RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump

[Schmelter, Ralf]

Hi everyone,

this change adds the option to write a hprof heap dump directly gzipped. Currently this is supported for the GC.heap_dump diagnostic command via the "-gz" flag.

Since gzip is not particular fast when compressing the data, the actual compression will be done parallel in a bunch of background threads.

The created file itself is not a single gzip stream, but a concatenation of streams of about 1 MB gzippped data. This makes it easier to parallelize the compression and it allows for an at least semi-efficient random access to the created file. I've adjusted the jhat library to be able to directly parse a such a gzipped hrpof  file, without the need of prior decompression.

bugreport: https://bugs.openjdk.java.net/browse/JDK-8237354
webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8237354/webrev.0/

If this get reviewed, a CSR is still needed for the change to the GC.heap_dump command.

Best regards,
Ralf
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