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

[Reingruber, Richard]

Hi Ralf,

> > 767: I think _current->in_used doesn't take the final 9 bytes into account that are written in
> > DumperSupport::end_of_dump() after the last dump segment has been finished.
> > You could call get_new_buffer() instead of the if clause.

> Wow, how did you found this? I've fixed it by making sure we flush the DumpWriter before calling the deactivate method.

Spending long hours on the review ;)
Ok with the fix.

> > ### src/java.base/share/native/libzip/zip_util.c
> > 1610: Will be hard to beat zlib_block_alloc() and zlib_block_free() performance wise. But have you
> >  measured the performance gain? In other words: is it worth it? :)

> This is not done for performance, but to make sure the allocation will not fail midway during writing the dump. Maybe it is not worth it, though.

Understood. The heap dump will succeed if you can allocate at least one WriteWork instance. Without
that you could get out of memory errors in the zlib which would make the dump fail. Ok!

> http://cr.openjdk.java.net/~rschmelter/webrevs/8237354/webrev.2/

Thanks for the clarifications and the changes in the new webrev.
Webrev.2 looks good to me.

Cheers, Richard.

-----Original Message-----
From: Schmelter, Ralf <ralf.schmelter at sap.com> 
Sent: Montag, 20. April 2020 10:14
To: Reingruber, Richard <richard.reingruber at sap.com>; Ioi Lam <ioi.lam at oracle.com>; Langer, Christoph <christoph.langer at sap.com>; Yasumasa Suenaga <suenaga at oss.nttdata.com>; serguei.spitsyn at oracle.com; hotspot-runtime-dev at openjdk.java.net runtime <hotspot-runtime-dev at openjdk.java.net>
Cc: serviceability-dev at openjdk.java.net
Subject: RE: RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump

Hi Richard,

thanks for the review. I have incorporated your remarks into a new webrev:
http://cr.openjdk.java.net/~rschmelter/webrevs/8237354/webrev.2/

Some remarks to specific points:

> ### src/hotspot/share/services/heapDumper.cpp
> 762: assert(_active, "Must be active");
>
> It appears to me that the assertion would fail, if an error occurred creating the CompressionBackend.

You are supposed to check for errors after creating the DumpWriter (which creates the CompressionBackend). And in case of an error, you directly destruct the object. I've added a comment to make that clear. 

> 767: I think _current->in_used doesn't take the final 9 bytes into account that are written in
> DumperSupport::end_of_dump() after the last dump segment has been finished.
> You could call get_new_buffer() instead of the if clause.

Wow, how did you found this? I've fixed it by making sure we flush the DumpWriter before calling the deactivate method.

> 1064: DumpWriter::DumpWriter()
>
> There doesn't seem to be enough error handling if _buffer cannot be allocated.
> E.g. DumpWriter::write_raw() at line 1091 will enter an endless loop.

As described above, this will not happen if we check for error after constructing the DumpWriter.

> ### src/java.base/share/native/libzip/zip_util.c
> 1610: Will be hard to beat zlib_block_alloc() and zlib_block_free() performance wise. But have you
>  measured the performance gain? In other words: is it worth it? :)

This is not done for performance, but to make sure the allocation will not fail midway during writing the dump. Maybe it is not worth it, though.

> 1655: The result of deflateBound() seems to depend on the header comment, which is not given
> here. Could this be an issue, because ZIP_GZip_Fully() can take a comment?

I've added a 1024 byte additional bytes to avoid the problem.

> ### test/lib/jdk/test/lib/hprof/parser/Reader.java
>
> 93: is the created GzipRandomAccess instance closed somewhere?

The object is not closed since it is still used by the Snapshot returned.

