[Mamba] Add stochastic rounding support (#35753)

Signed-off-by: Roi Koren <roik@nvidia.com>

tests/kernels/mamba/test_mamba_ssm.py

@@ -12,6 +12,7 @@ from vllm.model_executor.layers.mamba.ops.mamba_ssm import (
     selective_scan_fn,
     selective_state_update,
 )
+from vllm.platforms import current_platform
 from vllm.utils.torch_utils import set_random_seed
 from vllm.v1.attention.backends.utils import PAD_SLOT_ID
 
@@ -429,6 +430,59 @@ def test_selective_state_update(dim, dstate, has_z, itype):
     assert torch.allclose(out, out_ref, rtol=rtol, atol=atol)
 
 
+@pytest.mark.parametrize("philox_rounds", [0, 4])
+@pytest.mark.parametrize("has_z", [False, True])
+@pytest.mark.parametrize("dstate", [16, 64])
+@pytest.mark.parametrize("dim", [2048, 4096])
+@pytest.mark.skipif(
+    not (
+        current_platform.is_cuda() and current_platform.is_device_capability_family(100)
+    ),
+    reason="Stochastic rounding in triton is only supported"
+    " on compute capability 10.0 CUDA devices.",
+)
+def test_selective_state_update_stochastic_rounding(dim, dstate, has_z, philox_rounds):
+    device = "cuda"
+    rtol, atol = 5e-3, 1e-1
+    # set seed
+    set_random_seed(0)
+    batch_size = 1
+    state = torch.randn(batch_size, dim, dstate, dtype=torch.float16, device=device)
+    x = torch.randn(batch_size, dim, device=device, dtype=torch.bfloat16)
+    out = torch.empty_like(x)
+    dt = torch.randn(batch_size, dim, device=device, dtype=torch.bfloat16)
+    dt_bias = torch.rand(dim, device=device) - 4.0
+    A = -torch.rand(dim, dstate, device=device) - 1.0
+    B = torch.randn(batch_size, dstate, device=device)
+    C = torch.randn(batch_size, dstate, device=device)
+    D = torch.randn(dim, device=device)
+    z = torch.randn_like(x) if has_z else None
+    # Reference uses fp32 state to get ground truth
+    state_ref = state.float()
+    selective_state_update(
+        state,
+        x,
+        dt,
+        A,
+        B,
+        C,
+        D=D,
+        z=z,
+        dt_bias=dt_bias,
+        dt_softplus=True,
+        out=out,
+        enable_stochastic_rounding=True,
+        cache_philox_rounds=philox_rounds,
+    )
+    out_ref = selective_state_update_ref(
+        state_ref, x, dt, A, B, C, D=D, z=z, dt_bias=dt_bias, dt_softplus=True
+    )
+
+    assert state.dtype == torch.float16
+    assert torch.allclose(state, state_ref.to(torch.float16), rtol=rtol, atol=atol)
+    assert torch.allclose(out, out_ref, rtol=rtol, atol=atol)
+
+
 @pytest.mark.parametrize("itype", [torch.float32, torch.bfloat16])
 @pytest.mark.parametrize("has_z", [False, True])
 @pytest.mark.parametrize("dstate", [16, 64])

vllm/config/cache.py

@@ -115,6 +115,14 @@ class CacheConfig:
     - "align": only cache the mamba state of the last token of each scheduler step and
            when the token is at position i * block_size.
     """
+    enable_mamba_cache_stochastic_rounding: bool = False
+    """Enable stochastic rounding when writing SSM state to fp16 cache.
+    Uses random bits to unbias the rounding error, which can improve
+    numerical stability for long sequences."""
+    mamba_cache_philox_rounds: int = 0
+    """Number of Philox PRNG rounds for stochastic rounding random number
+    generation. 0 uses the Triton default. Higher values improve randomness
+    quality at the cost of compute."""
 
