RFR: 8256390: ZGC: Relocate in-place instead of having a heap reserve [v2]

[Stefan Karlsson]

On Tue, 17 Nov 2020 16:30:23 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`).
>
> Per Liden has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review Stefank

src/hotspot/share/gc/z/zStat.hpp line 536:

> 534:   static size_t free(size_t used);
> 535:   static size_t allocated(size_t used, size_t reclaiemd);
> 536:   static size_t garbage(size_t reclaiemd);

reclaiemd -> reclaimed

src/hotspot/share/gc/z/zStat.cpp line 1281:

> 1279: 
> 1280: size_t ZStatHeap::allocated(size_t used, size_t reclaimed) {
> 1281:   return (used + reclaimed) - _at_mark_start.used;

I don't know why used + reclaimed is the amount allocated. Is this correct? Could you add a comment?

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