Merge pull request #503 from TimDettmers/efficient_8bit_serialize

Make 8-bit serialization more memory-efficient (v2)

Author: TimDettmers

bitsandbytes/autograd/_functions.py

@@ -232,6 +232,19 @@ def supports_igemmlt(device: torch.device) -> bool:
     return True
 
 
+def _get_tile_size(format):
+    assert format in (
+        "col_turing",
+        "col_ampere",
+    ), f"please find this assert and manually enter tile size for {format}"
+    return (8, 32) if format == "col_turing" else (32, 32)
+
+
+def get_tile_inds(format, device):
+    transform = lambda x: F.transform(x.to(device), from_order="row", to_order=format)[0].to(x.device)
+    with torch.no_grad():
+        return get_inverse_transform_indices(transform, _get_tile_size(format)).to(device)
+
 @dataclass
 class MatmulLtState:
     _tile_indices: Optional[torch.Tensor] = None
@@ -267,20 +280,10 @@ def reset_grads(self):
         self.SBt = None
         self.CBt = None
 
-    def get_tile_size(self):
-        assert self.formatB in (
-            "col_turing",
-            "col_ampere",
-        ), f"please find this assert and manually enter tile size for {self.formatB}"
-        return (8, 32) if self.formatB == "col_turing" else (32, 32)
-
     @property
     def tile_indices(self):
         if self._tile_indices is None:
-            device = self.CxB.device
-            transform = lambda x: F.transform(x.to(device), from_order="row", to_order=self.formatB)[0].to(x.device)
-            with torch.no_grad():
-                self._tile_indices = get_inverse_transform_indices(transform, self.get_tile_size()).to(device)
+            self._tile_indices = get_tile_inds(self.formatB, self.CxB.device)
         return self._tile_indices
 
 

bitsandbytes/nn/modules.py

@@ -10,7 +10,7 @@
 
 import bitsandbytes as bnb
 import bitsandbytes.functional
-from bitsandbytes.autograd._functions import get_inverse_transform_indices, undo_layout
+from bitsandbytes.autograd._functions import undo_layout, get_tile_inds
 from bitsandbytes.optim import GlobalOptimManager
 from bitsandbytes.utils import OutlierTracer, find_outlier_dims
 
@@ -306,6 +306,17 @@ def to(self, *args, **kwargs):
             return new_param
 
 
+def maybe_rearrange_weight(state_dict, prefix, local_metadata, strict, missing_keys, unexpected_keys, error_msgs):
+    weight = state_dict.get(f"{prefix}weight")
+    if weight is None:
+        # if the state dict has no weights for this layer (e.g., LoRA finetuning), do nothing
+        return
+    weight_format = state_dict.pop(f"{prefix}weight_format", "row")
+
+    if weight_format != "row":
+        tile_indices = get_tile_inds(weight_format, weight.device)
+        state_dict[f"{prefix}weight"] = undo_layout(weight, tile_indices)
+
 
 class Linear8bitLt(nn.Linear):
     def __init__(self, input_features, output_features, bias=True, has_fp16_weights=True,
@@ -322,52 +333,55 @@ def __init__(self, input_features, output_features, bias=True, has_fp16_weights=
             self.state.use_pool = True
 
         self.weight = Int8Params(self.weight.data, has_fp16_weights=has_fp16_weights, requires_grad=has_fp16_weights)
+        self._register_load_state_dict_pre_hook(maybe_rearrange_weight)
 
     def _save_to_state_dict(self, destination, prefix, keep_vars):
-        if not self.state.has_fp16_weights and self.state.CB is None and self.state.CxB is not None:
-            # reorder weight layout back from ampere/turing to row
-            reorder_layout = True
-            weight_clone = self.weight.data.clone()
-        else:
-            reorder_layout = False
+        super()._save_to_state_dict(destination, prefix, keep_vars)
 
-        try:
-            if reorder_layout:
-                self.weight.data = undo_layout(self.state.CxB, self.state.tile_indices)
+        # we only need to save SCB as extra data, because CB for quantized weights is already stored in weight.data
+        scb_name = "SCB"
 
-            super()._save_to_state_dict(destination, prefix, keep_vars)
+        # case 1: .cuda was called, SCB is in self.weight
+        param_from_weight = getattr(self.weight, scb_name)
+        # case 2: self.init_8bit_state was called, SCB is in self.state
+        param_from_state = getattr(self.state, scb_name)
+        # case 3: SCB is in self.state, weight layout reordered after first forward()
+        layout_reordered = self.state.CxB is not None
 
-            # we only need to save SCB as extra data, because CB for quantized weights is already stored in weight.data
-            weight_name = "SCB"
+        key_name = prefix + f"{scb_name}"
+        format_name = prefix + "weight_format"
 
-            # case 1: .cuda was called, SCB is in self.weight
-            param_from_weight = getattr(self.weight, weight_name)
-            # case 2: self.init_8bit_state was called, SCB is in self.state
-            param_from_state = getattr(self.state, weight_name)
-
-            key_name = prefix + f"{weight_name}"
+        if not self.state.has_fp16_weights:
             if param_from_weight is not None:
                 destination[key_name] = param_from_weight if keep_vars else param_from_weight.detach()
-            elif not self.state.has_fp16_weights and param_from_state is not None:
+                destination[format_name] = "row"
+            elif param_from_state is not None and not layout_reordered:
                 destination[key_name] = param_from_state if keep_vars else param_from_state.detach()
-        finally:
-            if reorder_layout:
-                self.weight.data = weight_clone
+                destination[format_name] = "row"
+            elif param_from_state is not None:
+                destination[key_name] = param_from_state if keep_vars else param_from_state.detach()
+                destination[format_name] = self.state.formatB
 
     def _load_from_state_dict(self, state_dict, prefix, local_metadata, strict,
                               missing_keys, unexpected_keys, error_msgs):
         super()._load_from_state_dict(state_dict, prefix, local_metadata, strict, missing_keys, unexpected_keys,
                                       error_msgs)
-        for key in unexpected_keys:
+        unexpected_copy = list(unexpected_keys)
+
+        for key in unexpected_copy:
             input_name = key[len(prefix):]
             if input_name == "SCB":
                 if self.weight.SCB is None:
-                    # buffers not yet initialized, can't call them directly without
+                    # buffers not yet initialized, can't access them directly without quantizing first
                     raise RuntimeError("Loading a quantized checkpoint into non-quantized Linear8bitLt is "
                                        "not supported. Please call module.cuda() before module.load_state_dict()")
 
                 input_param = state_dict[key]
                 self.weight.SCB.copy_(input_param)
+
+                if self.state.SCB is not None:
+                    self.state.SCB = self.weight.SCB
+
                 unexpected_keys.remove(key)
 
     def init_8bit_state(self):