     # Will be set after profiling.
     num_gpu_blocks: int | None = field(default=None, init=False)
@@ -231,3 +239,29 @@ class CacheConfig:
                 "scaling factor."
             )
         return cache_dtype
+
+    def __post_init__(self):
+        if self.enable_mamba_cache_stochastic_rounding:
+            from vllm.platforms import current_platform
+
+            if not current_platform.is_cuda():
+                raise ValueError(
+                    "Stochastic rounding for Mamba cache is only supported "
+                    "on NVIDIA CUDA platforms. Please do not specify  "
+                    "`--enable-mamba-cache-stochastic-rounding`."
+                )
+            if not current_platform.is_device_capability_family(100):
+                raise ValueError(
+                    "Stochastic rounding for Mamba cache requires compute "
+                    "capability 10.0 (data center Blackwell). The `cvt.rs` PTX "
+                    "instruction is not supported on your GPU. Please do not specify "
+                    "`--enable-mamba-cache-stochastic-rounding`."
+                )
+            if self.mamba_ssm_cache_dtype != "float16":
+                raise ValueError(
+                    "Stochastic rounding for Mamba cache requires "
+                    "the SSM cache to be float16. Please set it explicitly, "
+                    "by specifying `--mamba-ssm-cache-dtype float16`, or disable "
+                    "stochastic rounding by not specifying "
+                    "`--enable-mamba-cache-stochastic-rounding`."
+                )

vllm/engine/arg_utils.py

@@ -604,6 +604,10 @@ class EngineArgs:
     mamba_ssm_cache_dtype: MambaDType = CacheConfig.mamba_ssm_cache_dtype
     mamba_block_size: int | None = get_field(CacheConfig, "mamba_block_size")
     mamba_cache_mode: MambaCacheMode = CacheConfig.mamba_cache_mode
+    enable_mamba_cache_stochastic_rounding: bool = (
+        CacheConfig.enable_mamba_cache_stochastic_rounding
+    )
+    mamba_cache_philox_rounds: int = CacheConfig.mamba_cache_philox_rounds
 
     additional_config: dict[str, Any] = get_field(VllmConfig, "additional_config")
 
@@ -1024,6 +1028,13 @@ class EngineArgs:
         cache_group.add_argument(
             "--mamba-cache-mode", **cache_kwargs["mamba_cache_mode"]
         )
+        cache_group.add_argument(
+            "--enable-mamba-cache-stochastic-rounding",
+            **cache_kwargs["enable_mamba_cache_stochastic_rounding"],
+        )
+        cache_group.add_argument(
+            "--mamba-cache-philox-rounds", **cache_kwargs["mamba_cache_philox_rounds"]
+        )
         cache_group.add_argument(
             "--kv-offloading-size", **cache_kwargs["kv_offloading_size"]
         )
@@ -1590,6 +1601,8 @@ class EngineArgs:
             mamba_ssm_cache_dtype=self.mamba_ssm_cache_dtype,
             mamba_block_size=self.mamba_block_size,
             mamba_cache_mode=self.mamba_cache_mode,
+            enable_mamba_cache_stochastic_rounding=self.enable_mamba_cache_stochastic_rounding,
+            mamba_cache_philox_rounds=self.mamba_cache_philox_rounds,
             kv_offloading_size=self.kv_offloading_size,
             kv_offloading_backend=self.kv_offloading_backend,
         )

vllm/model_executor/layers/mamba/mamba_mixer.py

@@ -428,6 +428,8 @@ class MambaMixer(MambaBase, PluggableLayer):
                 state_batch_indices=state_indices_tensor_d_input,
                 dst_state_batch_indices=state_indices_tensor_d_output,
                 out=scan_outputs_d,
+                enable_stochastic_rounding=self.cache_config.enable_mamba_cache_stochastic_rounding,
+                cache_philox_rounds=self.cache_config.mamba_cache_philox_rounds,
             )
             scan_outputs_d = scan_outputs_d.transpose(0, 1)
 

vllm/model_executor/layers/mamba/mamba_mixer2.py

@@ -888,6 +888,8 @@ class MambaMixer2(MambaBase, PluggableLayer):
                 num_accepted_tokens=num_accepted_tokens,
                 cu_seqlens=query_start_loc_d,
                 is_blackwell=self.is_blackwell,
+                enable_stochastic_rounding=self.cache_config.enable_mamba_cache_stochastic_rounding,
+                cache_philox_rounds=self.cache_config.mamba_cache_philox_rounds,
             )
 
     def get_state_dtype(self) -> tuple[torch.dtype, torch.dtype]:

