593bb2c40b
Summary: Add an option to indicates the variation of background threads. If the flag is not 0, for every 100 milliseconds, adjust thread pool size to a value within the range. Test Plan: run db_stress Reviewers: haobo, dhruba, igor, ljin Reviewed By: ljin Subscribers: leveldb, nkg- Differential Revision: https://reviews.facebook.net/D18759