Integrated: 8276055: ZGC: Defragment address space

[Per Liden]

On Wed, 27 Oct 2021 12:10:25 GMT, Per Liden <pliden at openjdk.org> wrote:

> In ZGC, small pages are allocated from the beginning of the address space (low addresses), while medium and large pages are allocated from the end of the address space (high addresses). However, small pages can sometimes end up on a high address if a medium or a large page was split into small pages. Over time this can cause the address space to become fragmented, with the risk of eventually running out of address space for new large allocations.
> 
> To mitigate this, we can defragment the address space by letting small pages at high addresses be remapped to lower addresses. That's what this patch does.
> 
> Testing:
>  - Tier 1-7 with ZGC on Linux/x86_64
>  - SPECjbb2015 shows no regression
>  - Manually ran test that tries to fragment the address space

This pull request has now been integrated.

Changeset: 1750a6e2
Author:    Per Liden <pliden at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/1750a6e2c06960b734f646018fc99b336bd966a5
Stats:     80 lines in 8 files changed: 63 ins; 0 del; 17 mod

8276055: ZGC: Defragment address space

Reviewed-by: eosterlund, stefank

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PR: https://git.openjdk.java.net/jdk/pull/6137