RFR: 8260267: ZGC: Reduce mark stack usage [v5]

[Hui Shi]

On Wed, 19 May 2021 08:40:20 GMT, Per Liden <pliden at openjdk.org> wrote:

> Updated PR with the following:
> 
> 1. Always mark before push. After additional testing and some discussions, it turns out that the overhead of marking before pushing isn't that high. Concurrent Mark times increase by about 1% or so on SPECjbb2015 and BigRamTester. So let's keep thing simple, and avoid introducing that arbitrary "high usage" threshold.
> 2. Only uncommit the part of the mark stack space that hasn't been used.
> 3. Print mark stack memory usage in the GC log.

This is great! entire code is unified with always before push. Thanks~!

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