diff --git a/pecos/utils/mmap_hashmap_util.py b/pecos/utils/mmap_hashmap_util.py
index 2138ec7..e8eeb2e 100644
--- a/pecos/utils/mmap_hashmap_util.py
+++ b/pecos/utils/mmap_hashmap_util.py
@@ -11,7 +11,7 @@
 import logging
 from abc import abstractmethod
 from pecos.core import clib
-from typing import Optional, Tuple, Any
+from typing import Optional, Tuple
 from ctypes import c_char_p, c_uint32, c_uint64, POINTER
 import numpy as np
 import os
@@ -95,20 +95,24 @@ class MmapHashmapBatchGetter(object):
     Batch getter for MmapHashmap opened for readonly.
     """
 
-    def __init__(self, mmap_r: MmapHashmap, max_batch_size: int, threads: int=1):
-        if not isinstance(mmap_r, MmapHashmap):
-            raise ValueError(f"Should get from MmapHashmap, got {type(mmap_r)}")
-        if mmap_r.mode not in ["r", "r_lazy"]:
-            raise ValueError(f"MmapHashmap should opened for readonly, got {mmap_r.mode}")
+    def __init__(self, mmap_r, max_batch_size: int, threads: int = 1):
+        if not isinstance(mmap_r, _MmapHashmapReadOnly):
+            raise ValueError(f"Should get from readonly MmapHashmap, got {type(mmap_r)}")
         if max_batch_size <= 0:
             raise ValueError(f"Max batch size should >0, got {max_batch_size}")
         if threads <= 0 and threads != -1:
             raise ValueError(f"Number of threads should >0 or =-1, got {threads}")
 
-        self.mmap_r = mmap_r
+        self.mmap_r: Optional[_MmapHashmapReadOnly] = mmap_r
         self.max_batch_size = max_batch_size
-        self.key_prealloc = self.mmap_r.map.get_keyalloc(max_batch_size)
-        self.threads_c_uint32 = c_uint32(min(os.cpu_count(), os.cpu_count() if threads == -1 else threads))
+        self.key_prealloc = mmap_r.get_keyalloc(max_batch_size)
+
+        # `os.cpu_count()` is not equivalent to the number of CPUs the current process can use.
+        # The number of usable CPUs can be obtained with len(os.sched_getaffinity(0))
+        n_usable_cpu = len(os.sched_getaffinity(0))
+        self.threads_c_uint32 = c_uint32(
+            min(n_usable_cpu, n_usable_cpu if threads == -1 else threads)
+        )
 
         # Pre-allocated space for returns
         self.vals = np.zeros(max_batch_size, dtype=np.uint64)
@@ -123,8 +127,12 @@ def get(self, keys, default_val):
                 ii) int2int: 1D numpy array of int64
             2) The return is a reused buffer, use or copy the data once you get it. It is not guaranteed to last.
         """
-        self.mmap_r.map.batch_get(
-            len(keys), self.key_prealloc.get_key_prealloc(keys), default_val, self.vals, self.threads_c_uint32
+        self.mmap_r.batch_get(
+            len(keys),
+            self.key_prealloc.get_key_prealloc(keys),
+            default_val,
+            self.vals,
+            self.threads_c_uint32,
         )
         return memoryview(self.vals)[: len(keys)]
 
@@ -159,7 +167,7 @@ def __contains__(self, key):
         pass
 
     @abstractmethod
-    def batch_get(self, n_keys, keys, default_val, vals):
+    def batch_get(self, n_keys, keys, default_val, vals, threads_c_uint32):
         pass
 
     @classmethod
@@ -200,7 +208,9 @@ def __getitem__(self, key_utf8):
     def __contains__(self, key_utf8):
         return self.fn_dict["contains"](self.map_ptr, key_utf8, len(key_utf8))
 
-    def batch_get(self, n_keys: int, keys_utf8: Tuple, default_val: int, vals, threads_c_uint32: c_uint32):
+    def batch_get(
+        self, n_keys: int, keys_utf8: Tuple, default_val: int, vals, threads_c_uint32: c_uint32
+    ):
         """
         Batch get values for UTF8 encoded bytes string keys.
         Return values are stored in vals.
@@ -227,7 +237,7 @@ def batch_get(self, n_keys: int, keys_utf8: Tuple, default_val: int, vals, threa
             keys_lens.ctypes.data_as(POINTER(c_uint32)),
             default_val,
             vals.ctypes.data_as(POINTER(c_uint64)),
-            threads_c_uint32
+            threads_c_uint32,
         )
         return vals
 
@@ -280,7 +290,7 @@ def batch_get(self, n_keys: int, keys, default_val: int, vals, threads_c_uint32:
             keys.ctypes.data_as(POINTER(c_uint64)),
             default_val,
             vals.ctypes.data_as(POINTER(c_uint64)),
-            threads_c_uint32
+            threads_c_uint32,
         )
         return vals
 
diff --git a/test/pecos/utils/test_mmap_hashmap_util.py b/test/pecos/utils/test_mmap_hashmap_util.py
index 4c7a622..59cf020 100644
--- a/test/pecos/utils/test_mmap_hashmap_util.py
+++ b/test/pecos/utils/test_mmap_hashmap_util.py
@@ -48,7 +48,7 @@ def test_str2int_mmap_hashmap(tmpdir):
     # Batch get with default
     max_batch_size = 5
     # max_batch_size > num of key
-    r_map_batch_getter = MmapHashmapBatchGetter(r_map, max_batch_size)
+    r_map_batch_getter = MmapHashmapBatchGetter(r_map.map, max_batch_size)
     ks = list(kv_dict.keys()) + ["ccccc".encode("utf-8")]  # Non-exist key
     vs = list(kv_dict.values()) + [10]
     assert r_map_batch_getter.get(ks, 10).tolist() == vs
@@ -99,7 +99,7 @@ def test_int2int_mmap_hashmap(tmpdir):
     # Batch get with default
     max_batch_size = 5
     # max_batch_size > num of key
-    r_map_batch_getter = MmapHashmapBatchGetter(r_map, max_batch_size)
+    r_map_batch_getter = MmapHashmapBatchGetter(r_map.map, max_batch_size)
     ks = list(kv_dict.keys()) + [1000]  # Non-exist key
     vs = list(kv_dict.values()) + [10]
     assert r_map_batch_getter.get(np.array(ks, dtype=np.int64), 10).tolist() == vs