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

[Ashutosh Mehra]

On Wed, 8 Feb 2023 17:31:59 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> I am not referring to heap size, but the G1 region size. A user can create a base static archive with 128M heap and explicitly ask for larger region size than 1M and then try to use that image with a smaller region size. This will cause the archive heap to be relocated.
>>  Although highly unlikely scenario, but it can easily be handled by changing the alignment to always use `MIN_GC_REGION_ALIGNMENT` instead of using the current region size.
>
> Since we have a small amount of archived objects, if we always use MIN_GC_REGION_ALIGNMENT to decide the requested location, it will always be 1MB below the top of the heap.  As a result, we will always relocate when running with larger heaps (or larger G1HeapRegionSize):
> 
> 
> $ java -Xshare:dump -Xmx128m -XX:G1HeapRegionSize=1m 
> $ java -Xmx128m -XX:G1HeapRegionSize=4m -Xlog:cds --version
> [.....]
> [0.002s][info][cds] CDS archive was created with max heap size = 128M, and the following configuration:
> [0.002s][info][cds]     narrow_klass_base = 0x0000000800000000, narrow_klass_shift = 0
> [0.002s][info][cds]     narrow_oop_mode = 0, narrow_oop_base = 0x0000000000000000, narrow_oop_shift = 0
> [0.002s][info][cds]     heap range = [0x00000000f8000000 - 0x0000000100000000]
> [0.002s][info][cds] The current max heap size = 128M, HeapRegion::GrainBytes = 4194304
> [0.002s][info][cds]     narrow_klass_base = 0x0000000800000000, narrow_klass_shift = 0
> [0.002s][info][cds]     narrow_oop_mode = 0, narrow_oop_base = 0x0000000000000000, narrow_oop_shift = 0
> [0.002s][info][cds]     heap range = [0x00000000f8000000 - 0x0000000100000000]
> [0.002s][info][cds] Heap region ca0 = 0x00000000fff00000 - 0x00000000fff83000 =   536576 bytes
> [0.002s][info][cds] Heap region oa0 = 0x00000000ffe00000 - 0x00000000ffe79000 =   495616 bytes
> [0.002s][info][cds] CDS heap data relocation delta = 0 bytes
> [0.002s][info][cds] CDS heap data needs to be relocated lower by a further 3145728 bytes to -3145728 to be aligned with HeapRegion::GrainBytes
> 
> 
> Because the archive regions are pinned, we want to always put them at the absolute top end of the heap to avoid fragmentation. 
> 
> With this PR, I think we should keep the current behavior (which has not been changed by this PR) -- the requested location of the archive objects should be optimized for the heap parameters specified during dump time.
> 
> However, after [JDK-8298048](https://bugs.openjdk.org/browse/JDK-8298048), we will use G1's "old" regions which aren't pinned. So we can afford to have a few empty regions above the archive regions.
> 
> In that case, for G1, the requested base should be (heap_top - max_region_size), where  max_region_size should be 8M. This will allow all heap sizes up to 16GB to map the archive regions without relocating the pointers.

yeah I realized what I am suggesting wouldn't work given that the two different archive regions are required to be  different G1 heap region. Relocation cannot be avoided in that case. Hopefully things will be simpler to reason about after JDK-8298048.
Thanks for explaining the current situation.

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