fused_moe_modular_kernel.md

# Fused MoE Modular Kernel

## Introduction

FusedMoEModularKernel is implemented [here](../../vllm/model_executor/layers/fused_moe/modular_kernel.py)

Based on the format of the input activations, FusedMoE implementations are broadly classified into 2 types.

* Contiguous / Standard / Non-Batched, and
* Batched

!!! note
    The terms Contiguous, Standard, and Non-Batched are used interchangeably throughout the document.

The input activation format completely depends on the All2All Dispatch being used.

* In the Contiguous variant, the All2All Dispatch returns the activations as a contiguous tensor of shape (M, K) along with TopK Ids and TopK weights of shape (M, num_topk). Look at `DeepEPHTPrepareAndFinalize` for an example.
* In the Batched variant, the All2All Dispatch returns the activations as a tensor of shape (num_experts, max_tokens, K). Here, the activations/tokens that subscribe to the same expert are batched together. Note that not all entries of the tensor are valid. The activations tensor is typically accompanied by an `expert_num_tokens` tensor of size `num_experts`, where `expert_num_tokens[i]` indicates the number of valid tokens that subscribe to the ith expert. Look at `DeepEPLLPrepareAndFinalize` for an example.

The FusedMoE operation is generally made of multiple operations, in both the Contiguous and Batched variants, as described in the diagrams below

![FusedMoE Non-Batched](../assets/design/fused_moe_modular_kernel/fused_moe_non_batched.png)

![FusedMoE Batched](../assets/design/fused_moe_modular_kernel/fused_moe_batched.png)

!!! note
    The main difference, in terms of operations, between the Batched and Non-Batched cases is the Permute / Unpermute operations. All other operations remain.

## Motivation

As can be seen from the diagrams, there are a lot of operations and there can be a variety of implementations for each operation. The set of ways the operations can be put together to make a valid FusedMoE implementation quickly becomes intractable. The Modular Kernel framework addresses this issue,  by grouping the operations into logical components. This broad categorization makes the combinations manageable and prevents code-duplication. This also decouples the All2All Dispatch & Combine implementations from the FusedMoE implementations and allows for their independent development and testing. Furthermore, the Modular Kernel framework introduces Abstract classes for the different components thus providing a well-defined skeleton for future implementations.

The rest of the document will focus on the Contiguous / Non-Batched case. Extrapolating to the Batched case should be straight-forward.

## ModularKernel Components

FusedMoEModularKernel splits the FusedMoE operation into 3 parts,

1. TopKWeightAndReduce
2. FusedMoEPrepareAndFinalize
3. FusedMoEPermuteExpertsUnpermute

### TopKWeightAndReduce

The TopK Weight Application and Reduction components happen right after the Unpermute operation and before the All2All Combine. Note that the `FusedMoEPermuteExpertsUnpermute` is responsible for the Unpermute and `FusedMoEPrepareAndFinalize` is responsible for the All2All Combine. There is value in doing the TopK Weight Application and Reduction in the `FusedMoEPermuteExpertsUnpermute`. But some implementations choose to do it `FusedMoEPrepareAndFinalize`. In order to enable this flexibility, we have a TopKWeightAndReduce abstract class.

Please find the implementations of TopKWeightAndReduce [here](../../vllm/model_executor/layers/fused_moe/topk_weight_and_reduce.py).

`FusedMoEPrepareAndFinalize::finalize()` method accepts a `TopKWeightAndReduce` argument that is invoked inside the method.
The `FusedMoEModularKernel` acts as a bridge between the `FusedMoEPermuteExpertsUnpermute` and `FusedMoEPerpareAndFinalize` implementations to determine where the TopK Weight Application and Reduction happens.

* `FusedMoEPermuteExpertsUnpermute::finalize_weight_and_reduce_impl` method returns `TopKWeightAndReduceNoOp` if the `FusedMoEPermuteExpertsUnpermute` implementation does the weight application and reduction itself.
* `FusedMoEPermuteExpertsUnpermute::finalize_weight_and_reduce_impl` method returns `TopKWeightAndReduceContiguous` / `TopKWeightAndReduceNaiveBatched` / `TopKWeightAndReduceDelegate` if the `FusedMoEPermuteExpertsUnpermute` implementation needs the `FusedMoEPrepareAndFinalize::finalize()` to do the weight application and reduction.

