RFR: 8314021: HeapDump: Optimize segmented heap file merging phase

[Yi Yang]

On Thu, 7 Sep 2023 00:56:40 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> This patch reduce ~16%(24s->20s) pahse 2 merge time during dumping 32g heap with 96threads and fixes a memory leak of compressor
>> 
>> You might argue why this is Linux-only optimization, because sendfile requires at least socket fd in other platforms([aix sendfile](https://www.ibm.com/docs/en/aix/7.1?topic=s-send-file-subroutine) [maxos sendfile](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/sendfile.2.html) [win32 TransmitFile](https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile)), while [only Linux](https://man7.org/linux/man-pages/man2/sendfile.2.html) supports both two file descriptors.
>
> The fix looks good to me in general, but I'm not sure about code organization.
> src/hotspot/share/runtime/os.hpp describes rules for os* files.
> It states:
> //   Platform-independent source files should not include these header files
> //   (although sadly there are some rare exceptions ...)
> And the change adds one more exception.
> I'd like to hear runtime guys opinion.

Hi @alexmenkov, can we move on since there are no more comments from runtime folks. We can still propose a follow-up change if someone have strong objection/concern on this "exception"

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PR Comment: https://git.openjdk.org/jdk/pull/15245#issuecomment-1713063232