RFR: 8320215: HeapDumper can use DumpWriter buffer during merge

[Alex Menkov]

On Wed, 29 May 2024 02:52:22 GMT, Yi Yang <yyang at openjdk.org> wrote:

> I remember experimenting with different buffer sizes and figuring out that 4KB was the sweet spot. We could potentially switch to 1MB, but it would be better if we had some benchmark numbers to back that up.

My experiments shown that with the fix time of the merge decreased ~5%. 
But this is not just change of the buffer size:
Before the fix merger (1) read `segment_fs` in 4K chunks, (2) wrote them (copying memory) to `_writer->_buffer` until the buffer is full (i.e. 256 read iterations) and then (3) flush `_writer->_buffer`;
With the fix (1) and (2) are replaced with single reading of 1MB directly to `_writer->_buffer` (no memory copy).

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PR Comment: https://git.openjdk.org/jdk/pull/18850#issuecomment-2138238839