### FusedMoEPrepareAndFinalize

The `FusedMoEPrepareAndFinalize` abstract class exposes `prepare`, `prepare_no_receive`  and `finalize` functions.
The `prepare` function is responsible for input activation Quantization and All2All Dispatch. If implemented, The `prepare_no_receive` is like `prepare` except it does not wait to receive results from other workers.  Instead it returns a "receiver" callback that must be invoked to wait for the final results of worker. It is not required that this method is supported by all `FusedMoEPrepareAndFinalize` classes, but if it is available, it can be used to interleave work with the initial all to all communication, e.g. interleaving shared experts with fused experts.  The `finalize` function is responsible for invoking the All2All Combine. Additionally the `finalize` function may or may not do the TopK weight application and reduction (Please refer to the TopKWeightAndReduce section)

![FusedMoEPrepareAndFinalize Blocks](../assets/design/fused_moe_modular_kernel/prepare_and_finalize_blocks.png)

### FusedMoEPermuteExpertsUnpermute

The `FusedMoEPermuteExpertsUnpermute` class is where the crux of the MoE operations happen. The `FusedMoEPermuteExpertsUnpermute` abstract class exposes a few important functions,

* apply()
* workspace_shapes()
* finalize_weight_and_reduce_impl()

#### apply()

The `apply` method is where the implementations perform

* Permute
* Matmul with weight W1
* Act + Mul
* Quantization
* Matmul with weight W2
* Unpermute
* Maybe TopK Weight Application + Reduction

#### workspace_shapes()

The core FusedMoE implementation performs a series of operations. It would be inefficient to create output memory for each of these operations separately. To that effect, implementations are required to declare 2 workspace shapes, the workspace datatype and the FusedMoE output shape as outputs of the workspace_shapes() method. This information is used to allocate the workspace tensors and the output tensor in `FusedMoEModularKernel::forward()` and passed on to the `FusedMoEPermuteExpertsUnpermute::apply()` method. The workspaces could then be used as intermediate buffers in the FusedMoE implementation.

#### finalize_weight_and_reduce_impl()

It is sometimes efficient to perform TopK weight application and Reduction inside the `FusedMoEPermuteExpertsUnpermute::apply()`. Find an example [here](https://github.com/vllm-project/vllm/pull/20228). We have a `TopKWeightAndReduce` abstract class to facilitate such implementations. Please refer to the TopKWeightAndReduce section.
`FusedMoEPermuteExpertsUnpermute::finalize_weight_and_reduce_impl()` returns the `TopKWeightAndReduce` object that the implementation wants the `FusedMoEPrepareAndFinalize::finalize()` to use.

![FusedMoEPermuteExpertsUnpermute Blocks](../assets/design/fused_moe_modular_kernel/fused_experts_blocks.png)

### FusedMoEModularKernel

`FusedMoEModularKernel` is composed of the `FusedMoEPrepareAndFinalize` and `FusedMoEPermuteExpertsUnpermute` objects.
`FusedMoEModularKernel` pseudocode/sketch,

```py
class FusedMoEModularKernel:
    def __init__(self,
                 prepare_finalize: FusedMoEPrepareAndFinalize,
                 fused_experts: FusedMoEPermuteExpertsUnpermute):

        self.prepare_finalize = prepare_finalize
        self.fused_experts = fused_experts

    def forward(self, DP_A):

        Aq, A_scale, _, _, _ = self.prepare_finalize.prepare(DP_A, ...)

        workspace13_shape, workspace2_shape, _, _ = self.fused_experts.workspace_shapes(...)

        # allocate workspaces
        workspace_13 = torch.empty(workspace13_shape, ...)
        workspace_2 = torch.empty(workspace2_shape, ...)

        # execute fused_experts
        fe_out = self.fused_experts.apply(Aq, A_scale, workspace13, workspace2, ...)

        # war_impl is an object of type TopKWeightAndReduceNoOp if the fused_experts implementations
        # performs the TopK Weight Application and Reduction.
        war_impl = self.fused_experts.finalize_weight_and_reduce_impl()

        output = self.prepare_finalize.finalize(fe_out, war_impl,...)

        return output
```

## How-To

### How To Add a FusedMoEPrepareAndFinalize Type

Typically a FusedMoEPrepareAndFinalize type is backed by an All2All Dispatch & Combine implementation / kernel. For example,

* DeepEPHTPrepareAndFinalize type is backed by DeepEP High-Throughput All2All kernels, and
* DeepEPLLPrepareAndFinalize type is backed by DeepEP Low-Latency All2All kernels.

