From dd6f073377f376fb1033195061a31c80b3b7e12f Mon Sep 17 00:00:00 2001 From: Qiaolin Yu Date: Tue, 26 May 2026 14:14:48 -0700 Subject: [PATCH] Reland "[perf][spec decoding] Skip full-vocab softmax in EAGLE draft when topk == 1 (#26235)" (#26397) --- .../eagle_draft_extend_cuda_graph_runner.py | 16 +++++++++-- .../sglang/srt/speculative/eagle_worker_v2.py | 27 ++++++++++++++++--- 2 files changed, 37 insertions(+), 6 deletions(-) diff --git a/python/sglang/srt/speculative/eagle_draft_extend_cuda_graph_runner.py b/python/sglang/srt/speculative/eagle_draft_extend_cuda_graph_runner.py index ad17631bc..d2992024e 100644 --- a/python/sglang/srt/speculative/eagle_draft_extend_cuda_graph_runner.py +++ b/python/sglang/srt/speculative/eagle_draft_extend_cuda_graph_runner.py @@ -30,12 +30,15 @@ from sglang.srt.model_executor.input_buffers import ForwardInputBuffers from sglang.srt.speculative.eagle_info import EagleDraftExtendInput from sglang.srt.speculative.spec_utils import fast_topk from sglang.srt.utils import ( + is_hip, require_attn_tp_gather, require_gathered_buffer, require_mlp_sync, require_mlp_tp_gather, ) +_is_hip = is_hip() + if TYPE_CHECKING: from sglang.srt.speculative.eagle_worker import EAGLEWorker @@ -401,8 +404,17 @@ class EAGLEDraftExtendCudaGraphRunner: forward_batch.positions, forward_batch, ) - probs = torch.softmax(ret.next_token_logits, dim=-1) - ret.topk_p, ret.topk_index = fast_topk(probs, self.topk, dim=-1) + # ROCm's argmax tie-breaks differently from CUDA's softmax+max + # path on FP8 logits, which corrupts MTP draft selection on AMD. + # Keep the fastpath CUDA-only. + if self.topk == 1 and not _is_hip: + ret.topk_index = torch.argmax( + ret.next_token_logits, dim=-1, keepdim=True + ) + ret.topk_p = torch.ones_like(ret.topk_index, dtype=torch.float32) + else: + probs = torch.softmax(ret.next_token_logits, dim=-1) + ret.topk_p, ret.topk_index = fast_topk(probs, self.topk, dim=-1) forward_batch.out_cache_loc = output_cache_loc_backup forward_batch.spec_info.hidden_states = hidden_states_backup diff --git a/python/sglang/srt/speculative/eagle_worker_v2.py b/python/sglang/srt/speculative/eagle_worker_v2.py index bb1b51686..5421282f9 100644 --- a/python/sglang/srt/speculative/eagle_worker_v2.py +++ b/python/sglang/srt/speculative/eagle_worker_v2.py @@ -487,8 +487,19 @@ class EagleDraftWorker(BaseDraftWorker): ).logits_output maybe_detect_nan(logits_output.next_token_logits, f"draft_forward step {i}") maybe_detect_inf(logits_output.next_token_logits, f"draft_forward step {i}") - probs = torch.softmax(logits_output.next_token_logits, dim=-1) - topk_p, topk_index = fast_topk(probs, self.topk, dim=-1) + if self.topk == 1 and not _is_hip: + # topk=1 → degenerate single-path tree; `topk_p` is unused + # downstream, so skip softmax and just argmax over logits. + # Gated to CUDA: on ROCm the argmax tie-break diverges from + # the softmax+max path on FP8 logits and corrupts MTP draft + # selection (DSV3.2 MTP GSM8K, see #26358). + topk_index = torch.argmax( + logits_output.next_token_logits, dim=-1, keepdim=True + ) + topk_p = torch.ones_like(topk_index, dtype=torch.float32) + else: + probs = torch.softmax(logits_output.next_token_logits, dim=-1) + topk_p, topk_index = fast_topk(probs, self.topk, dim=-1) maybe_detect_oob( topk_index, 0, @@ -660,8 +671,16 @@ class EagleDraftWorker(BaseDraftWorker): draft_logits_output.hidden_states = draft_logits_output.hidden_states[ select_index ] - probs = torch.softmax(draft_logits_output.next_token_logits, dim=-1) - ret_topk_p, ret_topk_index = fast_topk(probs, self.topk, dim=-1) + if self.topk == 1 and not _is_hip: + # Gated to CUDA: see #26358 — ROCm's argmax tie-break corrupts + # MTP draft selection on FP8 logits. + ret_topk_index = torch.argmax( + draft_logits_output.next_token_logits, dim=-1, keepdim=True + ) + ret_topk_p = torch.ones_like(ret_topk_index, dtype=torch.float32) + else: + probs = torch.softmax(draft_logits_output.next_token_logits, dim=-1) + ret_topk_p, ret_topk_index = fast_topk(probs, self.topk, dim=-1) ret_hidden_states = draft_logits_output.hidden_states # Construct the return values