Add option to FSDP wrap by groups of blocks

olmo/config.py

@@ -258,6 +258,13 @@ class ModelConfig(BaseConfig):
     The transformer block implementation.
     """
 
+    block_group_size: int = 1
+    """
+    The number of blocks to group together into a single parent block.
+    This is has no affect on the number of parameters in the model and is only used to wrap groups
+    of blocks together with a single FSDP wrapper during training.
+    """
+
     alibi: bool = False
     """
     If ``True``, use ALiBi embeddings. Mutually exclusive with ``rope``.
@@ -532,6 +539,12 @@ class FSDPWrapStrategy(StrEnum):
     Wrap each OLMo block with its own FSDP instance.
     """
 
+    by_block_group = "by_block_group"
+    """
+    Wrap each block group together into its own FSDP instance.
+    This requires :attr:`~ModelConfig.block_group_size` to be bigger than 1.
+    """
+
     size_based = "size_based"
     """
     Used PyTorch's default size-based auto wrap policy.

olmo/model.py

@@ -24,6 +24,7 @@
     ActivationType,
     BlockType,
     CheckpointType,
+    FSDPWrapStrategy,
     LayerNormType,
     ModelConfig,
 )
@@ -532,6 +533,8 @@ def forward(
         self,
         x: torch.Tensor,
         attention_bias: Optional[torch.FloatTensor] = None,
+        layer_past: Optional[Tuple[torch.Tensor, torch.Tensor]] = None,
+        use_cache: bool = False,
     ) -> Tuple[torch.Tensor, Optional[Tuple[torch.Tensor, torch.Tensor]]]:
         raise NotImplementedError
 
@@ -694,6 +697,28 @@ class OlmoGenerateOutput(NamedTuple):
     """
 
 
+class OlmoBlockGroup(nn.ModuleList):
+    def forward(
+        self,
+        x: torch.Tensor,
+        attention_bias: Optional[torch.FloatTensor] = None,
+        layers_past: Optional[List[Tuple[torch.Tensor, torch.Tensor]]] = None,
+        use_cache: bool = False,
+    ) -> Tuple[torch.Tensor, Optional[List[Tuple[torch.Tensor, torch.Tensor]]]]:
+        attn_key_values: Optional[List[Tuple[torch.Tensor, torch.Tensor]]] = [] if use_cache else None
+        for block_idx, block in enumerate(self):
+            layer_past = None if layers_past is None else layers_past[block_idx]
+            x, cache = block(x, attention_bias=attention_bias, layer_past=layer_past, use_cache=use_cache)
+            if attn_key_values is not None:
+                assert cache is not None
+                attn_key_values.append(cache)
+        return x, attn_key_values
+
+    def reset_parameters(self):
+        for block in self:
+            block.reset_parameters()
+
+
 def causal_attention_bias(seq_len: int, device: torch.device) -> torch.FloatTensor:
     att_bias = torch.triu(
         torch.ones(seq_len, seq_len, device=device, dtype=torch.float),
@@ -741,6 +766,12 @@ def __init__(self, config: ModelConfig, init_params: bool = True):
                     "Embedding size is not a multiple of 128! This could hurt throughput performance.", UserWarning
                 )
 
+        if not (
+            0 < self.config.block_group_size <= self.config.n_layers
+            and self.config.n_layers % self.config.block_group_size == 0
+        ):
+            raise OlmoConfigurationError("n layers must be divisible by block group size")
+
         torch.backends.cuda.enable_flash_sdp(self.config.flash_attention)
         torch.backends.cuda.enable_mem_efficient_sdp(False)  # this is super slow so make sure torch won't use it
 