Best regard,
Ralf


-----Original Message-----
From: Reingruber, Richard <richard.reingruber at sap.com> 
Sent: Tuesday, 14 April 2020 10:30
To: Schmelter, Ralf <ralf.schmelter at sap.com>; Ioi Lam <ioi.lam at oracle.com>; Langer, Christoph <christoph.langer at sap.com>; Yasumasa Suenaga <suenaga at oss.nttdata.com>; serguei.spitsyn at oracle.com; hotspot-runtime-dev at openjdk.java.net runtime <hotspot-runtime-dev at openjdk.java.net>
Cc: serviceability-dev at openjdk.java.net
Subject: RE: RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump

Hi Ralf,

thanks for providing this enhancement to parallel gzip-compress heap dumps!

I reckon it's safe to say that the coding is sophisticated. It would be awesome if you could sketch
the idea of how HeapDumper, DumpWriter and CompressionBackend work together to produce the gzipped
dump in a source code comment. Just enough to get started if somebody should ever have to track down
a bug -- an unlikely event, I know ;)

Please find the details of my review below.

Thanks, Richard.
// Not Reviewer

--

### src/hotspot/share/services/diagnosticCommand.cpp

510   _gzip_level("-gz-level", "The compression level from 0 (store) to 9 (best) when writing in gzipped format.",
511               "INT", "FALSE", "1") {

    "FALSE" should be probably false.

### src/hotspot/share/services/diagnosticCommand.hpp
Ok.

### src/hotspot/share/services/heapDumper.cpp

390: Typo: initized

415: Typo: GZipComressor

477: Could you please add a comment, how the "HPROF BLOCKSIZE" comment is helpful?

539: Member variables of WriteWork are missing the '_' prefix.

546: Just a comment: WriteWork::in_max is actually a compile time constant. Would be nice if it could be
     declared so. One could use templates for this, but then my favourite ide (eclipse cdt) doesn't
     show me references and call hierarchies anymore. So I don't think it is worth it.

591: Typo: Removes the first element. Returns NULL is empty.

663: _writer, _compressor, _lock could be const.

762: assert(_active, "Must be active");

     It appears to me that the assertion would fail, if an error occurred creating the CompressionBackend.

767: I think _current->in_used doesn't take the final 9 bytes into account that are written in
     DumperSupport::end_of_dump() after the last dump segment has been finished.
     You could call get_new_buffer() instead of the if clause.

903: Typo: Check if we don not waste more than _max_waste

1064: DumpWriter::DumpWriter()

     There doesn't seem to be enough error handling if _buffer cannot be allocated.
     E.g. DumpWriter::write_raw() at line 1091 will enter an endless loop.

2409: A comment, why Shenandoah is not supported, would be good.
      In general I'd say it is good and natural to use the GC work threads.

### src/hotspot/share/services/heapDumper.hpp
Ok.

### src/java.base/share/native/libzip/zip_util.c

I'm not familiar with zlib, but here are my .02€ :)

1610: Will be hard to beat zlib_block_alloc() and zlib_block_free() performance wise. But have you
      measured the performance gain? In other words: is it worth it? :)

1655: The result of deflateBound() seems to depend on the header comment, which is not given
      here. Could this be an issue, because ZIP_GZip_Fully() can take a comment?

1658: deflateEnd() should not be called if deflateInit2Wrapper() failed. I think this can lead
      otherwise to a double free() call.

### test/hotspot/jtreg/serviceability/dcmd/gc/HeapDumpCompressedTest.java

66: Maybe additionally check the exit value?

73: It's unclear to me, why this fails. Because the dump already exists? Because the level is
    invalid? Reading the comment I'd expect success, not failure.

### test/hotspot/jtreg/serviceability/dcmd/gc/HeapDumpCompressedTestEpsilon.java
Ok.

### test/hotspot/jtreg/serviceability/dcmd/gc/HeapDumpCompressedTestShenandoah.java
Ok.

### test/hotspot/jtreg/serviceability/dcmd/gc/HeapDumpCompressedTestZ.java
Ok.

### test/lib/jdk/test/lib/hprof/parser/GzipRandomAccess.java
Ok.

### test/lib/jdk/test/lib/hprof/parser/HprofReader.java
Ok.

### test/lib/jdk/test/lib/hprof/parser/Reader.java

93: is the created GzipRandomAccess instance closed somewhere?