RFR: 8234543: ZGC: Parallel pre-touch

[Per Liden]

I noticed that we didn't have a test for this on ZGC, so I added one:

Updated webrev: http://cr.openjdk.java.net/~pliden/8234543/webrev.1

/Per

On 11/21/19 10:32 AM, Per Liden wrote:
> When using -XX:+AlwaysPreTouch, ZGC is currently doing single threaded 
> pre-touch. This patch makes this a parallel operation. This improves 
> startup time, especially when using large heaps. For example, when using 
> a 3TB heap (-XX:+AlwaysPreTouch -Xms3T -Xmx3T), startup time is improved 
> by about 30x.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8234543
> Webrev: http://cr.openjdk.java.net/~pliden/8234543/webrev.0
> 
> /Per