RFR: 8319875: Add macOS implementation for jcmd System.map [v2]

[Simon Tooke]

On Thu, 5 Dec 2024 17:41:00 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>>> @tstuefe I ran an experiment with raw mmap, and there's no way to differentiate between one large allocation of 5*128MB and 5 smaller allocations of 128MB. I _could_ add code to fold these, but we risk loosing information.
>> 
>> What information would we loose?
>> 
>> As it is now, the display is somewhat confusing. We did not allocate the heap with multiple mmap calls, each one of 128MB in size; we use a single mmap call.
>> 
>> If you want to close the work for now and leave this glitch for later, we can do this too.
>
>> @tstuefe if it's up to me, I would leave the folding for a quick later PR (in fact I would start it right after this one goes in). I also would like to investigate the use of mach_make_memory_entry_64() which could be interesting on it's own.
>> 
>> Do you know how I can get the GitHub runner to start working? It seems one of them is misconfigured.
> 
> No idea, but it was broken for a while now, wasn't it? If you figure it out and fix it a lot of ppl would be thankful :)
> 
> Otherwise, since the code does not touch anything dangerous, I think its fine to check it in as long as the other platforms are green and you have executed the relevant test on MacOS for System.map and System.dump_map

@tstuefe , I've actually added the combining code here, and reconfirmed that both the System.map and System.dump_map tests pass on my arm M2.

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PR Comment: https://git.openjdk.org/jdk/pull/20953#issuecomment-2521151800