🐛 Bug Description

highfreq example process killed in linux

To Reproduce

Steps to reproduce the behavior:

1. cd qlib/examples/highfreq
2. python workflow.py dump_and_load_dataset

Screenshot

(qlib.venv) elife@rtx:~/work/Trading/qlib/examples/highfreq$ python workflow.py dump_and_load_dataset
2021-07-18 09:28:27.330 | WARNING | qlib.tests.data:qlib_data:149 - Data already exists: ~/.qlib/qlib_data/yahoo_cn_1min, the data download will be skipped
If downloading is required: exists_skip=False or change target_dir
[4241:MainThread](2021-07-18 09:28:27,330) INFO - qlib.Initialization - [config.py:287] - default_conf: client.
[4241:MainThread](2021-07-18 09:28:27,468) INFO - qlib.Initialization - [init.py:54] - qlib successfully initialized based on client settings.
[4241:MainThread](2021-07-18 09:28:27,468) INFO - qlib.Initialization - [init.py:55] - data_path=/home/elife/.qlib/qlib_data/yahoo_cn_1min
[4241:MainThread](2021-07-18 09:28:55,331) INFO - qlib.timer - [log.py:113] - Time cost: 27.673s | Loading data Done
Killed

Environment

Linux
x86_64
Linux-5.4.0-74-generic-x86_64-with-Ubuntu-18.04-bionic
#83~18.04.1-Ubuntu SMP Tue May 11 16:01:00 UTC 2021

Python version: 3.7.5 (default, Feb 23 2021, 13:22:40) [GCC 8.4.0]

Qlib version: 0.7.0.99
numpy==1.20.1
pandas==1.2.4
scipy==1.6.2
requests==2.25.1
sacred==0.8.2
python-socketio==3.1.2
redis==3.5.3
python-redis-lock==3.7.0
schedule==1.1.0
cvxpy==1.0.21
hyperopt==0.1.1
fire==0.4.0
statsmodels==0.12.2
xlrd==2.0.1
plotly==4.12.0
matplotlib==3.1.3
tables==3.6.1
pyyaml==5.4.1
mlflow==1.15.0
tqdm==4.50.2
loguru==0.5.3
lightgbm==3.2.1
tornado==6.1
joblib==1.0.1
fire==0.4.0
ruamel.yaml==0.17.4

commit 3b80876