Reduce the device bubble introduced by heavy loop synchronization in coalesced fetch/release(z3_leaf_module) (deepspeedai#6694)

depend on deepspeedai#6649

When performing fetch/release operations on Z3 leaf modules, the loop
time is excessively long in fine-grained module. Compared to non-leaf
modules, Z3 leaf modules may include a larger number of parameters.
Although each loop unit does not consume much time, the overall loop
length can be significant.

![image](https://github.com/user-attachments/assets/9891835a-2620-47f3-aba6-ea22b8905d1c)
**The fetch time is impacted by:**

Post-allgather operations (narrow， slice ，cat, difficult to avoid)
Memory pressure（record_stream/fetch event create&sync）
**The release time is impacted by:**
slice
Free parameter record_stream

Considering the fine-grained leaf modules, where each parameter is
relatively small, we can treat the parameters within each leaf module as
a unified entity to handle memory pressure. This approach can
approximately halve the CPU time required for fetch/release operations.

---------

Co-authored-by: Ma, Guokai <guokai.ma@gmail.com>
Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>

Author: 4 people

deepspeed/runtime/zero/mics.py

@@ -38,7 +38,7 @@ class MiCS_AllGatherCoalescedHandle(AllGatherCoalescedHandle):
     def __init__(self, allgather_handle, params: List[Parameter], partitions: List[Tensor], world_size: int) -> None:
         super().__init__(allgather_handle, params, partitions, world_size)
 
-    def wait(self) -> None:
+    def wait(self, **kwargs) -> None:
         """
         """
         # let the current stream to op

deepspeed/runtime/zero/parameter_offload.py

@@ -145,6 +145,16 @@ def __init__(
             module.ds_inflight_param_registry = InflightParamRegistry()
         self.__inflight_param_registry = module.ds_inflight_param_registry
 
+        self.fast_sharding_for_leaf_module = False
+
+        if zero_module_granularity_threshold > 0:
+            self.min_granularity_value = sys.maxsize
+            self.min_granularity_layer = None
+            self.granularity_info = set()
+            self.z3_leaf_layers = []
+            self._set_z3_leaf_modules_by_threshold(module, zero_module_granularity_threshold)
+            self.fast_sharding_for_leaf_module = True
+
         self.param_coordinator = PartitionedParameterCoordinator(
             prefetch_bucket_sz=self._prefetch_bucket_sz,
             max_reuse_distance_in_numel=self._max_reuse_distance_in_numel,
@@ -155,14 +165,7 @@ def __init__(
             timers=self.timers,
             zero_quantized_weights=self.zero_quantized_weights,
             zero_quantized_nontrainable_weights=self.zero_quantized_nontrainable_weights,
-        )
-
-        if zero_module_granularity_threshold > 0:
-            self.min_granularity_value = sys.maxsize
-            self.min_granularity_layer = None
-            self.granularity_info = set()
-            self.z3_leaf_layers = []
-            self._set_z3_leaf_modules_by_threshold(module, zero_module_granularity_threshold)
+            fast_sharding_for_leaf_module=self.fast_sharding_for_leaf_module)
 
         self.forward_hooks = []
         self.backward_hooks = []

deepspeed/runtime/zero/partition_parameters.py

@@ -55,7 +55,7 @@ def __init__(self, param: Parameter) -> None:
                                                  non_blocking=True).view(param.ds_shape)
         self.__param = param
 
-    def wait(self) -> None:
+    def wait(self, **kwargs) -> None:
         if not get_accelerator().resolves_data_dependency():
             get_accelerator().current_stream().synchronize()
         self.__param.ds_status = ZeroParamStatus.AVAILABLE
@@ -78,7 +78,7 @@ def __init__(self, params: List[Parameter]) -> None:
                                                      non_blocking=True).view(param.ds_shape)
 
     @instrument_w_nvtx
-    def wait(self) -> None:
+    def wait(self, **kwargs) -> None:
         if self.__complete:
             return
 
@@ -639,7 +639,7 @@ def __init__(self, handle, param: Parameter, quantization=None) -> None:
         self.__param = param
         self.__quantization = quantization
 
-    def wait(self) -> None:
+    def wait(self, handle_dependency=True) -> None:
         instrument_w_nvtx(self.__handle.wait)()
         if self.__quantization:
             instrument_w_nvtx(self.__quantization.quant_handle.wait)()
@@ -650,6 +650,8 @@ def wait(self) -> None:
 
 class AllGatherCoalescedHandle:
 
+    data_buffer = []
+
     def __init__(
         self,
         allgather_handle,
@@ -672,7 +674,7 @@ def __init__(
                 raise RuntimeError(f"expected param {param.ds_summary()} to not be available")
 
     @instrument_w_nvtx
-    def wait(self) -> None:
+    def wait(self, handle_dependency=True) -> None:
         if self.complete:
             return
 