#### Step 1: Add an All2All manager

The purpose of the All2All Manager is to set up the All2All kernel implementations. The `FusedMoEPrepareAndFinalize` implementations typically fetch a kernel-implementation "handle" from the All2All Manager to invoke the Dispatch and Combine functions. Please look at the All2All Manager implementations [here](../../vllm/distributed/device_communicators/all2all.py).

#### Step 2: Add a FusedMoEPrepareAndFinalize Type

This section describes the significance of the various functions exposed by the `FusedMoEPrepareAndFinalize` abstract class.

`FusedMoEPrepareAndFinalize::prepare()`: The prepare method implements the Quantization and All2All Dispatch. Typically the Dispatch function from the relevant All2All Manager is invoked.

`FusedMoEPrepareAndFinalize::has_prepare_no_receive()`: Indicates whether or not this subclass implements `prepare_no_receive`. Defaults to False.

`FusedMoEPrepareAndFinalize::prepare_no_receive()`: The prepare_no_receive method implements the Quantization and All2All Dispatch. It does not wait for the result of the dispatch operation but instead returns a thunk that can be invoked to wait for the final results. Typically the Dispatch function from the relevant All2All Manager is invoked.

`FusedMoEPrepareAndFinalize::finalize()`: Maybe perform TopK Weight Application and Reduction and All2All Combine. Typically the Combine function from the relevant All2AllManager is invoked.

`FusedMoEPrepareAndFinalize::activation_format()`: Return `FusedMoEActivationFormat.BatchedExperts` if the output of the prepare method (i.e. the All2All dispatch) is Batched. Return `FusedMoEActivationFormat.Standard` otherwise.

`FusedMoEPrepareAndFinalize::topk_indices_dtype()`: Data type of the TopK ids. Some All2All kernels have strict requirements pertaining to the data type of the TopK ids. This requirement is passed on to the `FusedMoe::select_experts` function so it could be respected. If there are no strict requirements return None.

`FusedMoEPrepareAndFinalize::max_num_tokens_per_rank()`: This is the maximum number of tokens that would be submitted to the All2All Dispatch at once.

`FusedMoEPrepareAndFinalize::num_dispatchers()`: Total number of dispatching units. This value determines the size of the Dispatch output. The Dispatch output is of shape (num_local_experts, max_num_tokens, K). Here max_num_tokens = num_dispatchers() * max_num_tokens_per_rank().

We suggest picking an already existing `FusedMoEPrepareAndFinalize` implementation that matches your All2All implementation closely and using it as a reference.

### How To Add a FusedMoEPermuteExpertsUnpermute Type

FusedMoEPermuteExpertsUnpermute performs the core of the FusedMoE operations. The various functions exposed by the abstract class and their significance is as follows,

`FusedMoEPermuteExpertsUnpermute::activation_formats()`: Return the supported Input and Output activation formats. i.e. Contiguous / Batched format.

`FusedMoEPermuteExpertsUnpermute::supports_chunking()`: Return True if the implementation supports chunking. Typically
implementations that input `FusedMoEActivationFormat.Standard` support chunking and `FusedMoEActivationFormat.BatchedExperts` do not.

`FusedMoEPermuteExpertsUnpermute::supports_expert_map()`: Return True if the implementation supports expert map.

`FusedMoEPermuteExpertsUnpermute::workspace_shapes()` /
`FusedMoEPermuteExpertsUnpermute::finalize_weight_and_reduce_impl` /
`FusedMoEPermuteExpertsUnpermute::apply`: Refer to `FusedMoEPermuteExpertsUnpermute` section above.

### FusedMoEModularKernel Initialization

`FusedMoEMethodBase` class has 3 methods that are collectively responsible in creating the `FusedMoEModularKernel` object. They are,

* maybe_make_prepare_finalize,
* select_gemm_impl, and
* init_prepare_finalize

#### maybe_make_prepare_finalize

The `maybe_make_prepare_finalize` method is responsible for constructing an instance of `FusedMoEPrepareAndFinalize` when appropriate based on the current all2all backend, e.g. when EP + DP is enabled.  The base class method currently constructs all the `FusedMoEPrepareAndFinalize` objects for the EP+DP case.  Derived classes can override this method to construct prepare/finalize objects for different scenarios, e.g. `ModelOptNvFp4FusedMoE` can construct a `FlashInferCutlassMoEPrepareAndFinalize` for the EP+TP case.
Please refer to the implementations in,

