Fix swa eviction frontier for bigram keys (#34870)

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -773,16 +773,21 @@ class UnifiedRadixCache(BasePrefixCache):
             if cl is not None:
                 effective_cache_len = min(effective_cache_len, cl)
 
-        # swa_evicted_seqlen is a raw-token length, but under EAGLE the insert key is
-        # bigram-indexed, so SWA would carve tombstones at the wrong offset (#34653).
-        if (
-            envs.SGLANG_OPT_UNIFIED_CACHE_FREE_OUT_OF_WINDOW_SLOTS.get()
-            and not self.tree_core.is_eagle
-        ):
+        radix_key = RadixKey(
+            token_ids[:effective_cache_len],
+            req.extra_key,
+            is_bigram=self.tree_core.is_eagle,
+            cache_salt=req.cache_salt,
+        )
+
+        if envs.SGLANG_OPT_UNIFIED_CACHE_FREE_OUT_OF_WINDOW_SLOTS.get():
+            # The frontier lands a page below page_floor(pre_len + 1), which has to
+            # be where the insert stops, or the leaf it creates keeps less than a
+            # sliding window of live SWA and the match after the insert rejects it.
+            # The insert stops at page_floor(len(radix_key)), and a bigram key is
+            # one shorter than the tokens it spans, so measure the key.
             for comp in self._components_tuple:
-                comp.free_out_of_window_slots(
-                    req, effective_cache_len - 1, insert_params
-                )
+                comp.free_out_of_window_slots(req, len(radix_key) - 1, insert_params)
 
         if effective_cache_len <= 0:
             req.prefix_indices = kv_indices_orig.to(dtype=torch.int64, copy=True)
@@ -794,12 +799,7 @@ class UnifiedRadixCache(BasePrefixCache):
 
         kv_indices = kv_indices_orig[:effective_cache_len]
 
-        radix_key = RadixKey(
-            token_ids[:effective_cache_len],
-            req.extra_key,
-            is_bigram=self.tree_core.is_eagle,
-            cache_salt=req.cache_salt,
-        ).page_aligned(self.page_size)
+        radix_key = radix_key.page_aligned(self.page_size)
         page_aligned_len = len(radix_key)
         values = kv_indices[:page_aligned_len].to(dtype=torch.int64, copy=True)