garbage-collection - 有人能解释一下 CMS 垃圾回收的 "Concurrent Abortable Preclean"阶段发生了什么吗？

Question

我经历了https://plumbr.eu/handbook/garbage-collection-algorithms-implementations/concurrent-mark-and-sweep解释 CMS GC 的链接，但无法理解 Concurrent Abortable Preclean 在做什么，如果有人详细解释它会有所帮助。

Answer 1

博文Understanding CMS GC Logs提到(不可中止的)预清理阶段的目的:

Precleaning is also a concurrent phase. Here in this phase we look at the objects in CMS heap which got updated by promotions from young generation or new allocations or got updated by mutators while we were doing the concurrent marking in the previous concurrent marking phase. By rescanning those objects concurrently, the precleaning phase helps reduce the work in the next stop-the-world “remark” phase.

所以那个阶段是一个优化。

然后可中止的预清理被描述为

After 'concurrent preclean' if the Eden occupancy is above CMSScheduleRemarkEdenSizeThreshold, we start 'concurrent abortable preclean' and continue precleanig until we have CMSScheduleRemarkEdenPenetration percentage occupancy in eden, otherwise we schedule 'remark' phase immediately.

另一个帖子 The Unspoken - CMS and PrintGCDetails进一步提到:

The remark phase is scheduled so that it does not occur back-to-back with a ParNew so as not to appear to be a pause that is the sum of the ParNew and the remark pause. A second precleaning phase is started and is aborted when the remark phase is ready to start. Aborting this second precleaning phase is the expected behavior. That it was aborted is not an indication of an error. Since the remark phase is waiting, why not preclean but don't delay the remark for the sake of precleaning.

换句话说，abortable preclean 是一种优化(preclean 优化)，用于在年轻的集合之间间隔 remark 暂停，并以有用的方式度过等待时间。