我想读卡桑德的文章时出错了：

WARN 19:17:29,839 Heap is 0.8888050654459693 full.  
    You may need to reduce memtable and/or cache sizes.  
    Cassandra will now flush up to the two largest memtables to free up memory.  
    Adjust flush_largest_memtables_at threshold in cassandra.yaml if you don't want Cassandra to do this automatically 
INFO 19:17:30,238 Unable to reduce heap usage since there are no dirty column families

当我把flush\u largest\u memtables\u减少到阈值时，没有任何变化，所以你能帮我减少memtable或缓存的大小吗？因为我不知道怎么做。