@@ -750,10 +781,20 @@ def __init__(self, config: ModelConfig, init_params: bool = True):
                     config.embedding_size or config.vocab_size, config.d_model, device=config.init_device
                 ),
                 emb_drop=Dropout(config.embedding_dropout),
-                blocks=nn.ModuleList([OlmoBlock.build(i, config, self.__cache) for i in range(config.n_layers)]),
                 ln_f=LayerNorm.build(config),
             )
         )
+
+        blocks = [OlmoBlock.build(i, config, self.__cache) for i in range(config.n_layers)]
+        if self.config.block_group_size > 1:
+            block_groups = [
+                OlmoBlockGroup(blocks[i : i + config.block_group_size])
+                for i in range(0, config.n_layers, config.block_group_size)
+            ]
+            self.transformer.update({"block_groups": nn.ModuleList(block_groups)})
+        else:
+            self.transformer.update({"blocks": nn.ModuleList(blocks)})
+
         if not (self.config.alibi or self.config.rope):
             self.transformer.update(
                 {"wpe": nn.Embedding(config.max_sequence_length, config.d_model, device=config.init_device)}
@@ -806,8 +847,12 @@ def reset_parameters(self):
             init_weights(self.config, self.transformer.ff_out)  # type: ignore
 
         # Let the blocks handle themselves.
-        for block in self.transformer.blocks:  # type: ignore
-            block.reset_parameters()  # type: ignore
+        if self.config.block_group_size == 1:
+            for block in self.transformer.blocks:
+                block.reset_parameters()
+        else:
+            for block_group in self.transformer.block_groups:
+                block_group.reset_parameters()
 
     def get_causal_attention_bias(self, seq_len: int, device: torch.device) -> torch.Tensor:
         if (causal_bias := self.__cache.get("causal_attention_bias")) is not None and causal_bias.shape[
@@ -944,15 +989,29 @@ def forward(
         attn_key_values: Optional[List[Tuple[torch.Tensor, torch.Tensor]]] = [] if use_cache else None
 
         # Apply blocks one-by-one.
-        for block, layer_past in zip(
-            self.transformer.blocks,  # type: ignore
-            past_key_values or [None] * self.config.n_layers,  # type: ignore
-        ):
-            # shape: (batch_size, seq_len, d_model)
-            x, cache = block(x, attention_bias=attention_bias, layer_past=layer_past, use_cache=use_cache)
-            if attn_key_values is not None:
-                assert cache is not None
-                attn_key_values.append(cache)
+        if self.config.block_group_size == 1:
+            for block_idx, block in enumerate(self.transformer.blocks):
+                layer_past = None if past_key_values is None else past_key_values[block_idx]
+                # shape: (batch_size, seq_len, d_model)
+                x, cache = block(x, attention_bias=attention_bias, layer_past=layer_past, use_cache=use_cache)
+                if attn_key_values is not None:
+                    assert cache is not None
+                    attn_key_values.append(cache)
+        else:
+            for group_idx, block_group in enumerate(self.transformer.block_groups):
+                layers_past = (
+                    None
+                    if past_key_values is None
+                    else past_key_values[
+                        group_idx * self.config.block_group_size : (group_idx + 1) * self.config.block_group_size
+                    ]
+                )
+                x, cache = block_group(
+                    x, attention_bias=attention_bias, layers_past=layers_past, use_cache=use_cache
+                )
+                if attn_key_values is not None:
+                    assert cache is not None
+                    attn_key_values.extend(cache)
 
         if last_logits_only:
             # shape: (batch_size, 1, d_model)
@@ -973,11 +1032,37 @@ def forward(
 
         return OlmoOutput(logits=logits, attn_key_values=attn_key_values)  # type: ignore[arg-type]
 
-    def fsdp_wrap_fn(self, module, recurse: bool = True, nonwrapped_numel: int = 0):
-        del nonwrapped_numel
-        if recurse:
-            return True  # always recurse
-        return isinstance(module, OlmoBlock)
+    def get_fsdp_wrap_policy(self, wrap_strategy: Optional[FSDPWrapStrategy] = None):
+        if wrap_strategy is None:
+            return None
+        if wrap_strategy == FSDPWrapStrategy.by_block:
+
+            def fsdp_wrap_fn(module, recurse: bool = True, nonwrapped_numel: int = 0):
+                del nonwrapped_numel
+                if recurse:
+                    return True  # always recurse for simplicity
+                return isinstance(module, OlmoBlock)
+
+            return fsdp_wrap_fn
+        elif wrap_strategy == FSDPWrapStrategy.by_block_group:
+            if self.config.block_group_size <= 1:
+                raise OlmoConfigurationError(
+                    "'by_block_group' FSDP wrapping strategy requires block group size greater than 1"
+                )
+
+            def fsdp_wrap_fn(module, recurse: bool = True, nonwrapped_numel: int = 0):
+                del nonwrapped_numel
+                if recurse:
+                    return True  # always recurse for simplicity
+                return isinstance(module, OlmoBlockGroup)
+
+            return fsdp_wrap_fn
+        elif wrap_strategy == FSDPWrapStrategy.size_based:
+            from torch.distributed.fsdp.wrap import size_based_auto_wrap_policy
+
+            return size_based_auto_wrap_policy
+        else:
+            raise NotImplementedError(wrap_strategy)
 
