diff --git a/tosfs/core.py b/tosfs/core.py
index d1aed62..293c27f 100644
--- a/tosfs/core.py
+++ b/tosfs/core.py
@@ -38,7 +38,6 @@
     ListObjectVersionsOutput,
     PartInfo,
     UploadPartCopyOutput,
-    UploadPartOutput,
 )
 
 from tosfs.consts import (
@@ -117,8 +116,8 @@ def __init__(
         default_fill_cache: bool = True,
         default_cache_type: str = "readahead",
         multipart_staging_dirs: str = tempfile.mkdtemp(),
-        multipart_size: int = 8388608,
-        multipart_thread_pool_size: int = 96,
+        multipart_size: int = 4096,
+        multipart_thread_pool_size: int = max(2, os.cpu_count() or 1),
         multipart_staging_buffer_size: int = 4096,
         multipart_threshold: int = 10485760,
         **kwargs: Any,
@@ -165,6 +164,25 @@ def __init__(
             Whether to fill the cache (default is True).
         default_cache_type : str, optional
             The default cache type (default is 'readahead').
+        multipart_staging_dirs : str, optional
+            The staging directories for multipart uploads (default is a temporary
+            directory). Separate the staging dirs with comma if there are many
+            staging dir paths.
+        multipart_size : int, optional
+            The max byte size which will buffer the staging data in-memory before
+            flushing to the staging file. Decrease the random write in local staging
+            disk dramatically if writing plenty of small files. (default is 4096).
+        multipart_thread_pool_size : int, optional
+            The size of the thread pool for multipart uploads (default is
+            max(2, os.cpu_count()).
+        multipart_staging_buffer_size : int, optional
+            The size of thread pool used for uploading multipart in parallel for the
+            given object storage. (default is 4096).
+        multipart_threshold : int, optional
+            The threshold which control whether enable multipart upload during
+            writing data to the given object storage, if the write data size is less
+            than threshold, will write data via simple put instead of multipart upload.
+             (default is 10 MB).
         kwargs : Any, optional
             Additional arguments.
 
@@ -192,7 +210,9 @@ def __init__(
         self.default_cache_type = default_cache_type
         self.max_retry_num = max_retry_num
 
-        self.multipart_staging_dirs = [d.strip() for d in multipart_staging_dirs.split(",")]
+        self.multipart_staging_dirs = [
+            d.strip() for d in multipart_staging_dirs.split(",")
+        ]
         self.multipart_size = multipart_size
         self.multipart_thread_pool_size = multipart_thread_pool_size
         self.multipart_staging_buffer_size = multipart_staging_buffer_size
@@ -2012,7 +2032,7 @@ def __init__(
         self.mode = mode
         self.autocommit = autocommit
         self.mpu: CreateMultipartUploadOutput = None
-        self.parts: Optional[list] = None
+        self.parts: list = []
         self.append_block = False
         self.buffer: Optional[io.BytesIO] = io.BytesIO()
 
@@ -2022,7 +2042,7 @@ def __init__(
         self.staging_buffer_size = fs.multipart_staging_buffer_size
         self.multipart_threshold = fs.multipart_threshold
         self.executor = ThreadPoolExecutor(max_workers=self.thread_pool_size)
-        self.staging_files = []
+        self.staging_files: list[str] = []
 
         if "a" in mode and fs.exists(path):
             head = retryable_func_executor(
@@ -2045,7 +2065,6 @@ def _initiate_upload(self) -> None:
             return
         logger.debug("Initiate upload for %s", self)
 
-
     def _upload_chunk(self, final: bool = False) -> bool:
         """Write one part of a multi-block file upload.
 
@@ -2078,7 +2097,9 @@ def _upload_chunk(self, final: bool = False) -> bool:
             self.parts = []
 
             self.mpu = retryable_func_executor(
-                lambda: self.fs.tos_client.create_multipart_upload(self.bucket, self.key),
+                lambda: self.fs.tos_client.create_multipart_upload(
+                    self.bucket, self.key
+                ),
                 max_retry_num=self.fs.max_retry_num,
             )
 
@@ -2117,12 +2138,14 @@ def _upload_multiple_chunks(self, bucket: str, key: str) -> None:
                     tmp.write(chunk)
                     self.staging_files.append(tmp.name)
 
-    def _upload_staged_files(self):
+    def _upload_staged_files(self) -> None:
         futures = []
         for i, staging_file in enumerate(self.staging_files):
             part_number = i + 1
             futures.append(
-                self.executor.submit(self._upload_part_from_file, staging_file, part_number)
+                self.executor.submit(
+                    self._upload_part_from_file, staging_file, part_number
+                )
             )
 
         for future in futures:
diff --git a/tosfs/stability.py b/tosfs/stability.py
index b573a3d..6a0e027 100644
--- a/tosfs/stability.py
+++ b/tosfs/stability.py
@@ -32,8 +32,10 @@
 
 from tosfs.exceptions import TosfsError
 
+CONFLICT_CODE = "409"
+
 TOS_SERVER_RETRYABLE_STATUS_CODES = {
-    "409",  # CONFLICT
+    CONFLICT_CODE,  # CONFLICT
     "429",  # TOO_MANY_REQUESTS
     "500",  # INTERNAL_SERVER_ERROR
 }
@@ -115,8 +117,8 @@ def _is_retryable_tos_server_exception(e: TosError) -> bool:
     if not isinstance(e, TosServerError):
         return False
 
-    # 409 is a conflict error, but not all conflict errors are retryable
-    if e.status_code == 409:
+    # not all conflict errors are retryable
+    if e.status_code == CONFLICT_CODE:
         return e.code not in TOS_SERVER_NOT_RETRYABLE_CONFLICT_ERROR_CODES
 
     return e.status_code in TOS_SERVER_RETRYABLE_STATUS_CODES