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

[Yi Yang]

On Fri, 11 Aug 2023 09:31:56 GMT, Yi Yang <yyang 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.

Can I have a review for this? This contains a memory leak and harmless optimziation.

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PR Comment: https://git.openjdk.org/jdk/pull/15245#issuecomment-1707732875