Integrate FlashAttention into Megatron-LM

README.md

@@ -333,6 +333,18 @@ Theoretical memory savings vary depending on the combination of the model's para
 | bf16 param, fp32 grads | 18 | 6 + 12/d |
 | fp32 param, fp32 grads | 16 | 8 + 8/d |
 
+## FlashAttention
+
+Usage: `--use-flash-attn`. Support attention head dimensions at most 128.
+
+[FlashAttention](https://github.com/HazyResearch/flash-attention) is a fast and
+memory-efficient algorithm to compute exact attention. It speeds up model
+training and reduces memory requirement.
+
+To install FlashAttention:
+```sh
+pip install flash-attn
+```
 
 ## GPT-3 Example
 

megatron/arguments.py

@@ -612,6 +612,9 @@ def _add_training_args(parser):
     group.add_argument('--no-bias-dropout-fusion', action='store_false',
                        help='Disable bias and dropout fusion.',
                        dest='bias_dropout_fusion')
+    group.add_argument('--use-flash-attn', action='store_true',
+                       help='use FlashAttention implementation of attention. '
+                       'https://arxiv.org/abs/2205.14135')
     group.add_argument('--optimizer', type=str, default='adam',
                        choices=['adam', 'sgd'],
                        help='Optimizer function')

megatron/model/transformer.py

@@ -15,6 +15,16 @@ from megatron.model.fused_softmax import FusedScaleMaskSoftmax
 from megatron.model.fused_bias_gelu import bias_gelu_impl
 from megatron.model.utils import attention_mask_func, openai_gelu, erf_gelu
 
+try:
+    from einops import rearrange
+except ImportError:
+    rearrange = None
+
+try:
+    from flash_attn.flash_attn_interface import flash_attn_unpadded_func
+except ImportError:
+    flash_attn_unpadded_func = None
+
 
 """ We use the following notation throughout this file:
      h: hidden size
@@ -306,6 +316,48 @@ class CoreAttention(MegatronModule):
         return context_layer
 
 
+class FlashSelfAttention(torch.nn.Module):
+    """Implement the scaled dot product attention with softmax.
+    Arguments
+    ---------
+        softmax_scale: The temperature to use for the softmax attention.
+                      (default: 1/sqrt(d_keys) where d_keys is computed at
+                      runtime)
+        attention_dropout: The dropout rate to apply to the attention
+                           (default: 0.0)
+    """
+    def __init__(self, causal=False, softmax_scale=None, attention_dropout=0.0,
+                 device=None, dtype=None):
+        super().__init__()
+        assert flash_attn_unpadded_func is not None, ('Please install FlashAttention first, '
+                                                      'e.g., with pip install flash-attn')
+        assert rearrange is not None, 'Please install einops first, e.g., with pip install einops'
+        self.causal = causal
+        self.softmax_scale = softmax_scale
+        self.dropout_p = attention_dropout
+
+    def forward(self, q, k, v):
+        """Implements the multihead softmax attention.
+        Arguments
+        ---------
+            q, k, v: The tensor containing the query, key, and value. (B, S, H, D)
+        """
+        assert q.dtype in [torch.float16, torch.bfloat16]
+        assert q.is_cuda
+        batch_size, seqlen = q.shape[0], q.shape[1]
+        q, k, v = [rearrange(x, 'b s ... -> (b s) ...') for x in [q, k, v]]
+        max_s = seqlen
+        cu_seqlens = torch.arange(0, (batch_size + 1) * seqlen, step=seqlen, dtype=torch.int32,
+                                  device=q.device)
+        output = flash_attn_unpadded_func(
+            q, k, v, cu_seqlens, cu_seqlens, max_s, max_s,
+            self.dropout_p if self.training else 0.0,
+            softmax_scale=self.softmax_scale, causal=self.causal
+        )
+        output = rearrange(output, '(b s) ... -> b s ...', b=batch_size)
+        return output
+
+
 class ParallelAttention(MegatronModule):
     """Parallel self-attention layer abstract class.
 
@@ -323,6 +375,21 @@ class ParallelAttention(MegatronModule):
         self.attention_type = attention_type
         self.attn_mask_type = attn_mask_type
         self.params_dtype = args.params_dtype
+        self.sequence_parallel = args.sequence_parallel
+
+        self.use_flash_attn = args.use_flash_attn
+        if self.use_flash_attn:
+            if flash_attn_unpadded_func is None:
+                raise ImportError('FlashAttention is not installed, please install with '
+                                  'pip install flash-attn')
+            assert attention_type == AttnType.self_attn, ('FlashAttention code path only supports '
+                                                          'self-attention for now')
+            assert self.attn_mask_type == AttnMaskType.causal, ('FlashAttention code path only '
+                                                                'supports causal mask for now')
+            headdim = args.hidden_size / args.num_attention_heads
+            assert headdim <= 128, 'FlashAttention only supports head dimension at most 128'
+            if rearrange is None:
+                raise ImportError('einops is not installed, please install with pip install einops')
 
         projection_size = args.kv_channels * args.num_attention_heads
 
@@ -365,6 +432,11 @@ class ParallelAttention(MegatronModule):
                                             self.attn_mask_type)
         self.checkpoint_core_attention = args.recompute_granularity == 'selective'
 
+        if self.use_flash_attn:
+            self.core_attention_flash = FlashSelfAttention(
+                causal=True, attention_dropout=args.attention_dropout
+            )
+
         # Output.
         self.dense = tensor_parallel.RowParallelLinear(
             projection_size,
@@ -487,12 +559,22 @@ class ParallelAttention(MegatronModule):
         # core attention computation
         # ==================================
 
-        if self.checkpoint_core_attention:
-            context_layer = self._checkpointed_attention_forward(
-                query_layer, key_layer, value_layer, attention_mask)
+        if not self.use_flash_attn:
+            if self.checkpoint_core_attention:
+                context_layer = self._checkpointed_attention_forward(
+                    query_layer, key_layer, value_layer, attention_mask)
+            else:
+                context_layer = self.core_attention(
+                    query_layer, key_layer, value_layer, attention_mask)
         else:
-            context_layer = self.core_attention(
-                query_layer, key_layer, value_layer, attention_mask)
+            q, k, v = [rearrange(x, 's b ... -> b s ...').contiguous()
+                       for x in (query_layer, key_layer, value_layer)]
+            if not self.sequence_parallel:
+                with tensor_parallel.get_cuda_rng_tracker().fork():
+                    context_layer = self.core_attention_flash(q, k, v)
+            else:
+                context_layer = self.core_attention_flash(q, k, v)
+            context_layer = rearrange(context_layer, 'b s h d -> s b (h d)').contiguous()
 
         # =================
         # Output. [sq, b, h]