fix error

ppocr/data/rec/dataset_traversal.py

@@ -247,10 +247,12 @@ class SimpleReader(object):
                     print("multiprocess is not fully compatible with Windows."
                           "num_workers will be 1.")
                     self.num_workers = 1
-                if self.batch_size * get_device_num() > img_num:
+                if self.batch_size * get_device_num(
+                ) * self.num_workers > img_num:
                     raise Exception(
-                        "The number of the whole data ({}) is smaller than the batch_size * devices_num ({})".
-                        format(img_num, self.batch_size * get_device_num()))
+                        "The number of the whole data ({}) is smaller than the batch_size * devices_num * num_workers ({})".
+                        format(img_num, self.batch_size * get_device_num() *
+                               self.num_workers))
                 for img_id in range(process_id, img_num, self.num_workers):
                     label_infor = label_infor_list[img_id_list[img_id]]
                     substr = label_infor.decode('utf-8').strip("\n").split("\t")