java - ZGC如何在不使用写屏障的情况下进行并发标记？

Question

最近看了一些关于ZGC的文章。
例如。 https://dinfuehr.github.io/blog/a-first-look-into-zgc/
并且它说ZGC中没有写障碍。

Both reading and writing in the heap is extremely common, so both GC-barriers need to be super efficient. That means just a few assembly instructions in the common case. Read barriers are an order of magnitude more likely than write-barriers (although this can certainly vary depending on the application), so read-barriers are even more performance-sensitive. Generational GC’s for example usually get by with just a write barrier, no read barrier needed. ZGC needs a read barrier but no write barrier. For concurrent compaction I haven’t seen a solution without read barriers.

Write-Barrier不需要并发紧凑，但是没有Write-Barrier怎么做并发标记呢？一些收集器，例如G1，使用三色标记和SATB+write Barrier来进行并发标记。

我从 OJ 那里读到还有另一种方法:

An alternative approach would be to keep a queue of all changes that could potentially violate the invariant, and then have a secondary “fixup” phase that runs after the main phase has finished. Different collectors can resolve this problem with tri-color marking in different ways, based on criteria such as performance or the amount of locking required.

我想知道它是如何工作的...搜索了一段时间后几乎没有关于它的信息...(也许我应该阅读一些源代码...

Answer 1

据我了解，不需要写入屏障，因为重定位集中的根引用是在并发重定位之前的 STW 阶段处理的。因此，你永远不会从堆栈、局部变量等引用旧对象。