RFR: 8309065: Move the logic to determine archive heap location from CDS to G1 GC [v3]

[Ashutosh Mehra]

On Wed, 7 Jun 2023 14:29:07 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>>> we map the archive region at the end of the heap, therefore, if heap size changed between dump time and runtime, we will always have to relocate
>> 
>> Not always. For small heap sizes under 2G with compressedoops the heap is mapped such that top of the heap is at 4G boundary. So by keeping the archive regions towards the end, we can get same offset even if the heap size changes. If we map towards the bottom, then the offsets would change as the heap size changes.
>> 
>> For example, the default archive is created with 128m. So executing:
>> 
>> $ java -Xlog:cds -Xmx128m -version
>> [0.003s][info][cds] The current max heap size = 128M, HeapRegion::GrainBytes = 1048576
>> [0.003s][info][cds]     narrow_klass_base = 0x0000000800000000, narrow_klass_shift = 0
>> [0.003s][info][cds]     narrow_oop_mode = 0, narrow_oop_base = 0x0000000000000000, narrow_oop_shift = 0
>> [0.003s][info][cds]     heap range = [0x00000000f8000000 - 0x0000000100000000]
>> [0.003s][info][cds] Preferred address to map heap data (to avoid relocation) is 0x00000000ffe00000
>> [0.003s][info][cds] Heap data mapped at 0x00000000ffe00000, size =  1071752 bytes
>> [0.003s][info][cds] CDS heap data relocation delta = 0 bytes
>> 
>> 
>> Changing the heap size to 256m:
>> 
>> $ java -Xlog:cds -Xmx256m -version
>> [0.003s][info][cds] The current max heap size = 256M, HeapRegion::GrainBytes = 1048576
>> [0.003s][info][cds]     narrow_klass_base = 0x0000000800000000, narrow_klass_shift = 0
>> [0.003s][info][cds]     narrow_oop_mode = 0, narrow_oop_base = 0x0000000000000000, narrow_oop_shift = 0
>> [0.003s][info][cds]     heap range = [0x00000000f0000000 - 0x0000000100000000]
>> [0.003s][info][cds] Preferred address to map heap data (to avoid relocation) is 0x00000000ffe00000
>> [0.003s][info][cds] Heap data mapped at 0x00000000ffe00000, size =  1071752 bytes
>> [0.003s][info][cds] CDS heap data relocation delta = 0 bytes
>> 
>> 
>> Notice that relocation delta is 0 in both cases and the heap data is mapped at same address.
>
> @ashu-mehra Ah, thank you. I remembered this wrongly as the heap having a preferred fixed start address when the fix point was actually the end of the zero-based encoding range.

> I didn't look but I assume the dumptime heap size is the default heap size?

I am wondering why do you think dumptime heap size is the default heap size?  When creating an archive the heap size can be anything. For default archive it is 128m.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14208#discussion_r1221706723