vllm.distributed.eplb ¶

@dataclass
class EplbState:
    """EPLB metrics."""

    physical_to_logical_map: torch.Tensor
    """
    Mapping from physical experts to logical experts.

    Shape: (num_moe_layers, num_physical_experts)

    # Example

    For a 2-layer MoE model with 6 physical experts and 4 logical experts on 3
    EP ranks, the mapping could look like this:

    ```
    [[0, 1, 2, 3, 0, 1],
     [0, 2, 0, 1, 0, 3]]
    ```
    """
    logical_to_physical_map: torch.Tensor
    """
    Mapping from logical experts to physical experts.

    This is a sparse matrix, where -1 indicates no mapping.

    Shape: (num_moe_layers, num_logical_experts, num_redundant_experts + 1)

    # Example

    For a 2-layer MoE model with 6 physical experts and 4 logical experts on 3
    EP ranks, the mapping could look like this:

    ```
    [[[0, 4, -1],
      [1, 5, -1],
      [2, -1, -1],
      [3, -1, -1]],
     [[0, 2, 4],
      [3, -1, -1],
      [1, -1, -1],
      [5, -1, -1]]]
    ```
    """
    logical_replica_count: torch.Tensor
    """
    Number of replicas for each logical expert.
    This is exactly the non-`-1` count in the `logical_to_physical_map`.

    Shape: (num_moe_layers, num_logical_experts)

    # Example
    For a 2-layer MoE model with 6 physical experts and 4 logical experts on 3
    EP ranks, the count could look like this:

    ```
    [[2, 2, 1, 1],
     [3, 1, 1, 1]]
    """

    expert_load_pass: torch.Tensor
    """
    Expert load during this forward pass. 
    We use the token count each expert processes as the load.

    Shape: (num_moe_layers, num_physical_experts)
    """
    expert_load_window: torch.Tensor
    """
    A sliding window of expert load.

    Shape: (window_size, num_moe_layers, num_physical_experts)

    NOTE: The expert_load_view now records load for all physical experts
    rather than just local experts. This ensures consistent load statistics
    across different dispatch methods (naive all-to-all, DeepEP, pplx-kernels).
    The recorded load will be multiplied by dp_size when using naive all-to-all
    due to each DP rank contributing the same token set to the calculation.
    See:
    https://github.com/vllm-project/vllm/pull/22167#pullrequestreview-3086143856
    """
    expert_load_window_step: int = 0
    """
    Current step in the sliding window.

    Different from `expert_rearrangement_step`, each EP rank may have its own
    `expert_load_window_step`.
    """
    expert_load_window_size: int = 0
    """
    Size of the expert load sliding window.
    This is a constant and is taken from the config.
    """

    expert_rearrangement_step: int = 0
    """
    Steps after last rearrangement.
    Will trigger a rearrangement if it exceeds the threshold.

    NOTE: Keep in mind that all EP ranks need to have the same
    `expert_rearrangement_step` value to ensure synchronization.
    Otherwise, the rearrangement will hang at collective
    communication calls.
    """
    expert_rearrangement_step_interval: int = 0
    """
    Interval for expert rearrangement steps.
    This is a constant and is taken from the config.
    """

    @staticmethod
    def build_initial_global_physical_to_logical_map(
        num_routed_experts: int,
        num_redundant_experts: int,
    ) -> Sequence[int]:
        """
        Build an initial expert arrangement using the following structure:
        [original routed experts, redundant experts]

        Returns:
            physical_to_logical_map (Sequence[int]): A list of integers,
                where each integer is the index of the logical expert
                that the corresponding physical expert maps to.
        """
        global_physical_to_logical_map = list(range(num_routed_experts))
        global_physical_to_logical_map += [
            i % num_routed_experts for i in range(num_redundant_experts)
        ]
        return global_physical_to_logical_map

