Reuse process pool to avoid unexpected memory increase (#85)
Reuse process pool to avoid unexpected memory increase (#85)

* fix multiprocessing in building histogram

* pylint

* Reuse process pool to avoid unexpected memory increase

* pylint

Co-authored-by: lilongyijia <457594070308f316fabde6e014723a26c545667d@bytedance.com>
bytedance/fedlearner,bytedance/fedlearner,bytedance/fedlearner,bytedance/fedlearner,bytedance/fedlearner,bytedance/fedlearner,bytedance/fedlearner