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

[David Holmes]

Hi Ralf,

One part of this caught my eye and now I look at the webrev I have some 
concerns. Introducing new threads to the VM is not something that should 
be done lightly and it has to be done very carefully - I need to look 
closer at this aspect. Further when using Mutexes/Monitors in such code 
you have to be extremely careful about how (or even if) those 
Mutex/Monitor get deleted. The code you have at present is not safe 
because you cannot know when other threads have completely exited the 
Monitor/Mutex code. The last thread to terminate will signal the 
destructing thread (blocked in wait) then release the monitor, allowing 
the destructing thread to acquire the monitor and then delete the _lock. 
But at the point at which the monitor becomes free and the destructor 
thread is unparked, the terminating thread may be context switched out 
and remain inside the Monitor code. The destructor thread then deletes 
the monitor and frees it. When the terminating thread resumes, if it 
touches any memory associated with the Monitor it could SEGV.

To safely delete a Monitor/Mutex you have to know for certain that all 
threads using it have completely ceased to use it. You cannot use that 
Monitor/Mutex as the means for determining that. It is a non-trivial 
problem to solve.

Cheers,
David
-----

On 11/02/2020 1:33 am, Schmelter, Ralf wrote:
> 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
>