    @classmethod
    def build(
        cls,
        model: MixtureOfExperts,
        device: torch.device,
        parallel_config: ParallelConfig,
        global_expert_load: Optional[torch.Tensor] = None,
        old_global_expert_indices: Optional[torch.Tensor] = None,
        rank_mapping: Optional[dict[int, int]] = None,
    ) -> "EplbState":
        """
        Build the initial EPLB state.
        """
        physical_to_logical_map_list = (
            cls.build_initial_global_physical_to_logical_map(
                model.num_routed_experts,
                model.num_redundant_experts,
            ))
        physical_to_logical_map = torch.tensor(
            physical_to_logical_map_list,
            device=device,
        )
        # Assuming 8 GPUs per node, this supports up to
        # (1023 + 1) / 8 = 128 nodes for now.
        # TODO(rui): make this configurable
        MAX_EXPERT_REDUNDANCY = 1023
        assert model.num_redundant_experts <= MAX_EXPERT_REDUNDANCY, (
            f"num_redundant_experts {model.num_redundant_experts} "
            f"must be less than or equal to {MAX_EXPERT_REDUNDANCY}")
        max_slots_per_logical_expert = MAX_EXPERT_REDUNDANCY + 1
        logical_to_physical_map = torch.full(
            (model.num_logical_experts, max_slots_per_logical_expert),
            -1,
            device=device,
        )
        logical_replica_count = torch.zeros(
            (model.num_logical_experts, ),
            device=device,
            dtype=torch.long,
        )

        for i in range(model.num_physical_experts):
            logical_idx = physical_to_logical_map[i]
            logical_to_physical_map[logical_idx,
                                    logical_replica_count[logical_idx]] = i
            logical_replica_count[logical_idx] += 1

        # Duplicate initial mapping for all layers
        physical_to_logical_map = physical_to_logical_map.unsqueeze(0).expand(
            model.num_moe_layers,
            -1,
        ).contiguous()
        logical_to_physical_map = logical_to_physical_map.unsqueeze(0).expand(
            model.num_moe_layers,
            -1,
            -1,
        ).contiguous()
        logical_replica_count = logical_replica_count.unsqueeze(0).expand(
            model.num_moe_layers,
            -1,
        ).contiguous()

        expert_load_pass = torch.zeros(
            (model.num_moe_layers, model.num_physical_experts),
            dtype=torch.int32,
            device=device,
        )
        expert_load_window_size = parallel_config.eplb_config.window_size
        expert_load_window = torch.zeros(
            (expert_load_window_size, model.num_moe_layers,
             model.num_physical_experts),
            dtype=torch.int32,
            device=device,
        )

        # Set the initial progress of rearrangement to 3/4
        eplb_step_interval = parallel_config.eplb_config.step_interval
        expert_rearrangement_step = max(
            0, eplb_step_interval - eplb_step_interval // 4)

        if global_expert_load is not None:
            ep_group = get_ep_group().device_group
            assert global_expert_load.shape == (model.num_moe_layers,
                                                model.num_logical_experts)
            assert global_expert_load.dtype == torch.int64

            num_replicas = model.num_physical_experts
            num_groups = model.num_expert_groups
            num_nodes = get_node_count()
            num_gpus = ep_group.size()

            if num_gpus % num_nodes != 0:
                num_nodes = 1
                logger.warning_once(
                    f"num_gpus % num_nodes != 0, "
                    "not using hierarchical rearrangement algorithm.\n"
                    f"{num_gpus=}, {num_nodes=}")

            # Get new expert mappings
            (
                new_physical_to_logical_map,
                new_logical_to_physical_map,
                new_logical_replica_count,
            ) = (rebalance_experts(
                global_expert_load,
                num_replicas,
                num_groups,
                num_nodes,
                num_gpus,
            ))

            max_physical_slots = new_logical_to_physical_map.shape[-1]
            assert max_physical_slots <= logical_to_physical_map.shape[-1]
            new_logical_to_physical_map = torch.nn.functional.pad(
                new_logical_to_physical_map,
                (0, logical_to_physical_map.shape[-1] - max_physical_slots),
                value=-1,
            )
            physical_to_logical_map = new_physical_to_logical_map.to(device)
            logical_to_physical_map.copy_(new_logical_to_physical_map)
            logical_replica_count.copy_(new_logical_replica_count)

        model.set_eplb_state(
            expert_load_pass,
            logical_to_physical_map,
            logical_replica_count,
        )
        if global_expert_load is not None:
            rearrange_expert_weights_inplace(
                old_global_expert_indices,
                new_physical_to_logical_map,
                model.expert_weights,
                ep_group,
                False,
                rank_mapping,
            )
            expert_rearrangement_step = 0

        return cls(
            physical_to_logical_map,
            logical_to_physical_map,
            logical_replica_count,
            expert_load_pass,
            expert_load_window,
            expert_load_window_size=expert_load_window_size,
            expert_rearrangement_step=expert_rearrangement_step,
            expert_rearrangement_step_interval=eplb_step_interval,
        )

    def step(self,
             model: MixtureOfExperts,
             is_dummy: bool = False,
             is_profile: bool = False,
             log_stats: bool = False) -> None:
        """
        Step the EPLB state.

