RFR: 8234338: ZGC: Improve small heap usage

[Erik Österlund]

Hi Per,

Looks good.

Thanks,
/Erik

> On 19 Nov 2019, at 19:54, Per Liden <per.liden at oracle.com> wrote:
> 
> When using small heaps (like less than 128M), the heap reserve tends to take up a relatively large portion of the heap. We have quite a lot of tests that explicitly set the heap size to small values (like -Xmx8M, -Xmx16M, etc). Today, these tests often fail with OOME when using ZGC. While ZGC isn't really that focused on tiny/small heaps, we still want to make testing of ZGC easy without having to adjust these tests to use a larger heap.
> 
> There are at least two things we can do when ZGC is given a small heap:
> 1) Dynamically scale the medium ZPage size, and even disable medium pages all together when using tiny heaps.
> 2) Stop using per-CPU small pages for allocations, and instead switch to using a single small page.
> 
> With this patch, ZGC can scale down to 8M.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8234338
> Webrev: http://cr.openjdk.java.net/~pliden/8234338/webrev.0
> 
> Testing: Tier1-7 using ZGC. A new small heap test was also added.
> 
> /Per