[NSA] Avoid repeated NSA MQA logits memory queries (#25299)

python/sglang/srt/layers/attention/nsa/nsa_indexer.py

@@ -176,6 +176,12 @@ def rotate_activation(x: torch.Tensor) -> torch.Tensor:
 
 
 class Indexer(MultiPlatformOp):
+    _MQA_LOGITS_BYTES_PER_ELEM = 4
+    _MQA_LOGITS_STATIC_SKIP_ELEMS = 8_000_000
+    _MQA_LOGITS_FREE_MEM_FRACTION = 0.5
+    _MQA_LOGITS_TOTAL_MEM_FRACTION = 0.3
+    _mqa_logits_budget_bytes: Dict[int, int] = {}
+
     def __init__(
         self,
         hidden_size: int,
@@ -552,24 +558,59 @@ class Indexer(MultiPlatformOp):
             topk_result = torch.cat([topk_result, padding], dim=0)
         return topk_result
 
+    def _get_mqa_logits_budget_bytes(self, device_index: int) -> int:
+        cached_budget = self._mqa_logits_budget_bytes.get(device_index)
+        if cached_budget is not None:
+            return cached_budget
+
+        total_mem = torch.cuda.get_device_properties(device_index).total_memory
+
+        total_mem_budget = int(total_mem * self._MQA_LOGITS_TOTAL_MEM_FRACTION)
+        mem_fraction_static = get_global_server_args().mem_fraction_static
+        if mem_fraction_static is None:
+            static_budget = total_mem_budget
+        else:
+            static_free_mem = int(total_mem * max(0.0, 1.0 - mem_fraction_static))
+            static_budget = min(
+                int(static_free_mem * self._MQA_LOGITS_FREE_MEM_FRACTION),
+                total_mem_budget,
+            )
+        static_budget = max(1, static_budget)
+
+        # Keep the static serving-memory guard during CUDA graph capture without
+        # caching it. The first non-capture prefill path will cache the real
+        # free-memory budget below.
+        if get_is_capture_mode():
+            return static_budget
+
+        # Match the original free-memory guard: logits_bytes * 2 > free_mem.
+        # torch.cuda.mem_get_info synchronizes the host, so cache the result,
+        # capped by the workload-independent serving-memory headroom.
+        free_mem, _ = torch.cuda.mem_get_info(device_index)
+        budget_bytes = min(
+            int(free_mem * self._MQA_LOGITS_FREE_MEM_FRACTION), static_budget
+        )
+
+        budget_bytes = max(1, budget_bytes)
+        self._mqa_logits_budget_bytes[device_index] = budget_bytes
+        return budget_bytes
+
     def _should_chunk_mqa_logits(
-        self, num_q: int, num_k: int, device: torch.device
+        self, num_q: int, num_k: int, device_index: int
     ) -> Tuple[bool, int]:
         """
         Detect whether we need to chunk the MQA logits computation to avoid OOM
-        Return: (need_chunk, free_mem)
+        Return: (need_chunk, logits_budget_bytes)
         """
         # Quick static check for normal batches
-        if num_q * num_k < 8_000_000:  # 8M elements ≈ 32MB logits
+        if num_q * num_k < self._MQA_LOGITS_STATIC_SKIP_ELEMS:
             return False, 0
 
-        free_mem, total_mem = torch.cuda.mem_get_info(device)
-        bytes_per_elem = 4  # float32
-        logits_bytes = num_q * num_k * bytes_per_elem
+        logits_bytes = num_q * num_k * self._MQA_LOGITS_BYTES_PER_ELEM
+        logits_budget_bytes = self._get_mqa_logits_budget_bytes(device_index)
 
-        # Logits should not exceed 50% of free memory or 30% of total memory
-        need_chunk = (logits_bytes * 2 > free_mem) or (logits_bytes > total_mem * 0.3)
-        return need_chunk, free_mem
+        need_chunk = logits_bytes > logits_budget_bytes
+        return need_chunk, logits_budget_bytes
 
     def _get_topk_ragged(
         self,
@@ -618,6 +659,8 @@ class Indexer(MultiPlatformOp):
         batch_size = len(block_tables)
         token_nums, _, _ = q_fp8.shape
         device = q_fp8.device
+        device_index = device.index
+        assert device_index is not None, "q_fp8 must be on an indexed CUDA device"
 
         topk_result = torch.full(
             (token_nums, self.index_topk), -1, device=device, dtype=torch.int32
@@ -650,7 +693,9 @@ class Indexer(MultiPlatformOp):
         token_to_batch_idx = metadata.get_token_to_batch_idx()
         q_offset = ks.shape[0]
         k_offset = k_fp8.shape[0]
-        need_chunk, free_mem = self._should_chunk_mqa_logits(q_offset, k_offset, device)
+        need_chunk, logits_budget_bytes = self._should_chunk_mqa_logits(
+            q_offset, k_offset, device_index
+        )
 
         if not need_chunk:
             assert q_fp8[:q_offset].shape[0] != 0
@@ -678,14 +723,14 @@ class Indexer(MultiPlatformOp):
             topk_result[:q_offset] = raw_topk_result
             return topk_result
 
-        # Chunk path
-        bytes_per_elem = 4  # float32
-        bytes_per_row = k_offset * bytes_per_elem
-        # Reserve 50% of free memory for logits
-        max_rows = max(1, int((free_mem * 0.5) // max(bytes_per_row, 1)))
+        bytes_per_row = k_offset * self._MQA_LOGITS_BYTES_PER_ELEM
+        max_rows = max(1, int(logits_budget_bytes // max(bytes_per_row, 1)))
         max_rows = min(max_rows, q_offset)
 
         global_topk_offset = metadata.attn_metadata.topk_indices_offset
+        cu_seqlens_q_full = None
+        if global_topk_offset is None:
+            cu_seqlens_q_full = torch.ones(q_offset, dtype=torch.int32, device=device)
 
         assert (
             seq_lens_expanded.shape[0] == q_offset
@@ -733,10 +778,7 @@ class Indexer(MultiPlatformOp):
             else:
                 # PAGED path: treat each token as a length-1 sequence
                 topk_offset_chunk = None
-                B_chunk = logits_chunk.shape[0]
-                cu_seqlens_q_chunk = torch.ones(
-                    B_chunk, dtype=torch.int32, device=device
-                )
+                cu_seqlens_q_chunk = cu_seqlens_q_full[start:end]
                 batch_idx_chunk = token_to_batch_idx[start:end]
 
             raw_topk_chunk = metadata.topk_transform(