# Copyright 2022 Cerebras Systems.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import torch
import torch.nn as nn
from cerebras.modelzoo.common.registry import registry
from cerebras.modelzoo.trainer import summarize_scalar
[docs]@registry.register_loss("LoadBalancingLoss")
class LoadBalancingLoss(nn.Module):
def __init__(
self,
num_experts,
top_k,
):
super(LoadBalancingLoss, self).__init__()
self.num_experts = num_experts
self.top_k = top_k
[docs] def forward(
self,
router_weights_list,
expert_mask_list,
attention_mask=None,
):
"""
router_weights: Num hidden layers * [[batch_size, seq_len, experts]]
expert_mask: Num hidden layers * [[batch_size, seq_len, experts]].
"""
tokens_per_expert = torch.zeros_like(router_weights_list[0][:, 0, :])
router_prob_per_expert = torch.zeros_like(
router_weights_list[0][:, 0, :]
)
for router_weights, expert_mask in zip(
router_weights_list, expert_mask_list
):
if attention_mask is not None:
extended_attention_mask = (
attention_mask[:, :, None]
.broadcast_to(expert_mask.shape)
.to(router_weights.dtype)
)
# Compute the percentage of tokens routed to each experts
tokens_per_expert += torch.sum(
expert_mask * extended_attention_mask, dim=1
) / torch.sum(extended_attention_mask, dim=1)
router_prob_per_expert += torch.sum(
router_weights * extended_attention_mask, dim=1
) / torch.sum(extended_attention_mask, dim=1)
else:
# Compute the percentage of tokens routed to each experts
tokens_per_expert += torch.mean(expert_mask, dim=1)
# Compute the average probability of routing to these experts
router_prob_per_expert += torch.mean(router_weights, dim=1)
tokens_per_expert /= len(router_weights_list)
router_prob_per_expert /= len(router_weights_list)
for expert_idx in range(self.num_experts):
summarize_scalar(
f"expert_stats/tokens_per_expert/expert_{expert_idx}",
torch.mean(tokens_per_expert[:, expert_idx]),
)
summarize_scalar(
f"expert_stats/router_prob_per_expert/expert_{expert_idx}",
torch.mean(router_prob_per_expert[:, expert_idx]),
)
return (self.num_experts**2) * torch.mean(
router_prob_per_expert * tokens_per_expert
)