为什么JVM CMS（并发标记和清除）需要两次暂停GC？

I wonder why CMS needs two phases (and so two pauses) of marks: i.e. initial mark and remark. Can we simply do one mark and then perform sweep? I imagine this can be a faster pause. Can someone help explain what is the main purpose of the second mark and why we need it? Thanks!I wonder why CMS needs two phases (and so two p