diff --git a/data_juicer/config/config.py b/data_juicer/config/config.py
index f252f4999..c8f8f9ded 100644
--- a/data_juicer/config/config.py
+++ b/data_juicer/config/config.py
@@ -567,12 +567,14 @@ def update_op_process(cfg, parser):
 
         # update op params of cfg.process
         internal_op_para = temp_cfg.get(op_in_process_name)
-
-        cfg.process[i] = {
-            op_in_process_name:
-            None if internal_op_para is None else
-            namespace_to_dict(internal_op_para)
-        }
+        if internal_op_para is not None:
+            num_proc = internal_op_para.get('num_proc')
+            if 'num_proc' in internal_op_para:
+                internal_op_para['num_proc'] = num_proc or cfg.np
+            internal_op_para = namespace_to_dict(internal_op_para)
+        else:
+            internal_op_para = None
+        cfg.process[i] = {op_in_process_name: internal_op_para}
 
     # check the op params via type hint
     temp_parser = copy.deepcopy(parser)
diff --git a/data_juicer/core/ray_data.py b/data_juicer/core/ray_data.py
index 95c8874c9..d224a9b0a 100644
--- a/data_juicer/core/ray_data.py
+++ b/data_juicer/core/ray_data.py
@@ -71,7 +71,7 @@ def process_batch_arrow(table: pa.Table) -> pa.Table:
 
 
 def get_num_gpus(op, op_proc):
-    if op.use_cuda():
+    if not op.use_cuda():
         return 0
     proc_per_gpu = op_proc / cuda_device_count()
     return 1.0 / proc_per_gpu
diff --git a/data_juicer/ops/filter/video_ocr_area_ratio_filter.py b/data_juicer/ops/filter/video_ocr_area_ratio_filter.py
index 9cece53d2..2a647541d 100644
--- a/data_juicer/ops/filter/video_ocr_area_ratio_filter.py
+++ b/data_juicer/ops/filter/video_ocr_area_ratio_filter.py
@@ -94,6 +94,7 @@ def __init__(self,
 
     def get_reader(self, rank):
         if self.use_cuda():
+            rank = 0 if rank is None else rank
             device = f'cuda:{rank % cuda_device_count()}'
             self.reader.detector = self.reader.detector.to(device)
             self.reader.device = device
diff --git a/data_juicer/utils/process_utils.py b/data_juicer/utils/process_utils.py
index 2dd1e7d3a..33d0a9f68 100644
--- a/data_juicer/utils/process_utils.py
+++ b/data_juicer/utils/process_utils.py
@@ -55,13 +55,16 @@ def calculate_np(name,
                  num_proc=None,
                  use_cuda=False):
     """Calculate the optimum number of processes for the given OP"""
+    eps = 1e-9  # about 1 byte
+
     if num_proc is None:
         num_proc = psutil.cpu_count()
+
     if use_cuda:
         cuda_mem_available = get_min_cuda_memory() / 1024
         op_proc = min(
             num_proc,
-            math.floor(cuda_mem_available / (mem_required + 0.1)) *
+            math.floor(cuda_mem_available / (mem_required + eps)) *
             cuda_device_count())
         if use_cuda and mem_required == 0:
             logger.warning(f'The required cuda memory of Op[{name}] '
@@ -84,9 +87,9 @@ def calculate_np(name,
         cpu_available = psutil.cpu_count()
         mem_available = psutil.virtual_memory().available
         mem_available = mem_available / 1024**3
-        op_proc = min(op_proc, math.floor(cpu_available / cpu_required))
+        op_proc = min(op_proc, math.floor(cpu_available / cpu_required + eps))
         op_proc = min(op_proc,
-                      math.floor(mem_available / (mem_required + 0.1)))
+                      math.floor(mem_available / (mem_required + eps)))
         if op_proc < 1.0:
             logger.warning(f'The required CPU number:{cpu_required} '
                            f'and memory:{mem_required}GB might '
diff --git a/tests/config/test_config_funcs.py b/tests/config/test_config_funcs.py
index 23f2f0122..cf7e21f3d 100644
--- a/tests/config/test_config_funcs.py
+++ b/tests/config/test_config_funcs.py
@@ -46,7 +46,7 @@ def test_yaml_cfg_file(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'cpu_required': 1,
                         'mem_required': 0,
                     }
@@ -61,7 +61,7 @@ def test_yaml_cfg_file(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'stats_export_path': None,
                         'cpu_required': 1,
                         'mem_required': 0,
@@ -127,7 +127,7 @@ def test_mixture_cfg(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'stats_export_path': None,
                         'cpu_required': 1,
                         'mem_required': 0,
@@ -143,7 +143,7 @@ def test_mixture_cfg(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'stats_export_path': None,
                         'cpu_required': 1,
                         'mem_required': 0,
@@ -159,7 +159,7 @@ def test_mixture_cfg(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'stats_export_path': None,
                         'cpu_required': 1,
                         'mem_required': 0,
@@ -175,7 +175,7 @@ def test_mixture_cfg(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'stats_export_path': None,
                         'cpu_required': 1,
                         'mem_required': 0,
@@ -191,7 +191,7 @@ def test_mixture_cfg(self):
                         'audio_key': 'audios',
                         'video_key': 'videos',
                         'accelerator': None,
-                        'num_proc': None,
+                        'num_proc': 4,
                         'stats_export_path': None,
                         'cpu_required': 1,
                         'mem_required': 0,