RFR: 8258576: Try to get zerobased CCS if heap is above 32 and CDS is disabled [v2]

[Richard Reingruber]

On Sun, 20 Dec 2020 12:08:01 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Hi Richard, 
>> 
>> I don't want to block this. Feel free to commit this. Since you want to keep the change minimal to aid backporting, I would refrain from changing the logic which initiatializes CompressedOops. Instead, I would just use 4G instead of heapBaseMinAddress on aarch64. 
>> 
>> Cheers, Thomas
>
> BTW, mid-term I'd like to implement this: https://bugs.openjdk.java.net/browse/JDK-8258743. I already have a promising prototype for Windows, which was kind of a by product of the recent CDS reservation changes. What do you think, does that make sense to you?

Hi Thomas,

> BTW, mid-term I'd like to implement this:
> https://bugs.openjdk.java.net/browse/JDK-8258743. I already have a promising
> prototype for Windows, which was kind of a by product of the recent CDS
> reservation changes. What do you think, does that make sense to you?

Well, I would think that in most cases the trial and error method will succeed
at first try. This doesn't leave much room for enhancement. Building a vmmap
looks like more effort too me. Just my .02€

> I don't want to block this. Feel free to commit this. Since you want to keep
> the change minimal to aid backporting,

Ok. There may be room for further enhancements after this. Also I reckon it
could be better to place ccs just below 4G instead of placing it at 2G
(HeapBaseMinAddress) for less fragmentation of the 32bit addressable range if
possible. I'm not sure how important this would be though.

> I would refrain from changing the logic which initiatializes
> CompressedOops. Instead, I would just use 4G instead of heapBaseMinAddress on
> aarch64.

Hm, I think you meant CompressedKlassPointers and the issue related to
CompressedKlassPointers::is_valid_base(), right? I won't do changes for
that. I've created https://bugs.openjdk.java.net/browse/JDK-8258756 for
tracking.

Thanks for the review,
Richard.

PS: I won't integrate until next year since I'll be off for holidays now

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PR: https://git.openjdk.java.net/jdk/pull/1815