docstr: preload the calendar for cache
docstr_tok: ["preload","the","calendar","for","cache"]
code: def _prepare_calender_cache(self):
      """preload the calendar for cache"""
  
      # This code used the copy-on-write feature of Linux to avoid calculating the calendar multiple times in the subprocess
      # This code may accelerate, but may be not useful on Windows and Mac Os
      Cal.calendar(freq="1min")
      get_calendar_day(freq="1min")
  
file_path: qlib/examples/highfreq/workflow.py
imports: import fire
  import qlib
  import pickle
  from qlib.constant import REG_CN
  from qlib.config import HIGH_FREQ_CONFIG
  from qlib.utils import init_instance_by_config
  from qlib.data.dataset.handler import DataHandlerLP
  from qlib.data.ops import Operators
  from qlib.data.data import Cal
  from qlib.tests.data import GetData
  from highfreq_ops import get_calendar_day, DayLast, FFillNan, BFillNan, Date, Select, IsNull, Cut
label_desc: Use image node_id 2 for calling the HighfreqWorkflow obj's underlying member method code with example usage: obj._prepare_calender_cache() without return types