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

[Ioi Lam]

On 2/19/20 7:24 AM, Schmelter, Ralf wrote:
> Hi Ioi,
>
>> This seems to be an edge case (where your environment has more
>> RAM than disk)
> I would not say it's an edge case. Especially in a cloud environment, your container does not need much free diskspace, since the data is stored in a database and logging goes to stdout.
>
>> I think it would be better to handle this outside of the JVM
>> (using a named pipe and and external program such as the parallel gzip
>> "pigz") to limit the maintenance overhead of the JVM.
> But then you would have to implement writing the heap dump to a named pipe (and not only on Unix, but on Windows too). And you would still want to do the writing in background threads, so most of the code would stay. You need something like netcat on Windows. And it doesn't cover writing a heap dump on OOM via the VM flag.
>
> And you should to compress the hprof file in a specific way, since it will make it much faster to random access the gzipped hprof file directly.
>
> Note that I think it is a good idea to be able to write the dump to non-file destination. But removing the compression will not save much code and will make the handling messier.

I was thinking of doing something like this:

$ mkfifo /tmp/pipe
$ cat /tmp/pipe | gzip -c - > /tmp/zipped &
$ jcmd $PID GC.heap_dump filename=/tmp/pipe

You can replace the "> /tmp/zipped" part with a program that reads from 
stdin and send it over the network.

I tried the above with a recent JDK build (with your changes in 
JDK-8234510: Remove file seeking requirement for writing a heap dump), 
but it doesn't seem to work, probably because we need to change this 
code a little bit

http://hg.openjdk.java.net/jdk/jdk/file/7ef41e83066b/src/hotspot/share/services/heapDumper.cpp#l465

DumpWriter::DumpWriter(const char* path) : _fd(-1), _bytes_written(0), 
_pos(0),
                                            _in_dump_segment(false), 
_error(NULL) {
     ...
     _fd = os::create_binary_file(path, false);    // don't replace 
existing file <<<

I also saw a post saying that the JVM can write to named pipes on Windows:

https://stackoverflow.com/questions/634564/how-to-open-a-windows-named-pipe-from-java

There's no built-in mkfifo command on Windows, but the above link points 
to a .NET example that creates a named pipe and uses that to communicate 
with the JVM.

I don't know whether this will be a better solution than your proposed 
changes, but I think it should be explored as a possible alternative. It 
does seem to require a little work to get your whole data collection 
system working, but it also seems more flexible and extensible.

Thanks
- Ioi


>
> Best regards,
> Ralf
>
>
> -----Original Message-----
> From: Ioi Lam <ioi.lam at oracle.com>
> Sent: Mittwoch, 19. Februar 2020 01:16
> To: serguei.spitsyn at oracle.com; Schmelter, Ralf <ralf.schmelter at sap.com>; hotspot-runtime-dev at openjdk.java.net runtime <hotspot-runtime-dev at openjdk.java.net>
> Cc: Laurence Cable <larry.cable at oracle.com>; serviceability-dev at openjdk.java.net
> Subject: Re: RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump
>
> Hi Ralf,
>
> We are usually pretty picky about adding new features into the JVM. This
> seems to be an edge case (where your environment has more RAM than
> disk). I think it would be better to handle this outside of the JVM
> (using a named pipe and and external program such as the parallel gzip
> "pigz") to limit the maintenance overhead of the JVM.
>
> This would also have the benefit that you can do it with almost no local
> storage -- you can read from the named pipe, optionally compress the
> data, and send that over the network.
>
> Thanks
> - Ioi