[MLX] Size the attention KV pool at the compute dtype for quantized models (#30097)

Co-authored-by: siming-deng <deng_siming@apple.com>

python/sglang/srt/hardware_backend/mlx/model_runner.py

@@ -512,9 +512,17 @@ class MlxModelRunner:
         if hasattr(sample_attn, "k_proj") and hasattr(sample_attn.k_proj, "weight"):
             dtype = sample_attn.k_proj.weight.dtype
         if dtype not in _MLX_KV_FLOAT_DTYPES:
-            # QuantizedLinear stores packed weights as integers, while the KV
-            # cache stores dequantized projection outputs.
-            dtype = mx.float32
+            # QuantizedLinear packs weights as integers, but the KV cache
+            # stores dequantized projection outputs, which are produced in
+            # the compute dtype carried by the quantization scales.  Storing
+            # at that dtype instead of float32 halves pool bytes per slot
+            # and keeps prefix-hit forwards in the same dtype as the no-hit
+            # path (a float32 pool promoted every post-hit concat).
+            scales = getattr(sample_attn.k_proj, "scales", None)
+            if scales is not None and scales.dtype in _MLX_KV_FLOAT_DTYPES:
+                dtype = scales.dtype
+            else:
+                dtype = mx.float32
         return n_kv_heads, head_dim, dtype
 
     def _get_attn_config(self) -> tuple[int, int, mx.Dtype]:

test/registered/unit/hardware_backend/mlx/test_attention_patching.py

@@ -1149,6 +1149,7 @@ class TestMlxOverlapScheduler(unittest.TestCase):
         self.assertTrue(torch.equal(schedule_batch.input_ids, token_ids))
         self.assertIs(scheduler.processed_batch, batch_copy)
         self.assertIs(scheduler.processed_result, scheduler.tp_worker.result)
+        self.assertEqual(scheduler.forward_ct, 1)
 
     def test_overlap_loop_materializes_prefill_input_ids(self):
         # Regression: the MLX overlap loop must materialize batch.input_ids

test/registered/unit/hardware_backend/mlx/test_mlx_pool_dtype.py

@@ -0,0 +1,121 @@
+"""Unit tests for MLX attention-KV pool dtype inference.
+
+The shared pool stores dequantized projection outputs, so its dtype must
+follow the model's compute dtype, not the packed integer dtype of
+QuantizedLinear weights.  Storing float32 for quantized models (the old
+fallback) doubled pool bytes per slot for no precision gain — the source
+values are bf16/fp16 — and made prefix-hit forwards run in float32 (the
+pool gather promoted the concat) while no-hit forwards ran in the compute
+dtype.  These tests pin the inference rules:
+
+- unquantized projections: weight dtype (unchanged behavior);
+- quantized projections: the quantization ``scales`` dtype (== compute
+  dtype);
+- quantized without usable scales: float32 (the conservative fallback).
+"""
+
+from __future__ import annotations
+
+import importlib.util
+import unittest
+
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=4, suite="base-a-test-cpu")
+
+_HAS_MLX = (
+    importlib.util.find_spec("mlx") is not None
+    and importlib.util.find_spec("mlx_lm") is not None
+)
+_SKIP_REASON = "requires mlx + mlx_lm"
+
+if _HAS_MLX:
+    import mlx.core as mx
+    import mlx.nn as nn
+    from mlx_lm.models import qwen2
+
+    from sglang.srt.hardware_backend.mlx.kv_cache import (
+        MlxModelCacheLayout,
+        find_attention_layers,
+    )
+    from sglang.srt.hardware_backend.mlx.model_runner import MlxModelRunner
+
+
+def _tiny_qwen2_model():
+    """Randomly initialized 2-layer dense qwen2 (plain full attention)."""
+    args = qwen2.ModelArgs(
+        model_type="qwen2",
+        hidden_size=64,
+        num_hidden_layers=2,
+        intermediate_size=128,
+        num_attention_heads=4,
+        num_key_value_heads=2,
+        rms_norm_eps=1e-6,
+        vocab_size=128,
+        rope_theta=10000.0,
+    )
+    return qwen2.Model(args)
+
+
+def _runner_for(model):
+    layers, attrs = find_attention_layers(model)
+    runner = MlxModelRunner.__new__(MlxModelRunner)
+    runner._cache_layout = MlxModelCacheLayout.from_attention_discovery(layers, attrs)
+    return runner
+
+
+@unittest.skipUnless(_HAS_MLX, _SKIP_REASON)
+class TestPoolDtypeInference(CustomTestCase):
+    def test_unquantized_model_uses_weight_dtype(self):
+        model = _tiny_qwen2_model()
+        model.set_dtype(mx.float16)
+        _, _, dtype = _runner_for(model)._get_attn_config()
+        self.assertEqual(dtype, mx.float16)
+
+    def test_quantized_model_uses_scales_dtype(self):
+        model = _tiny_qwen2_model()
+        model.set_dtype(mx.bfloat16)
+        nn.quantize(model, group_size=64, bits=4)
+        attn = model.model.layers[0].self_attn
+        self.assertNotIn(attn.k_proj.weight.dtype, {mx.float16, mx.bfloat16})
+        self.assertEqual(attn.k_proj.scales.dtype, mx.bfloat16)
+        _, _, dtype = _runner_for(model)._get_attn_config()
+        self.assertEqual(dtype, mx.bfloat16)
+
+    def test_quantized_fp16_model_uses_scales_dtype(self):
+        model = _tiny_qwen2_model()
+        model.set_dtype(mx.float16)
+        nn.quantize(model, group_size=64, bits=4)
+        _, _, dtype = _runner_for(model)._get_attn_config()
+        self.assertEqual(dtype, mx.float16)
+
+    def test_quantized_without_usable_scales_falls_back_to_float32(self):
+        model = _tiny_qwen2_model()
+        model.set_dtype(mx.bfloat16)
+        nn.quantize(model, group_size=64, bits=4)
+        for layer in model.model.layers:
+            # Simulate a packed layer whose scales are not a float array
+            # (e.g. an exotic quant format): the conservative fallback must
+            # hold.
+            layer.self_attn.k_proj.scales = layer.self_attn.k_proj.scales.astype(
+                mx.uint32
+            )
+        _, _, dtype = _runner_for(model)._get_attn_config()
+        self.assertEqual(dtype, mx.float32)
+
+    def test_pool_bytes_per_slot_halves_for_bf16_quantized_model(self):
+        # The practical effect: bytes/slot uses dtype.size, so bf16 halves
+        # the fp32 fallback and the auto-sized pool fits ~2x the tokens.
+        model = _tiny_qwen2_model()
+        model.set_dtype(mx.bfloat16)
+        nn.quantize(model, group_size=64, bits=4)
+        n_kv_heads, head_dim, dtype = _runner_for(model)._get_attn_config()
+        num_layers = 2
+        bytes_per_slot = 2 * num_layers * n_kv_heads * head_dim * dtype.size
+        fp32_bytes_per_slot = 2 * num_layers * n_kv_heads * head_dim * 4
+        self.assertEqual(bytes_per_slot * 2, fp32_bytes_per_slot)
+
+
+if __name__ == "__main__":
+    unittest.main()