     def activation_checkpointing_fn(self, module):
         return isinstance(module, OlmoBlock)
@@ -1185,20 +1270,61 @@ def from_checkpoint(
         return model.eval()
 
     def _make_state_dict_compatible(self, state_dict: Dict[str, torch.Tensor]) -> Dict[str, torch.Tensor]:
+        import re
+        from fnmatch import fnmatch
+
+        # Remove "_fsdp_wrapped_module." prefix from all keys. We don't want this prefix when the model is
+        # not wrapped in FSDP. And when the model is wrapped in FSDP, loading this state dict will still work
+        # fine without the prefixes. This also simplifies the other steps below.
+        for key in list(state_dict.keys()):
+            state_dict[key.replace("_fsdp_wrapped_module.", "")] = state_dict.pop(key)
+
         # For backwards compatibility prior to fixing https://github.com/allenai/LLM/issues/222
-        prefix = ""
-        if next(iter(state_dict.keys())).startswith((fsdp_prefix := "_fsdp_wrapped_module.")):
-            prefix = fsdp_prefix
         if self.config.block_type == BlockType.sequential:
-            for block_idx in range(self.config.n_layers):
-                norm_w_key = f"{prefix}transformer.blocks.{block_idx}.norm.weight"
-                norm_b_key = f"{prefix}transformer.blocks.{block_idx}.norm.bias"
-                if norm_w_key in state_dict:
-                    norm_w = state_dict.pop(norm_w_key)
-                    state_dict[f"{prefix}transformer.blocks.{block_idx}.attn_norm.weight"] = norm_w
-                    state_dict[f"{prefix}transformer.blocks.{block_idx}.ff_norm.weight"] = norm_w.clone()
-                if norm_b_key in state_dict:
-                    norm_b = state_dict.pop(norm_b_key)
-                    state_dict[f"{prefix}transformer.blocks.{block_idx}.attn_norm.bias"] = norm_b
-                    state_dict[f"{prefix}transformer.blocks.{block_idx}.ff_norm.bias"] = norm_b.clone()
+            for key in list(state_dict.keys()):
+                if fnmatch(key, "transformer.*.norm.weight"):
+                    tensor = state_dict.pop(key)
+                    state_dict[key.replace("norm.weight", "attn_norm.weight")] = tensor
+                    state_dict[key.replace("norm.weight", "ff_norm.weight")] = tensor.clone()
+                elif fnmatch(key, "transformer.*.norm.bias"):
+                    tensor = state_dict.pop(key)
+                    state_dict[key.replace("norm.bias", "attn_norm.bias")] = tensor
+                    state_dict[key.replace("norm.bias", "ff_norm.bias")] = tensor.clone()
+
+        # For loading a state dict that was saved with a different `block_group_size`.
+        if "transformer.block_groups.0.0.attn_out.weight" in state_dict.keys():
+            state_dict_block_group_size = len(
+                [k for k in state_dict.keys() if fnmatch(k, "transformer.block_groups.0.*.attn_out.weight")]
+            )
+        else:
+            state_dict_block_group_size = 1
+        if self.config.block_group_size != state_dict_block_group_size:
+            log.info(
+                f"Regrouping state dict blocks from group size {state_dict_block_group_size} to "
+                f"group size {self.config.block_group_size}"
+            )
+            # For simplicity we're first going to flatten out the block groups in the state dict (if necessary)
+            # and then (re-)group them into the right block sizes.
+            if state_dict_block_group_size > 1:
+                for key in list(state_dict.keys()):
+                    if (m := re.match(r"transformer.block_groups\.(\d+)\.(\d+)\..*", key)) is not None:
+                        group_idx, group_block_idx = int(m.group(1)), int(m.group(2))
+                        block_idx = (group_idx * state_dict_block_group_size) + group_block_idx
+                        state_dict[
+                            key.replace(f"block_groups.{group_idx}.{group_block_idx}.", f"blocks.{block_idx}.")
+                        ] = state_dict.pop(key)
+
+            if self.config.block_group_size > 1:
+                # Group the state dict blocks into the right block size.
+                for key in list(state_dict.keys()):
+                    if (m := re.match(r"transformer.blocks\.(\d+)\..*", key)) is not None:
+                        block_idx = int(m.group(1))
+                        group_idx, group_block_idx = (
+                            block_idx // self.config.block_group_size,
+                            block_idx % self.config.block_group_size,
+                        )
+                        state_dict[
+                            key.replace(f"blocks.{block_idx}.", f"block_groups.{group_idx}.{group_block_idx}.")
+                        ] = state_dict.pop(key)
+
         return state_dict

