Hi,
Generally I agree with Ioi, but I think it is not a problem only for gzipped
heap dump.
For example, Compiler.codelist and Compiler.CodeHeap_Analytics might be large
text.
In addition, some users want to redirect the result from jcmd to other command
or log collector.
So I think it would be better if jcmd provides stdout redurect option to all
subocmmands. E.g.
$ jcmd <PID> GC.heap_dump -stdout | gzip -c - > heapdump.hprof.gz
Thanks,
Yasumasa
On 2020/02/20 2:40, Ioi Lam wrote:
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....@oracle.com>
Sent: Mittwoch, 19. Februar 2020 01:16
To: serguei.spit...@oracle.com; Schmelter, Ralf <ralf.schmel...@sap.com>;
hotspot-runtime-...@openjdk.java.net runtime <hotspot-runtime-...@openjdk.java.net>
Cc: Laurence Cable <larry.ca...@oracle.com>; serviceability-dev@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