        Args:
            model (MixtureOfExperts): The MoE model.
            is_dummy (bool): If `True`, this is a dummy step and the load
                metrics recorded in this forward pass will not count.
                Defaults to `False`.
            is_profile (bool): If `True`, perform a dummy rearrangement
                with maximum communication cost. This is used in
                `profile_run` to reserve enough memory
                for the communication buffer.
            log_stats (bool): If `True`, log the expert load metrics.

        # Stats
            The metrics are all summed up across layers.
            - `avg_tokens`: The average load across ranks.
            - `max_tokens`: The maximum load across ranks.
            - `balancedness`: The ratio of average load to maximum load.
        """

        if is_profile:
            self.rearrange(model, is_profile=True)
            return

        if is_dummy:
            # Do not record load metrics for dummy steps
            self.expert_load_pass.zero_()

        if log_stats:
            # total_expert_load_pass: (num_moe_layers, num_physical_experts)
            total_expert_load_pass = self.expert_load_pass.clone()

            # Collect load metrics from all ranks
            ep_group = get_ep_group().device_group
            all_reduce(total_expert_load_pass, group=ep_group)

            # num_tokens_per_rank: (num_moe_layers, num_ranks)
            num_tokens_per_rank = total_expert_load_pass.reshape(
                total_expert_load_pass.shape[0], ep_group.size(),
                -1).sum(dim=-1).float()

            # Compute balancedness ratio:
            # for each layer:
            #   (mean load across ranks) / (max load across ranks)
            avg_tokens_tensor = num_tokens_per_rank.mean(dim=0).sum(dim=0)
            max_tokens_tensor = num_tokens_per_rank.max(dim=0).values.sum(
                dim=0)

            # Just to make type checker happy
            tokens_tensors: list[float] = torch.stack(
                [avg_tokens_tensor, max_tokens_tensor]).tolist()
            avg_tokens, max_tokens = tokens_tensors
            balancedness = avg_tokens / max_tokens if max_tokens > 0 else 0.0

            if ep_group.rank() == 0:
                logger.info(
                    "EPLB step: avg_tokens=%.2f, max_tokens=%d, "
                    "balancedness=%.4f", avg_tokens, max_tokens, balancedness)

        # Update the expert load sliding window
        if not is_dummy:
            self.expert_load_window[self.expert_load_window_step] = (
                self.expert_load_pass.clone())
            self.expert_load_window_step += 1
            if self.expert_load_window_step >= self.expert_load_window_size:
                self.expert_load_window_step = 0
            self.expert_load_pass.zero_()

        # Step the expert rearrangement step
        # Note that even if this is a dummy step, we still increment the
        # rearrangement step and perform rearrangement to ensure all ranks are
        # performing collective communication.
        self.expert_rearrangement_step += 1
        if (self.expert_rearrangement_step
                >= self.expert_rearrangement_step_interval):
            self.expert_rearrangement_step = 0
            self.rearrange(model)

    def rearrange(
        self,
        model: MixtureOfExperts,
        is_profile: bool = False,
        execute_shuffle: bool = True,
        global_expert_load: Optional[torch.Tensor] = None,
        rank_mapping: Optional[dict[int,
                                    int]] = None) -> Optional[torch.Tensor]:
        """
        Rearrange the experts according to the current load.
        """

        ep_group = get_ep_group().device_group
        ep_rank = ep_group.rank()

        time_start = None
        is_main_rank = ep_rank == 0
        if is_main_rank:
            torch.cuda.synchronize()
            time_start = time.perf_counter()
            logger.info("Rearranging experts %s...",
                        "(profile)" if is_profile else "")

        if global_expert_load is None:
            # Map the physical expert load to global logical experts
            logical_expert_load_window = torch.zeros(
                self.expert_load_window_size,
                model.num_moe_layers,
                model.num_logical_experts,
                dtype=self.expert_load_window.dtype,
                device=self.expert_load_window.device,
            )
            logical_expert_load_window.scatter_add_(
                dim=-1,
                index=self.physical_to_logical_map.unsqueeze(0).expand_as(
                    self.expert_load_window).long(),
                src=self.expert_load_window,
            )

            if not execute_shuffle:
                metadata = torch.tensor(
                    [
                        model.num_moe_layers, model.num_logical_experts,
                        self.physical_to_logical_map.shape[1]
                    ],
                    dtype=torch.int32,
                    device="cpu",
                )
                torch.distributed.broadcast(metadata,
                                            group=get_ep_group().cpu_group,
                                            group_src=0)

