benchmark of fbgemm op - permute_multi_embedding (#2158)

Summary:

X-link: pytorch/FBGEMM#2771

# context
* added both **op-level** and **fn-level** benchmarks for the KT.regroup implementations
* analyze the op-level and fn-level performance in runtime and memory usage
* findings are that:
**a**. In the fn-level performance, the `permute_multi_embedding` (new op) outperforms both the native-pytorch implementation and the `permute_pooled_embs_auto_grad` (current Prod) by 50% GPU runtime and 33% memory usage
**b**. In the op-level performance, the new op is slightly slower than the current prod (by ~5% GPU runtime)
* conclusion: **we should use the new op**

# other considerations
The good:
1. the algorithm is designed in a way that it doesn't need to know in advance whether the 1-to-N mapping exists in the permutes. 
2. `_all_keys_used_once` is no longer needed
3. no longer need a torch.cat before calling the old operator
4. no need to use `_pin_and_move` for the meta data (arguments), it will be handled inside the operator, it's more friendly to tracing.

The same bad:
1. it requires several HtoD communications (move tensor to device):
a) [resolved] 3 tensors, which are `permutes`, `input_lengths`, and `output_lengths`. Those tensors needs to be on the device so that the cuda kernels has access to it.
b) [resolved] 2 lists of (scalar_t*) pointers, input and output tensor lists.
c) [resolved] Didn't find a good way to let the kernel knows the address of the lists of input/output tensors, because the lists are also need to be on the device. 
2. tensor.contiguous for the backward function, it looks like the grad from the backward are somehow not contiguous.

# benchmark
* op-level results: new op is ~5% slower in GPU runtime
```
INFO:root:size: 1024 x 136896; permute_multi_embedding: 2.25 ms; permute_pooled_embs: 2.15 ms; delta: 4.5%
INFO:root:size: 1024 x 108432; permute_multi_embedding: 1.79 ms; permute_pooled_embs: 1.7 ms; delta: 5.3%
INFO:root:size: 1024 x 277232; permute_multi_embedding: 4.54 ms; permute_pooled_embs: 4.37 ms; delta: 3.9%
INFO:root:size: 1024 x 244352; permute_multi_embedding: 4.01 ms; permute_pooled_embs: 3.83 ms; delta: 4.9%
INFO:root:size: 1024 x 524224; permute_multi_embedding: 8.62 ms; permute_pooled_embs: 8.25 ms; delta: 4.5%
INFO:root:size: 1024 x 564080; permute_multi_embedding: 9.27 ms; permute_pooled_embs: 8.92 ms; delta: 3.9%
```
* fn-level results: new op is 50%+ faster in GPU runtime and uses 33% fewer GPU memory
```
  _regroup_keyed_tenors               | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   2.8 ms | Memory (P90): 1011.0
  KeyedTensor.regroup                 | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   5.0 ms | Memory (P90): 1517.0
  KTRegroupAsDict                     | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   4.9 ms | Memory (P90): 1517.0
  permute_multi_embs                  | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   2.2 ms | Memory (P90): 1011.0
  _regroup_keyed_tenors_dup           | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   2.5 ms | Memory (P90): 1011.0
  KeyedTensor.regroup_dup             | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   2.5 ms | Memory (P90): 1011.0
  KTRegroupAsDict_dup                 | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   2.5 ms | Memory (P90): 1011.0
  permute_multi_embs_dup              | B: 1024     | F: 1020     | device: cuda     | Runtime (P90):   3.2 ms | Memory (P90): 1011.0
```

# traces
* [files](https://drive.google.com/drive/folders/1_9hOtQUQeFICBVxQtusvpQ_VajduFUmR?usp=sharing)
```
[[email protected] /data/sandcastle/boxes/fbsource (ae677c240)]$ ll *.json
-rw-rw-r-- 1 hhy hhy 8062993 Jun 21 23:26 trace-KeyedTensor.regroup_dup.json
-rw-rw-r-- 1 hhy hhy  949610 Jun 21 23:26 trace-KeyedTensor.regroup.json
-rw-rw-r-- 1 hhy hhy 5140143 Jun 21 23:26 trace-KTRegroupAsDict_dup.json
-rw-rw-r-- 1 hhy hhy  350370 Jun 21 23:26 trace-KTRegroupAsDict.json
-rw-rw-r-- 1 hhy hhy  581033 Jun 21 23:26 trace-permute_multi_embs_dup.json
-rw-rw-r-- 1 hhy hhy  582607 Jun 21 23:26 trace-permute_multi_embs.json
-rw-rw-r-- 1 hhy hhy 8025337 Jun 21 23:26 trace-_regroup_keyed_tenors_dup.json
-rw-rw-r-- 1 hhy hhy 8041586 Jun 21 23:26 trace-_regroup_keyed_tenors.json
```
* native-pytorch
 {F1713052022} 
* current prod
 {F1713052648} 
* new op
 {F1713052907} 
* runtime
|item|CPU runtime|GPU runtime|GPU memory|notes|
|---|---|---|---|---|
|**native-pytorch**|3.9 ms|3.1 ms|1.0 K|CPU-bounded|
|**prod op**|2.1 ms|4.9 ms|1.5 K|GPU-boudned due to torch.cat|
|**new op**|2.0 K|2.2 ms|1.0 K|both CPU and GPU runtime outperformed|

Differential Revision: D58906839

Author: TroyGarden

torchrec/sparse/jagged_tensor.py

@@ -170,6 +170,21 @@ def _all_keys_used_once(
     return len(key_set) == len(group_set) == len(flat_keys) == len(flat_groups)
 
 
+@torch.fx.wrap
+def permute_multi_embedding(
+    keyed_tensors: List["KeyedTensor"], groups: List[List["str"]]
+) -> List[torch.Tensor]:
+    keys, lengths, values = _desugar_keyed_tensors(keyed_tensors)
+    permutes, in_lengths, out_lengths = _multi_remap_to_groups(keys, lengths, groups)
+    permuted_values = torch.ops.fbgemm.permute_multi_embedding(
+        values,
+        permutes,
+        in_lengths,
+        out_lengths,
+    )
+    return permuted_values
+
+
 @torch.fx.wrap
 def _fbgemm_permute_pooled_embs(
     keyed_tensors: List["KeyedTensor"], groups: List[List["str"]]

torchrec/sparse/tests/jagged_tensor_benchmark.py

@@ -21,6 +21,7 @@
     _regroup_keyed_tensors,
     KeyedJaggedTensor,
     KeyedTensor,
+    permute_multi_embedding,
 )
 from torchrec.sparse.tests.utils import build_groups, build_kts
 
@@ -245,6 +246,17 @@ def main(
                         {"keyed_tensors": kts},
                         profile,
                     )
+                    bench(
+                        "permute_multi_embs" + dup,
+                        labels,
+                        batch_size,
+                        n_dense + n_sparse,
+                        device_type,
+                        run_backward,
+                        permute_multi_embedding,
+                        {"keyed_tensors": kts, "groups": groups},
+                        profile,
+                    )
 
 
 if __name__ == "__main__":