Make the scratch pad tensor UVA (pytorch#2844)

Summary:
Pull Request resolved: pytorch#2844

Before this diff, the scratch pad in SSD TBE (see D55998215 for more
detail) was a CPU tensor which was later transferred to GPU to allow
the TBE kernels to access it.  The scratch pad tranfer was highly
inefficient since TBE over provisioned the scratch pad buffer
allocation (as it did not know the exact number of cache missed rows)
causing extra data transfer.  Such the extra data transfer could be
large since the number of cache missed rows was normally much smaller
than value that TBE over provisioned.

There are two ways to avoid the extra data transfer:

(1) Let TBE have the exact number of cache missed rows on host which
requires device-to-host data transfer which will cause a sync point
between host and device (not desirable in most trainings).
However, this will allow TBE to use `cudaMemcpy` which will utilize
the DMA engine and will allow the memory copy to overlap efficiently
with other compute kernels.

(2) Make the scratch pad accessible by both CPU and GPU.  In other
words, make the scratch pad a UVA tensor.  This does not require
device and host synchornization.  However, the memory copy has to be
done through CUDA load/store which requires a kernel to run on SMs.
Thus, the memory copy and compute kernel overlapping will require a
careful SMs management.

Based on the tradeoffs explained above, we chose to implement (2)
to avoid the host and device sync point.

Differential Revision: D58631974

fbgemm_gpu/fbgemm_gpu/tbe/ssd/training.py

@@ -278,7 +278,7 @@ def __init__(
         self.ssd_event_evict_sp = torch.cuda.Event()
 
         self.timesteps_prefetched: List[int] = []
-        self.ssd_scratch_pads: List[Tuple[Tensor, Tensor, Tensor]] = []
+        self.ssd_scratch_pads: List[Tuple[Tensor, Tensor, Tensor, bool]] = []
         # TODO: add type annotation
         # pyre-fixme[4]: Attribute must be annotated.
         self.ssd_prefetch_data = []
@@ -397,48 +397,71 @@ def to_pinned_cpu(self, t: torch.Tensor) -> torch.Tensor:
 
     def evict(
         self,
-        evicted_rows: Tensor,
-        evicted_indices: Tensor,
+        rows: Tensor,
+        indices: Tensor,
         actions_count_cpu: Tensor,
-        eviction_stream: torch.cuda.Stream,
+        stream: torch.cuda.Stream,
         pre_event: torch.cuda.Event,
         post_event: torch.cuda.Event,
+        is_rows_uvm: bool,
     ) -> None:
         """
         Evict data from the given input tensors to SSD via RocksDB
+
+        Args:
+            rows (Tensor): The 2D tensor that contains rows to evict
+            indices (Tensor): The 1D tensor that contains the row indices that
+                             the rows will be evicted to
+            actions_count_cpu (Tensor): A scalar tensor that contains the
+                                        number of rows that the evict function
+                                        has to process
+            stream (Stream): The CUDA stream that cudaStreamAddCallback will
+                             synchronize the host function with.  Moreover, the
+                             asynchronous D->H memory copies will operate on
+                             this stream
+            pre_event (Event): The CUDA event that the stream has to wait on
+            post_event (Event): The CUDA event that the current will record on
+                                when the eviction is done
+            is_rows_uvm (bool): A flag to indicate whether `rows` is a UVM
+                                tensor (which is accessible on both host and
+                                device)
+        Returns:
+            None
         """
-        with torch.cuda.stream(eviction_stream):
-            eviction_stream.wait_event(pre_event)
+        with torch.cuda.stream(stream):
+            stream.wait_event(pre_event)
 
-            evicted_rows_cpu = self.to_pinned_cpu(evicted_rows)
-            evicted_indices_cpu = self.to_pinned_cpu(evicted_indices)
+            rows_cpu = rows if is_rows_uvm else self.to_pinned_cpu(rows)
+            indices_cpu = self.to_pinned_cpu(indices)
 
-            evicted_rows.record_stream(eviction_stream)
-            evicted_indices.record_stream(eviction_stream)
+            rows.record_stream(stream)
+            indices.record_stream(stream)
 
             self.ssd_db.set_cuda(
-                evicted_indices_cpu, evicted_rows_cpu, actions_count_cpu, self.timestep
+                indices_cpu, rows_cpu, actions_count_cpu, self.timestep
             )
 
             # TODO: is this needed?
             # Need a way to synchronize
             #  actions_count_cpu.record_stream(self.ssd_stream)
-            eviction_stream.record_event(post_event)
+            stream.record_event(post_event)
 
     def _evict_from_scratch_pad(self, grad: Tensor) -> None:
         assert len(self.ssd_scratch_pads) > 0, "There must be at least one scratch pad"
-        (inserted_rows_gpu, post_bwd_evicted_indices, actions_count_cpu) = (
+        (inserted_rows, post_bwd_evicted_indices, actions_count_cpu, do_evict) = (
             self.ssd_scratch_pads.pop(0)
         )
-        torch.cuda.current_stream().record_event(self.ssd_event_backward)
-        self.evict(
-            inserted_rows_gpu,
-            post_bwd_evicted_indices,
-            actions_count_cpu,
-            self.ssd_stream,
-            self.ssd_event_backward,
-            self.ssd_event_evict_sp,
-        )
+        if do_evict:
+            torch.cuda.current_stream().record_event(self.ssd_event_backward)
+            self.evict(
+                inserted_rows,
+                post_bwd_evicted_indices,
+                actions_count_cpu,
+                self.ssd_stream,
+                self.ssd_event_backward,
+                self.ssd_event_evict_sp,
+                is_rows_uvm=True,
+            )
 
