
+            observer="memoryless_minmax",
+        )
+        
+        # MinMax
+        _, _, _, minmax_mse, _ = _run_observer_test(
+            tensor, "memoryless_minmax", "channel", True, 8, None, None
+        )
+        
+        # MSE
+        _, _, _, mse_mse, _ = _run_observer_test(
+            tensor, "memoryless_mse", "channel", True, 8, None, None
+        )
+        
+        # Channel quantization: MSE not guaranteed better, allow 10% slack
+        _assert_mse_comparison(mse_mse, minmax_mse, "channel", True, is_real_weights=False)
+
+
+@pytest.mark.slow
+@pytest.mark.parametrize(
+    "strategy,symmetric,num_bits",
+    [
+        ("channel", True, 8),
+        ("channel", False, 8),
+        ("tensor_group", True, 4),
+        ("tensor_group", False, 4),
+    ],
+)
+def test_mse_vs_minmax_on_real_model_weights(strategy, symmetric, num_bits):
+    """
+    Test that MSE observer produces quantization error <= MinMax observer
+    on actual model weights from a real neural network.
+    
+    This test loads weights from a small model to verify observer behavior
+    on real weight distributions, which may differ from synthetic data.
+    """
+    try:
+        from transformers import AutoModelForCausalLM
+    except ImportError:
+        pytest.skip("transformers not available")
+
+    # Use a small, publicly available model for testing
+    model_id = "nm-testing/tinysmokellama-3.2"
+    
+    try:
+        # Load model and extract a weight tensor
+        # Use no_grad context to avoid unnecessary gradient computation
+        with torch.no_grad():
+            model = AutoModelForCausalLM.from_pretrained(
+                model_id, torch_dtype=torch.float32
+            )
+            
+            # Get a representative weight tensor (e.g., from first Linear layer)
+            weight_tensor = None
+            for name, module in model.named_modules():
+                if isinstance(module, torch.nn.Linear) and weight_tensor is None:
+                    weight_tensor = module.weight.data.clone()
+                    break
+        
+        if weight_tensor is None:
+            pytest.skip("No Linear layer found in model")
+        
+        # Flatten or reshape to 2D if needed for testing
+        if weight_tensor.dim() > 2:
+            weight_tensor = weight_tensor.view(-1, weight_tensor.shape[-1])
+        elif weight_tensor.dim() == 1:
+            weight_tensor = weight_tensor.unsqueeze(0)
+        
+        # Limit size for faster testing
+        if weight_tensor.shape[0] > 512:
+            weight_tensor = weight_tensor[:512, :]
+        if weight_tensor.shape[1] > 512:
+            weight_tensor = weight_tensor[:, :512]
+        
+    except Exception as e:
+        pytest.skip(f"Could not load model {model_id}: {e}")
+    
+    group_size = 32 if strategy == "tensor_group" else None
+    
+    # Create separate modules for tensor_group to avoid shared mutable state
+    module_minmax = None
+    module_mse = None
+    if strategy == "tensor_group":
+        module_minmax = torch.nn.Linear(weight_tensor.shape[1], weight_tensor.shape[0])
+        module_minmax.weight.data = weight_tensor.T
+        module_mse = torch.nn.Linear(weight_tensor.shape[1], weight_tensor.shape[0])
+        module_mse.weight.data = weight_tensor.T
+    
+    # Test with MinMax observer
+    _, _, _, minmax_mse, _ = _run_observer_test(
+        weight_tensor, "memoryless_minmax", strategy, symmetric, num_bits, group_size, module_minmax
+    )
+    
+    # Test with MSE observer
+    _, _, _, mse_mse, _ = _run_observer_test(
+        weight_tensor, "memoryless_mse", strategy, symmetric, num_bits, group_size, module_mse
+    )
+    
+    # For channel and tensor_group strategies, MSE is not guaranteed to be better
+    # Allow 20% slack for real model weights (more structure & extreme channels)
+    _assert_mse_comparison(mse_mse, minmax_mse, strategy, symmetric, is_real_weights=True)
+