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

[David Holmes]

Hi Ralf,

On 13/03/2020 9:43 pm, Schmelter, Ralf wrote:
> Hi,
> 
> I have updated the webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8237354/webrev.1/
> 
> It has the following significant changes:
> 
> - The jcmd now uses two separate flags. The -gz flag is now a boolean flag which toggles the compression  on/off. And the new -gz-level flag can be used to change the compression level. If tried to change the jlong flag coding to allow the old behavior (only one flag, which acts both as a boolean flag and a jlong flag), but decided against it, since it changes the semantic of a jlong flag. And I don't expect the -gz-level flag to be used all that much.
> 
> - I no longer use my own threads. Instead I use the WorkGang returned from CollectedHeap:: get_safepoint_workers(). This works fine, apart from Shenandoah GC, which runs into assertions when calling the CollectedHeap::object_iterate() method from a worker thread. I'm not sure if the assertion is too strong, but since the GC is currently experimental, I switch back to single threading in this case (as would be the case for serial GC or epsilon GC). Using the worker threads removes the problems the original code had regarding destruction of the monitor used.

I'm glad to see you are no longer using your own threads, and I 
apologise that I have not yet been able to look further into the thread 
lifecycle issues you encountered. However I'm not clear how this solves 
the problem of destroying the monitor while it can still be being 
accessed - is the dumping occurring at a safepoint in the WorkGang threads?

Thanks,
David
-----

> - The reported number of bytes is now the one written to disk.
> 
> Best regards,
> Ralf
> 
> -----Original Message-----
> From: Ioi Lam <ioi.lam at oracle.com>
> Sent: Dienstag, 25. Februar 2020 18:03
> To: Langer, Christoph <christoph.langer at sap.com>; Schmelter, Ralf <ralf.schmelter 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 Christoph,
> 
> This sounds fair. I will remove my objection :-)
> 
> Thanks
> - Ioi
>