@@ -704,24 +706,30 @@ def wait(self) -> None:
                     partitions.append(part_to_copy)
             param.data = instrument_w_nvtx(torch.cat)(partitions).view(param.ds_shape)
             param.ds_status = ZeroParamStatus.AVAILABLE
-
-            for part_to_copy in partitions:
-                if not get_accelerator().is_synchronized_device():
+            if not get_accelerator().is_synchronized_device() and handle_dependency:
+                for part_to_copy in partitions:
                     part_to_copy.record_stream(get_accelerator().current_stream())
 
             param_offset += ds_tensor_numel
 
         self.complete = True
+        if not get_accelerator().is_synchronized_device() and not handle_dependency:
+            # if the device needs to handle dependencies and opts for explicit processing outside the function.
+            AllGatherCoalescedHandle.data_buffer.append(partitions)
+
+    @staticmethod
+    def free_buffer():
+        AllGatherCoalescedHandle.data_buffer = []
 
 
 class MultipleAllGatherHandles:
 
     def __init__(self, handles: List[AllGatherCoalescedHandle]):
         self.handles = handles
 
-    def wait(self) -> None:
+    def wait(self, handle_dependency=True) -> None:
         for handle in self.handles:
-            handle.wait()
+            handle.wait(handle_dependency)
 
 
 class AllReduceCoalescedHandle:
@@ -1377,13 +1385,13 @@ def all_gather_coalesced(params: Iterable[Parameter],
                             quantization=quant_info,
                         )
 
-        def partition(param_list=None, hierarchy=0, has_been_updated=False):
+        def partition(param_list=None, hierarchy=0, has_been_updated=False, free_data=True):
             cls = param
             print_rank_0(f"{'--'*hierarchy}----Partitioning param {debug_param2name_id_shape_device(cls)}",
                          force=False)
             if param_list is None:
                 param_list = [cls]
-            self._partition(param_list, has_been_updated=has_been_updated)
+            self._partition(param_list, has_been_updated=has_been_updated, free_data=True)
 
         def reduce_gradients_at_owner(param_list=None, hierarchy=0):
             cls = param
@@ -1527,20 +1535,20 @@ def _all_gather(self, param_list, async_op=False, hierarchy=None):
 
         return handles
 
-    def _partition(self, param_list, force=False, has_been_updated=False):
+    def _partition(self, param_list, force=False, has_been_updated=False, free_data=True):
         for param in param_list:
             print_rank_0(f"Before Partitioning Param {param.ds_id}", force=False)
             if self.zero_param_process_group is not None:
                 self._partition_param_sec(param)
-            self._partition_param(param, has_been_updated=has_been_updated)
+            self._partition_param(param, has_been_updated=has_been_updated, free_data=True)
 
             param.ds_status = ZeroParamStatus.NOT_AVAILABLE
             # if param.ds_tensor is not None:
             #    assert id(param.data) == id(param.ds_tensor.data), \
             #    "After the parameters are initially partitioned, make sure we are not recreating the partition."
             #print_rank_0(f"After Partitioning Param {param.ds_id} {param.ds_tensor.size()} {param.ds_tensor}",force=False)
     @instrument_w_nvtx
-    def _partition_param(self, param, buffer=None, has_been_updated=False):
+    def _partition_param(self, param, buffer=None, has_been_updated=False, free_data=True):
         assert param.ds_status is not ZeroParamStatus.INFLIGHT, f" {param} Cannot partition a param in flight"
         global reuse_buffers
         print_rank_0(f"Param id {param.ds_id} status is {param.ds_status}", force=False)
@@ -1565,7 +1573,8 @@ def _partition_param(self, param, buffer=None, has_been_updated=False):
 
                 see_memory_usage(f'Before partitioning param {param.ds_id} {param.shape}', force=False)
                 # param.data does not store anything meaningful in partitioned state
-                free_param(param)
+                if free_data:
+                    free_param(param)
                 see_memory_usage(f'After partitioning param {param.ds_id} {param.shape}', force=False)
 
                 if param.ds_tensor.final_location == OffloadDeviceEnum.nvme:

deepspeed/runtime/zero/partitioned_param_coordinator.py

@@ -76,18 +76,17 @@ class __ParamInTrace:
         param: Parameter
         step_id_last_used_at: int
 