scripts/train.py

@@ -13,9 +13,8 @@
 import wandb
 from packaging import version
 from torch.distributed.fsdp import FullyShardedDataParallel as FSDP
-from torch.distributed.fsdp.wrap import size_based_auto_wrap_policy
 
-from olmo.config import CheckpointType, FSDPWrapStrategy, TrainConfig
+from olmo.config import CheckpointType, TrainConfig
 from olmo.data import build_train_dataloader
 from olmo.eval import build_evaluators
 from olmo.exceptions import OlmoCliError, OlmoConfigurationError
@@ -116,12 +115,7 @@ def main(cfg: TrainConfig) -> None:
 
     # Wrap the model in FSDP.
     log.info("Wrapping model with FDSP...")
-    wrap_policy = None
-    if cfg.fsdp.wrapping_strategy == FSDPWrapStrategy.by_block:
-        wrap_policy = olmo_model.fsdp_wrap_fn
-    elif cfg.fsdp.wrapping_strategy == FSDPWrapStrategy.size_based:
-        wrap_policy = size_based_auto_wrap_policy
-
+    wrap_policy = olmo_model.get_fsdp_wrap_policy(cfg.fsdp.wrapping_strategy)
     if version.parse(torch.__version__) >= version.parse("2.1.0"):
         # This prevents any parameters from being initialized twice
         def dummy_init_fn(module: torch.nn.Module) -> None:
@@ -130,7 +124,6 @@ def dummy_init_fn(module: torch.nn.Module) -> None:
         param_init_fn = dummy_init_fn
     else:
         param_init_fn = None
-
     fsdp_model = FSDP(
         olmo_model,
         sharding_strategy=cfg.fsdp.sharding_strategy,

tests/model_test.py

@@ -4,7 +4,7 @@
 from torch.nn import CrossEntropyLoss
 
 from olmo import BlockType, LayerNorm, Olmo, Tokenizer, TrainConfig
-from olmo.config import PaddingDirection
+from olmo.config import ModelConfig, PaddingDirection
 from olmo.data import DataCollator
 from olmo.model import AMDLayerNorm
 
@@ -432,3 +432,24 @@ def test_layer_norm(train_config: TrainConfig, elementwise_affine: bool, include
 
     y_actual = amd_ln(x)
     torch.testing.assert_close(y_actual, y_expected)
+
+
+def test_block_groups():
+    model_with_block_groups = Olmo(ModelConfig(d_model=128, n_heads=2, n_layers=9, block_group_size=3)).eval()
+    model_without_block_groups = Olmo(ModelConfig(d_model=128, n_heads=2, n_layers=9, block_group_size=1)).eval()
+
+    # We should be able to load the state dict from one model into the other, and vice-versa.
+    model_with_block_groups.load_state_dict(
+        model_with_block_groups._make_state_dict_compatible(model_without_block_groups.state_dict())
+    )
+    model_without_block_groups.load_state_dict(
+        model_without_block_groups._make_state_dict_compatible(model_with_block_groups.state_dict())
+    )
+
+    # Check that output is exactly the same.
+    input_ids = torch.randint(0, model_with_block_groups.config.vocab_size, (2, 16))
+    with torch.no_grad():
+        block_groups_output = model_with_block_groups(input_ids)
+        no_block_groups_output = model_without_block_groups(input_ids)
+
+    torch.testing.assert_close(block_groups_output, no_block_groups_output)