perf: PagedCache 持久前缀缓存 + LRU 逐出

- astrai/inference/cache.py: refcount 归零时保留 hash 映射，页加入 LRU evictable 池
- alloc() 无空闲页时从 LRU 逐出，优先释放 _free_mask
- lookup_prefix/inc_ref 触发 _touch 更新 LRU 序
- record_page 设置 pin 标记并从 LRU 移除

astrai/inference/cache.py

@@ -22,14 +22,16 @@ def page_hash(token_ids: List[int], page_idx: int, page_size: int) -> int:
 
 
 class PagedCache:
-    """Paged KV cache with page-table-indirected read/write.
+    """Paged KV cache with page-table-indirected read/write and persistent prefix caching.
 
     Combines:
       - Page pool (ref-counted alloc/free via bitmask)
       - KV tensor storage (k_cache, v_cache)
       - Prefix-cache hash lookup (page_content_hash -> physical_page_idx)
+      - LRU eviction for persistent cross-batch prefix caching
 
-    Call :meth:`bind` to obtain a batch view for the attention layers.
+    Pages with recorded hashes persist after refcount reaches 0 (pinned).
+    They are evicted via LRU only when alloc() finds no free pages.
     """
 
     def __init__(
@@ -57,6 +59,24 @@ class PagedCache:
         )
         self._page_to_hash: Dict[int, int] = {}
         self._hash_to_page: Dict[int, int] = {}
+        self._lru: List[int] = []
+        self._pin: List[bool] = [False] * n_pages
+
+    def _touch(self, idx: int) -> None:
+        if self._refs[idx] == 0 and idx in self._lru:
+            self._lru.remove(idx)
+            self._lru.append(idx)
+
+    def _evict_one(self) -> int:
+        while self._lru:
+            idx = self._lru.pop(0)
+            h = self._page_to_hash.pop(idx, None)
+            if h is not None:
+                self._hash_to_page.pop(h, None)
+            self._pin[idx] = False
+            self._refs[idx] = 1
+            return idx
+        return -1
 
     def record_page(
         self, page_idx: int, token_ids: List[int], logical_page_idx: int
@@ -67,6 +87,9 @@ class PagedCache:
             self._hash_to_page.pop(old_h, None)
         self._page_to_hash[page_idx] = h
         self._hash_to_page[h] = page_idx
+        self._pin[page_idx] = True
+        if page_idx in self._lru:
+            self._lru.remove(page_idx)
 
     def lookup_prefix(self, token_ids: List[int]) -> List[int]:
         full_pages = len(token_ids) // self.page_size
@@ -76,20 +99,25 @@ class PagedCache:
             p = self._hash_to_page.get(h)
             if p is None:
                 break
+            self._touch(p)
             hits.append(p)
         return hits
 
     def inc_ref(self, idx: int) -> None:
         self._refs[idx] += 1
+        if self._refs[idx] == 1 and idx in self._lru:
+            self._lru.remove(idx)
 
     def alloc(self) -> int:
+        if self._free_mask:
             lsb = self._free_mask & -self._free_mask
-        if lsb == 0:
-            return -1
             idx = lsb.bit_length() - 1
             self._free_mask ^= lsb
             self._refs[idx] = 1
+            if idx in self._lru:
+                self._lru.remove(idx)
             return idx
+        return self._evict_one()
 
     def alloc_n(self, n: int) -> List[int]:
         pages = [self.alloc() for _ in range(n)]
@@ -103,10 +131,15 @@ class PagedCache:
     def free(self, idx: int) -> None:
         self._refs[idx] -= 1
         if self._refs[idx] == 0:
+            h = self._page_to_hash.get(idx)
+            if h is not None and self._pin[idx]:
+                self._lru.append(idx)
+            else:
                 self._free_mask |= 1 << idx
                 h = self._page_to_hash.pop(idx, None)
                 if h is not None:
                     self._hash_to_page.pop(h, None)
+                self._pin[idx] = False
 
     def bind(self, page_table: Tensor, total_len: int = 0) -> "CacheView":
         return CacheView(self, page_table, total_len)