RFR: 8307058: Implementation of Generational ZGC [v6]

[Fei Yang]

On Fri, 5 May 2023 06:50:55 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> We emailed to erik to discuss this issue two months ago, and maybe he missed it.
>> ZForwardingTest does not guarantee a successful invoke of os::commit_memory for ZAddressHeapBase, and we saw some conflicts between ZAddressHeapBase and the metadata address space on the RISC-V hardware of 39-bits virtual address. There is no failure in the normal initialization phase of JVM, because the commit order of them is guaranteed.
>
> Could you provide the values for `reserved`, `ZAddressHeapBase`, and `ZAddressOffsetMax`, when this test is failing. I'd like to know if we can make a workaround for you, or if we have to turn off the test for riscv.

@stefank : I ran this gtest for 5 times and here is what I got.

ZAddressHeapBase :     0x800000000
ZAddressOffsetMax:     0x800000000
ZGranuleSize          :     0x200000

In os::pd_attempt_reserve_memory_at() which is called by os::attempt_reserve_memory_at(), return value by anon_mmap() [1] is one of: ```0x3f8d5ff000, 0x3f649fe000, 0x3f5d3ff000, 0x3f68077000 and 0x3f555ff000```

So seems that those values are not in the range [ZAddressHeapBase, ZAddressHeapBase+ZAddressOffsetMax).

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/os_linux.cpp#L3334

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PR Review Comment: https://git.openjdk.org/jdk/pull/13771#discussion_r1187278971