RFR: 8296344: Remove dependency on G1 for writing the CDS archive heap [v4]

[Ioi Lam]

On Tue, 7 Feb 2023 16:13:48 GMT, Ashutosh Mehra <duke at openjdk.org> wrote:

>> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Simplified relocation by using buffered address instead of requested address
>
> src/hotspot/share/cds/archiveHeapWriter.cpp line 388:
> 
>> 386:   assert(is_aligned(_open_bottom, HeapRegion::GrainBytes), "sanity");
>> 387: 
>> 388:   _requested_closed_region_bottom = align_down(heap_end - closed_region_byte_size, HeapRegion::GrainBytes);
> 
> Instead of aligning to `HeapRegion::GrainBytes`, shouldn't it be aligned to `MIN_GC_REGION_ALIGNMENT`?
> With `HeapRegion::GrainBytes`, if the cds archive is created with large region size (say 32M) and later mapped with smaller region size (say 1M), then the archive heap would need to be relocated. Using `MIN_GC_REGION_ALIGNMENT` would avoid the relocation in this case.

When we dump with a large heap size (e.g.,16GB) but with with a small heap size (e.g., 256M), we are likely to need to relocate anyway, because the narrowOop encoding would be different (3-bit shift vs no shift).

The design decision we made in the past was: the layout of the archived objects should be optimized for the heap size specified at dump time. Hence, the default CDS archive in the JDK is dumped with -Xmx128M so that it will be optimized for small workloads such as microservices. See https://github.com/openjdk/jdk/blob/2a579ab8392d30a35f044954178c788d16d4b800/make/Images.gmk#L117-L121

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PR: https://git.openjdk.org/jdk/pull/12304