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

[Schmelter, Ralf]

Hi Yasumasa,

>   You can use `DCmdArgument<jlong>` for -gz option.

That is what I originally tried. But then you always have to supply a compression level (just specifying -gz doesn't work). Since I would expect most users never caring about the compression level, I switched to a string option, which can handle this pattern.

> _nr_of_threads, _id_to_write, _current in CompressionBackend should be added `volatile` at least.

I don't think that is needed. Apart from the initialization, they are only changed under lock protection. 

> BTW how much processing time is different between single threaded and multi threaded?

I've benchmarked an example, which creates a ~31 GB uncompressed hprof file, with a VM which doesn't use any background threads. Here are the size of the create files, the compression level and the time spend:

Uncompressed, 31.6 G, 71 sec
gzipped level 1, 7.57 G, 463 sec (x6.5)
gzipped level 3, 7.10 G, 609 sec (x8.6)
gzipped level 6, 6.49 G, 1415 sec (x19.9)

So even the fastest gzip compression makes writing the dump at least 5 times as slow.

> Also I want to know what number is set to ParallelGCThreads.
> ParallelGCThreads seems to affect to thread num for GZip compression.

Originally, I've tried to use the WorkGang (CollectedHeap:: get_safepoint_workers()) of the GC to do the work. But this wouldn't work because Shenandoah could not iterate the heap from a worker thread. So I've opted to start the needed threads itself for the time of the heap dump. I've used ParallelGCThreads as the maximum number of threads, since this is what would be used for a GC too. So it should not clog up the machine more than a GC. Maybe it would be even better to additionally limit the threads by the compression level.

Best regards,
Ralf Schmelter

-----Original Message-----
From: Yasumasa Suenaga <suenaga at oss.nttdata.com> 
Sent: Samstag, 8. Februar 2020 14:46
To: Schmelter, Ralf <ralf.schmelter at sap.com>; OpenJDK Serviceability <serviceability-dev at openjdk.java.net>
Cc: yasuenag at gmail.com
Subject: Re: RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump

Hi Ralf,


- diagnosticCommand.cpp
  You can use `DCmdArgument<jlong>` for -gz option.
  If you want to use lesser type (e.g. int, unsigned char), I guess you need to modify GenDCmdArgument class.

- heapDumper.cpp
  _nr_of_threads, _id_to_write, _current in CompressionBackend should be added `volatile` at least.
  (Other values need to be checked)


BTW how much processing time is different between single threaded and multi threaded?
Also I want to know what number is set to ParallelGCThreads.
ParallelGCThreads seems to affect to thread num for GZip compression.


Thanks,

Yasumasa