If the clock offset is too high (more than 250ms), we should use another transaction model, Google Percolator is a good fit before the unforeseeable improvement

If the clock offset is too high (more than 250ms), we should use another transaction model, Google Percolator is a good fit before the unforeseeable improvement in the hardware. Based on the monitoring of clock offset on cloud, TiDB chose to use the timestamp oracle to allocate timestamp, which is much faster.