Integrated: 8256390: ZGC: Relocate in-place instead of having a heap reserve

[Per Liden]

On Mon, 16 Nov 2020 13:45:51 GMT, Per Liden <pliden at openjdk.org> wrote:

> The heap reserve is a portion of the heap that is set aside to cope with “emergency situations”, like when ZGC needs to compact the heap when it’s already full. However, due to a number of factors (mutator relocation, page pinning, etc) there is no guarantee that relocation will always succeed. In fact, we can end up in situations where we fail to reclaim memory even when 90% of the heap is garbage (JDK-8254346 is an example of where this happens).
> 
> So, even if the heap reserve serves its purpose well in most cases, it still doesn't offer any guarantees that relocation succeeds in all cases. To get the guarantees we want we could, instead of having a heap reserve, turn to doing in-place relocation. In other words, if a worker fails to allocate a new page, then it compacts the page currently being relocated, and then use that page as the target for new allocations. In addition, if a mutator fails to relocate an object, then it waits for a worker to relocate the object, instead of pinning the page. This patch does exactly that, and hence makes ZGC more resilient in situations where memory is scares. It also means that ZGC works even better when using small heap (where memory tends to be scares).
> 
> Testing: Tier1-7 on linux-x64, Tier-1-3 on all Oracle platforms, 50+ runs of gc-test-suite on linux-x64 (both with and without `-XX:+ZStressRelocateInPlace`).

This pull request has now been integrated.

Changeset: 372595cc
Author:    Per Liden <pliden at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/372595cc
Stats:     1332 lines in 35 files changed: 646 ins; 397 del; 289 mod

8256390: ZGC: Relocate in-place instead of having a heap reserve

Reviewed-by: stefank, eosterlund

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PR: https://git.openjdk.java.net/jdk/pull/1228