* `ModelOptNvFp4FusedMoE`

#### select_gemm_impl

The `select_gemm_impl` method is undefined in the base class. It is the responsibility of the derived class to implement a method that constructs a valid/appropriate `FusedMoEPermuteExpertsUnpermute` object.
Please refer to the implementations in,

* `UnquantizedFusedMoEMethod`
* `CompressedTensorsW8A8Fp8MoEMethod`
* `CompressedTensorsW8A8Fp8MoECutlassMethod`
* `Fp8MoEMethod`
* `ModelOptNvFp4FusedMoE`
derived classes.

#### init_prepare_finalize

Based on the input and env settings, the `init_prepare_finalize` method creates the appropriate `FusedMoEPrepareAndFinalize` object. The method then queries `select_gemm_impl` for the appropriate `FusedMoEPermuteExpertsUnpermute` object and builds the `FusedMoEModularKernel` object

Please take a look at [init_prepare_finalize](https://github.com/vllm-project/vllm/blob/1cbf951ba272c230823b947631065b826409fa62/vllm/model_executor/layers/fused_moe/layer.py#L188).
**Important**: The `FusedMoEMethodBase` derived classes use the `FusedMoEMethodBase::fused_experts` object in their `apply` methods. When settings permit the construction of a valid `FusedMoEModularKernel` object, we override `FusedMoEMethodBase::fused_experts` with it. This essentially makes the derived classes agnostic to what FusedMoE implementation is used.

### How To Unit Test

We have `FusedMoEModularKernel` unit tests at [test_modular_kernel_combinations.py](../../tests/kernels/moe/test_modular_kernel_combinations.py).

The unit test iterates through all combinations of `FusedMoEPrepareAndFinalize` and `FusedMoEPremuteExpertsUnpermute` types and if they are
compatible, runs some correctness tests.
If you are adding some `FusedMoEPrepareAndFinalize` / `FusedMoEPermuteExpertsUnpermute` implementations,

1. Add the implementation type to `MK_ALL_PREPARE_FINALIZE_TYPES` and `MK_FUSED_EXPERT_TYPES` in [mk_objects.py](../../tests/kernels/moe/modular_kernel_tools/mk_objects.py) respectively.
2. Update `Config::is_batched_prepare_finalize()`, `Config::is_batched_fused_experts()`, `Config::is_standard_fused_experts()`,
`Config::is_fe_16bit_supported()`,  `Config::is_fe_fp8_supported()`, `Config::is_fe_block_fp8_supported()`,
`Config::is_fe_supports_chunking()` methods in [/tests/kernels/moe/modular_kernel_tools/common.py](../../tests/kernels/moe/modular_kernel_tools/common.py)

Doing this will add the new implementation to the test suite.

### How To Check `FusedMoEPrepareAndFinalize` & `FusedMoEPermuteExpertsUnpermute` Compatibility

The unit test file [test_modular_kernel_combinations.py](../../tests/kernels/moe/test_modular_kernel_combinations.py) can also be executed as a standalone script.
Example: `python3 -m tests.kernels.moe.test_modular_kernel_combinations --pf-type DeepEPLLPrepareAndFinalize --experts-type BatchedTritonExperts`
As a side effect, this script can be used to test `FusedMoEPrepareAndFinalize` & `FusedMoEPermuteExpertsUnpermute` compatibility. When invoked
with incompatible types, the script will error.

### How To Profile

Please take a look at [profile_modular_kernel.py](../../tests/kernels/moe/modular_kernel_tools/profile_modular_kernel.py)
The script can be used to generate Torch traces for a single `FusedMoEModularKernel::forward()` call for any compatible
`FusedMoEPrepareAndFinalize` and `FusedMoEPermuteExpertsUnpermute` types.
Example: `python3 -m tests.kernels.moe.modular_kernel_tools.profile_modular_kernel --pf-type DeepEPLLPrepareAndFinalize --experts-type BatchedTritonExperts`

## FusedMoEPrepareAndFinalize Implementations

See [Fused MoE Kernel features](./moe_kernel_features.md#fused-moe-modular-all2all-backends) for a list of all the available modular prepare and finalize subclasses.

## FusedMoEPermuteExpertsUnpermute

See [Fused MoE Kernel features](./moe_kernel_features.md#fused-moe-experts-kernels) for a list of all the available modular experts.