vllm/model_executor/layers/mamba/ops/mamba_ssm.py

@@ -28,6 +28,21 @@ else:
         return dt
 
 
+@triton.jit
+def convert_rs_fp16x2(x: tl.tensor, rand: tl.tensor) -> tl.tensor:
+    y = tl.inline_asm_elementwise(
+        asm="""{
+cvt.rs.f16x2.f32 $0, $2, $1, $3;
+}""",
+        constraints="=r,r,r,r,r",
+        args=(x, rand),
+        dtype=tl.float16,
+        is_pure=True,
+        pack=2,
+    )
+    return y
+
+
 @triton.heuristics({"HAS_DT_BIAS": lambda args: args["dt_bias_ptr"] is not None})
 @triton.heuristics({"HAS_D": lambda args: args["D_ptr"] is not None})
 @triton.heuristics({"HAS_Z": lambda args: args["z_ptr"] is not None})
@@ -48,6 +63,7 @@ else:
 def _selective_scan_update_kernel(
     # Pointers to matrices
     state_ptr,
+    rand_seed_ptr,
     x_ptr,
     dt_ptr,
     dt_bias_ptr,
@@ -113,6 +129,8 @@ def _selective_scan_update_kernel(
     IS_SPEC_DECODING: tl.constexpr,
     IS_VARLEN: tl.constexpr,
     BLOCK_SIZE_DSTATE: tl.constexpr,
+    USE_RS_ROUNDING: tl.constexpr,
+    PHILOX_ROUNDS: tl.constexpr,
 ):
     pid_m = tl.program_id(axis=0)
     pid_b = tl.program_id(axis=1)
@@ -267,7 +285,35 @@ def _selective_scan_update_kernel(
             z_ptr += stride_z_batch
 
     if not IS_SPEC_DECODING:
-        tl.store(dst_state_ptrs, state.to(dst_state_ptrs.dtype.element_ty), mask=mask)
+        if USE_RS_ROUNDING:
+            # Load random seed
+            rand_seed = tl.load(rand_seed_ptr)
+            # Generate random offsets for each element in state
+            if HAS_STATE_BATCH_INDICES:
+                rand_offsets = (
+                    state_batch_idx * stride_state_batch + pid_h * stride_state_head
+                )
+            else:
+                rand_offsets = pid_b * stride_state_batch + pid_h * stride_state_head
+            rand_offsets += (
+                offs_m[:, None] * stride_state_dim
+                + offs_n[None, :] * stride_state_dstate
+            )
+            # Generate random 32-bits for each element in state
+            if PHILOX_ROUNDS > 0:
+                rand = tl.randint(rand_seed, rand_offsets, PHILOX_ROUNDS)
+            else:
+                rand = tl.randint(rand_seed, rand_offsets)
+            # Convert state to fp16 with RS rounding
+            state = convert_rs_fp16x2(state, rand)
+            tl.static_assert(state.dtype == tl.float16, "state must be fp16")
+            tl.static_assert(
+                dst_state_ptrs.dtype.element_ty == tl.float16,
+                "dst_state_ptrs must be fp16",
+            )
+        else:
+            state = state.to(dst_state_ptrs.dtype.element_ty)
+        tl.store(dst_state_ptrs, state, mask=mask)
 
 
 def selective_state_update(
@@ -288,6 +334,8 @@ def selective_state_update(
     num_accepted_tokens=None,
     cu_seqlens=None,
     is_blackwell=False,
+    enable_stochastic_rounding=False,
+    cache_philox_rounds=0,
 ):
     """
     Argument:
@@ -419,9 +467,16 @@ def selective_state_update(
         and dt.stride(-1) == 0
         and dt_bias.stride(-1) == 0
     )
+    rand_seed = (
+        torch.randint(0, 2**32, (1,), device=state.device)
+        if enable_stochastic_rounding
+        else None
+    )
+
     with torch.accelerator.device_index(x.device.index):
         _selective_scan_update_kernel[grid](
             state,
+            rand_seed,
             x,
             dt,
             dt_bias,
@@ -476,6 +531,8 @@ def selective_state_update(
             tie_hdim,
             BLOCK_SIZE_M,
             num_warps=num_warps,
+            USE_RS_ROUNDING=enable_stochastic_rounding,
+            PHILOX_ROUNDS=cache_philox_rounds,
         )
 
 

vllm/model_executor/models/plamo2.py

@@ -445,6 +445,8 @@ class Plamo2MambaMixer(MambaBase, PluggableLayer):
                 dt_softplus=True,
                 state_batch_indices=state_indices_tensor_d,
                 out=preallocated_ssm_out_d.view(num_decodes, -1, self.head_dim),
+                enable_stochastic_rounding=self.cache_config.enable_mamba_cache_stochastic_rounding,
+                cache_philox_rounds=self.cache_config.mamba_cache_philox_rounds,
             )
 
         # 4. Final linear projection