-    def __init__(
-        self,
-        prefetch_bucket_sz: int,
-        max_reuse_distance_in_numel: int,
-        max_available_parameters_in_numel: int,
-        allgather_stream: get_accelerator().Stream,
-        inflight_param_registry: InflightParamRegistry,
-        prefetch_nvme: bool = False,
-        timers=None,
-        zero_quantized_weights=False,
-        zero_quantized_nontrainable_weights=False,
-    ) -> None:
+    def __init__(self,
+                 prefetch_bucket_sz: int,
+                 max_reuse_distance_in_numel: int,
+                 max_available_parameters_in_numel: int,
+                 allgather_stream: get_accelerator().Stream,
+                 inflight_param_registry: InflightParamRegistry,
+                 prefetch_nvme: bool = False,
+                 timers=None,
+                 zero_quantized_weights=False,
+                 zero_quantized_nontrainable_weights=False,
+                 fast_sharding_for_leaf_module=False) -> None:
         # mapping of param -> handle for each param that is currently in flight
         self.__inflight_param_registry = inflight_param_registry
         # keeps track of the number of submodules invoked so far.
@@ -130,6 +129,10 @@ def __init__(
         self.__max_ongoing_fetch_events: int = 2
         self.__profiler = PartitionedParameterProfiler(timers if ENABLE_PROFILER else None)
 
+        # whether to enable fast fetch for the z3 leaf module.
+        # this will improve fetch speed but will not break down leaf module parameters to alleviate memory pressure.
+        self.fast_sharding_for_leaf_module = fast_sharding_for_leaf_module
+
     """Tracing and Tracking
     TODO. consider performing trace before initializing PartitionedParameterCoordinator
     and passing trace results into constructor. This way all the code in here can
@@ -308,6 +311,7 @@ def fetch_sub_module(self, current_submodule: Module, forward: bool) -> None:
         wait_numel = 0
         wait_event_name = __class__.FORWARD_FETCH_WAIT if forward else __class__.BACKWARD_FETCH_WAIT
         self.__profiler.start_event(wait_event_name)
+        fast_fetch = self.fast_sharding_for_leaf_module and z3_leaf_module(current_submodule)
         # wait for parameters in the immediately needed submodule to become available
         for param in params_to_fetch:
             param.ds_active_sub_modules.add(current_submodule.id)
@@ -321,16 +325,18 @@ def fetch_sub_module(self, current_submodule: Module, forward: bool) -> None:
                     if len(self.__ongoing_fetch_events) > self.__max_ongoing_fetch_events:
                         self.__ongoing_fetch_events.popleft().synchronize()
 
-                    self.__inflight_param_registry.pop(param).wait()
+                    self.__inflight_param_registry.pop(param).wait(handle_dependency=not fast_fetch)
 
-                    if not get_accelerator().handles_memory_backpressure():
+                    if not get_accelerator().handles_memory_backpressure() and not fast_fetch:
                         event = get_accelerator().Event()
                         event.record()
                         self.__ongoing_fetch_events.append(event)
 
             assert param.ds_status == ZeroParamStatus.AVAILABLE, param.ds_summary()
         if not get_accelerator().resolves_data_dependency():
             get_accelerator().current_stream().wait_stream(self.__allgather_stream)
+        if fast_fetch:
+            AllGatherCoalescedHandle.free_buffer()
         self.__profiler.stop_event(wait_event_name, wait_numel)
 
         # kick off parameter prefetches for upcoming modules
@@ -412,10 +418,20 @@ def release_sub_module(self, submodule: Module) -> None:
         be released."""
         params_to_release = (self.__params_to_release(submodule, self.__step_id) if self.is_complete_trace() else set(
             p.ds_id for p in iter_params(submodule, recurse=z3_leaf_module(submodule))))
+
+        free_data = not z3_leaf_module(submodule) or not self.fast_sharding_for_leaf_module
+        if not free_data:
+            # wait for the computation to finish and launch as early as possible.
+            empty_buffer = torch.empty(1, device=get_accelerator().current_device())
+
         for param in iter_params(submodule, recurse=z3_leaf_module(submodule)):
             param.ds_active_sub_modules.discard(submodule.id)
             if param.ds_id in params_to_release and not param.is_external_param:
-                self.__release_param(param)
+                self.__release_param(param, free_data)
+            if not free_data:
+                if param.ds_id in params_to_release and not param.is_external_param:
+                    # empty buffer ensures that all computations are complete
+                    param.data = empty_buffer
 
     @instrument_w_nvtx
     @torch.no_grad()
@@ -490,11 +506,11 @@ def __all_gather_params_(self, params: Set[Parameter], forward: bool, quantize:
 
     @compiler.disable
     @instrument_w_nvtx
-    def __release_param(self, param: Parameter) -> None:
+    def __release_param(self, param: Parameter, free_data: bool = True) -> None:
         if param.ds_status == ZeroParamStatus.AVAILABLE and not param.ds_active_sub_modules:
             if logger.isEnabledFor(logging.DEBUG):
                 debug_rank0(f"-release: {param.ds_summary()}")
-            param.partition()
+            param.partition(free_data=free_data)
             self.__n_available_params -= param.ds_numel
 
     @instrument_w_nvtx