     def _compute_cache_ptrs(
         self,
@@ -447,7 +470,7 @@ def _compute_cache_ptrs(
         linear_index_inverse_indices: torch.Tensor,
         unique_indices_count_cumsum: torch.Tensor,
         cache_set_inverse_indices: torch.Tensor,
-        inserted_rows_gpu: torch.Tensor,
+        inserted_rows: torch.Tensor,
         unique_indices_length: torch.Tensor,
         inserted_indices: torch.Tensor,
         actions_count_cpu: torch.Tensor,
@@ -468,7 +491,7 @@ def _compute_cache_ptrs(
                     unique_indices_count_cumsum,
                     cache_set_inverse_indices,
                     self.lxu_cache_weights,
-                    inserted_rows_gpu,
+                    inserted_rows,
                     unique_indices_length,
                     inserted_indices,
                 )
@@ -477,14 +500,19 @@ def _compute_cache_ptrs(
         with record_function("## ssd_scratch_pads ##"):
             # Store scratch pad info for post backward eviction
             self.ssd_scratch_pads.append(
-                (inserted_rows_gpu, post_bwd_evicted_indices, actions_count_cpu)
+                (
+                    inserted_rows,
+                    post_bwd_evicted_indices,
+                    actions_count_cpu,
+                    linear_cache_indices.numel() > 0,
+                )
             )
 
         # pyre-fixme[7]: Expected `Tensor` but got `Tuple[typing.Any, Tensor,
         #  typing.Any, Tensor]`.
         return (
             lxu_cache_ptrs,
-            inserted_rows_gpu,
+            inserted_rows,
             post_bwd_evicted_indices,
             actions_count_cpu,
         )
@@ -522,42 +550,50 @@ def prefetch(self, indices: Tensor, offsets: Tensor) -> Optional[Tensor]:
         evicted_rows = self.lxu_cache_weights[
             assigned_cache_slots.clamp(min=0).long(), :
         ]
-        inserted_rows = torch.empty(
-            evicted_rows.shape,
-            dtype=self.lxu_cache_weights.dtype,
-            pin_memory=True,
-        )
+
+        if linear_cache_indices.numel() > 0:
+            inserted_rows = torch.ops.fbgemm.new_managed_tensor(
+                torch.zeros(
+                    1, device=self.current_device, dtype=self.lxu_cache_weights.dtype
+                ),
+                evicted_rows.shape,
+            )
+        else:
+            inserted_rows = torch.empty(
+                evicted_rows.shape,
+                dtype=self.lxu_cache_weights.dtype,
+                device=self.current_device,
+            )
 
         current_stream = torch.cuda.current_stream()
 
+        inserted_indices_cpu = self.to_pinned_cpu(inserted_indices)
+
         # Ensure the previous iterations l3_db.set(..) has completed.
         current_stream.wait_event(self.ssd_event_evict)
         current_stream.wait_event(self.ssd_event_evict_sp)
-
-        self.ssd_db.get_cuda(
-            self.to_pinned_cpu(inserted_indices), inserted_rows, actions_count_cpu
-        )
+        if linear_cache_indices.numel() > 0:
+            self.ssd_db.get_cuda(inserted_indices_cpu, inserted_rows, actions_count_cpu)
         current_stream.record_event(self.ssd_event_get)
-        # TODO: T123943415 T123943414 this is a big copy that is (mostly) unnecessary with a decent cache hit rate.
-        # Should we allocate on HBM?
-        inserted_rows_gpu = inserted_rows.cuda(non_blocking=True)
 
         torch.ops.fbgemm.masked_index_put(
             self.lxu_cache_weights,
             assigned_cache_slots,
-            inserted_rows_gpu,
+            inserted_rows,
             actions_count_gpu,
         )
 
-        # Evict rows from cache to SSD
-        self.evict(
-            evicted_rows,
-            evicted_indices,
-            actions_count_cpu,
-            self.ssd_stream,
-            self.ssd_event_get,
-            self.ssd_event_evict,
-        )
+        if linear_cache_indices.numel() > 0:
+            # Evict rows from cache to SSD
+            self.evict(
+                evicted_rows,
+                evicted_indices,
+                actions_count_cpu,
+                self.ssd_stream,
+                self.ssd_event_get,
+                self.ssd_event_evict,
+                is_rows_uvm=False,
+            )
 
         # TODO: keep only necessary tensors
         self.ssd_prefetch_data.append(
@@ -567,7 +603,7 @@ def prefetch(self, indices: Tensor, offsets: Tensor) -> Optional[Tensor]:
                 linear_index_inverse_indices,
                 unique_indices_count_cumsum,
                 cache_set_inverse_indices,
-                inserted_rows_gpu,
+                inserted_rows,
                 unique_indices_length,
                 inserted_indices,
                 actions_count_cpu,
@@ -593,7 +629,7 @@ def forward(
         prefetch_data = self.ssd_prefetch_data.pop(0)
         (
             lxu_cache_ptrs,
-            inserted_rows_gpu,
+            inserted_rows,
             post_bwd_evicted_indices,
             actions_count_cpu,
         ) = self._compute_cache_ptrs(*prefetch_data)
@@ -635,7 +671,7 @@ def forward(
             # codegen/genscript/optimizer_args.py
             ssd_tensors={
                 "row_addrs": lxu_cache_ptrs,
-                "inserted_rows": inserted_rows_gpu,
+                "inserted_rows": inserted_rows,
                 "post_bwd_evicted_indices": post_bwd_evicted_indices,
                 "actions_count": actions_count_cpu,
             },