如果history length 比较大，在purge的时候会导致脏页增多，purge的堆栈如下
mysqld!trx_undo_truncate_start(trx_rseg_t*, unsigned long, unsigned long, unsigned long long) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0undo.cc:1191) mysqld!trx_purge_truncate_rseg_history(trx_rseg_t*, purge_iter_t const*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0purge.cc:582) mysqld!trx_purge_truncate_history(purge_iter_t*, ReadView const*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0purge.cc:1233) mysqld!trx_purge_truncate() (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/trx/trx0purge.cc:1816) mysqld!trx_purge(unsigned long, unsigned long, bool) (Unknown Source:0) mysqld!srv_do_purge(unsigned long, unsigned long*) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/srv/srv0srv.cc:2636) mysqld!::srv_purge_coordinator_thread(void *) (/Users/xiaoyu.bai/Downloads/mysql-5.7.29/storage/innobase/srv/srv0srv.cc:2809) libsystem_pthread.dylib!_pthread_start (Unknown Source:0) libsystem_pthread.dylib!thread_start (Unknown Source:0)