            # Perform all-reduce to get the expert load across all ranks
            global_expert_load_window = logical_expert_load_window.sum(dim=0)
            all_reduce(global_expert_load_window, group=ep_group)

            if not execute_shuffle:
                # (num_moe_layers, old_num_physical_experts)
                old_global_expert_indices = self.physical_to_logical_map
                torch.distributed.broadcast(old_global_expert_indices,
                                            group=ep_group,
                                            group_src=0)
                return global_expert_load_window
        else:
            assert execute_shuffle
            global_expert_load_window = global_expert_load

        # TODO(bowen): Treat differently for prefill and decode nodes
        num_replicas = model.num_physical_experts
        num_groups = model.num_expert_groups
        if rank_mapping is not None and len(rank_mapping) == ep_group.size():
            # NOTE(yongji): scale down, we need to rebalance the experts on
            # remaining GPUs, transfer the experts while we haven't shutdown
            # the GPUs to be released.
            cpu_group = get_ep_group().cpu_group
            num_nodes = _node_count_with_rank_mapping(cpu_group, rank_mapping)
            num_gpus = sum(new_rank != -1
                           for new_rank in rank_mapping.values())
            num_replicas = num_replicas // ep_group.size(
            ) * num_gpus  # handle num replicas change
        else:
            num_nodes = get_node_count()
            num_gpus = ep_group.size()

        if num_gpus % num_nodes != 0:
            self.num_nodes = 1
            logger.warning_once(
                f"num_gpus % num_nodes != 0, "
                "not using hierarchical rearrangement algorithm.\n"
                f"{num_gpus=}, {num_nodes=}")

        # Get new expert mappings
        (
            new_physical_to_logical_map,
            new_logical_to_physical_map,
            new_logical_replica_count,
        ) = (rebalance_experts(
            global_expert_load_window,
            num_replicas,
            num_groups,
            num_nodes,
            num_gpus,
        ))

        # Update expert weights
        rearrange_expert_weights_inplace(
            self.physical_to_logical_map,
            new_physical_to_logical_map,
            model.expert_weights,
            ep_group,
            is_profile,
            rank_mapping,
        )

        if not is_profile:
            if self.physical_to_logical_map.shape[
                    1] != new_physical_to_logical_map.shape[1]:
                self.physical_to_logical_map = new_physical_to_logical_map.to(
                    self.physical_to_logical_map.device)
            else:
                self.physical_to_logical_map.copy_(new_physical_to_logical_map)
            max_physical_slots = new_logical_to_physical_map.shape[-1]
            assert max_physical_slots <= self.logical_to_physical_map.shape[-1]
            new_logical_to_physical_map = torch.nn.functional.pad(
                new_logical_to_physical_map,
                (0,
                 self.logical_to_physical_map.shape[-1] - max_physical_slots),
                value=-1,
            )
            self.logical_to_physical_map.copy_(new_logical_to_physical_map)
            self.logical_replica_count.copy_(new_logical_replica_count)

        if is_main_rank:
            assert time_start is not None
            torch.cuda.synchronize()
            time_end = time.perf_counter()
            logger.info(
                "Rearranged experts%sin %.2f seconds.",
                " (profile) " if is_profile else " ",
                time_end - time_start,
            )
        return None

    @staticmethod
    def recv_state() -> tuple[torch.Tensor, torch.Tensor]:
        """
        Receive the expert load and old placement from the master rank.
        """
        ep_group = get_ep_group()
        metadata = torch.empty(3, dtype=torch.int32, device="cpu")
        torch.distributed.broadcast(metadata,
                                    group=ep_group.cpu_group,
                                    group_src=0)
        num_moe_layers, num_logical_experts, num_old_physical_experts = (
            metadata.tolist())
        global_expert_load = torch.zeros(
            (num_moe_layers, num_logical_experts),
            dtype=torch.int64,
            device=ep_group.device,
        )
        all_reduce(global_expert_load, group=ep_group.device_group)
        old_global_expert_indices = torch.empty(
            (num_moe_layers, num_old_physical_experts),
            dtype=torch.int64,
            device=ep_group.device,
        )
        torch.distributed.broadcast(old_global_expert_indices,
                                    group=ep_group.device_group,
                                    group_src=0)

        return global_expert_load, old_global_expert_indices