RFR: 8260267: ZGC: Reduce mark stack usage [v4]

[Per Liden]

> Under some conditions, striped marking can cause excessive mark stack memory usage. For example, when the object graph has one or more N:1 relations ships, where N is a large number.
> 
> To avoid pushing mark stack entries to already marked objects, and thus reduce mark stack memory usage, this patch adds does two things:
>  * GC threads will under some conditions mark the object before pushing, else (if the condition to mark failed)
>  * GC threads will check if the object is already marked before pushing.
> 
> The conditions for marking before pushing is:
>  * We are using a single stripe, in which cases striped marking is essentially disabled anyway.
>  * We see excessive mark stack usage. In other words, the mark stack allocator indicates "high usage".
> 
> To make this easier to review, this PR contains two commits:
>  * `ZGC: Reduce mark stack usage` - This patch adds the infrastructure needed to mark before push, but only does so if we are using a single stripe.
>  * `ZGC: Mark before push if mark stack usage is high` - This patch adds the condition to mark before push if the mark stack allocator indicates "high usage". As a bonus, this patch will also uncommit memory used for mark stacks after the marking has completed.
> 
> Testing:
>  * Tier1-7
>  * SPECjbb2015, score and marking times unaffected
>  * casparcwang at tencent.com reports that this patch, in combination with PR #3455, solves the problem for Apache Zookeeper.

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Review comments from StefanJ

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3460/files
  - new: https://git.openjdk.java.net/jdk/pull/3460/files/77a65449..f1a98aba

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3460&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3460&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3460.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3460/head:pull/3460

PR: https://git.openjdk.java.net/jdk/pull/3460