[TRTLLM-7728][perf] improve batched sampling perf for contiguous batches (#7908)

Signed-off-by: ixlmar <[email protected]>

Author: ixlmar

tensorrt_llm/_torch/pyexecutor/sampler.py

@@ -396,6 +396,7 @@ def _group_requests_by_sampling_strategy(
         requests: Iterable[LlmRequest],
         *,
         pin_memory: bool = False) -> dict[Strategy, torch.Tensor]:
+    # NB: Client code relies on request indices in returned torch.Tensor being sorted.
     strategy_dict: dict[Strategy, list[int]] = defaultdict(list)
     for req_index, req in enumerate(requests):
         strategy_dict[_request_strategy(req)].append(req_index)
@@ -1372,12 +1373,20 @@ def _sample_batched_by_strategy(
                     len(speculation_group_indices), dtype=torch.int32)
 
             group_logits_cuda_indices = logits_cuda_indexer[group_req_indices]
-            if group_logits_cuda_indices.numel() != logits_cuda.size(0):
+            # NB: Assuming that group_req_indices are sorted
+            group_req_1st_index, group_req_last_index = group_req_indices[
+                0], group_req_indices[-1]
+            if group_req_last_index - group_req_1st_index + 1 == len(
+                    group_req_indices):
+                # Avoid data movement if indices are contiguous
+                group_logits_cuda = logits_cuda[
+                    req_offsets[group_req_1st_index]:(
+                        req_offsets[group_req_last_index] +
+                        req_num_steps[group_req_last_index])]
+            else:
                 group_logits_cuda_indices_cuda = group_logits_cuda_indices.to(
                     device=logits_cuda.device, non_blocking=True)
                 group_logits_cuda = logits_cuda[group_logits_cuda_indices_cuda]
-            else:
-                group_logits_cuda = logits_cuda
 
             # Indexer for accessing tokens in 'group_logits_cuda' (and 'group_next_tokens_cuda')
             # corresponding to the requests in 'group_req_indices'.