RFR: 8252842: Extend jmap to support parallel heap dump

[Lin Zang]

On Wed, 27 Jan 2021 19:58:39 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>>> This patch introduces a new "parallel" option to jmap, so after this patch, there can be at most 4 options for jmap that can be passed to JDK.
>>> 
>>> The issue is that there is a limitation in old version of jmap that at most 3 arguments can be accepted. As I commented in File JMap.java. and I can not simply enlarge the limitation to 4 because that will cause old version of jmap hang when working on new JDK.
>> 
>> A year or so ago we ran into this same issue. I'll see if I can find the CR for it.
>
> It looks like [JDK-8215622](https://bugs.openjdk.java.net/browse/JDK-8215622) introduced a 4th argument to the attach API, and this caused hangs when and older JVMs tried to attach. That was fixed by [JDK-8219721](https://bugs.openjdk.java.net/browse/JDK-8219721), and seemed to revert the arg count back to 3, although it's not too clear to me how this was accomplished. The webrev is here:
> 
> http://cr.openjdk.java.net/~xiaofeya/8219721/webrev.01/
> 
> Also at the same time the following was filed, which has not yet been addressed:
> 
> [JDK-8219896](https://bugs.openjdk.java.net/browse/JDK-8219896) Parameter size mismatch between client and VM sides of the Attach API
> 
> @linzang JDK-8215622 and JDK-8219721 were your CRs, so maybe you can elaborate on them a bit more.

Dear @plummercj，
You are right, the same issue has been encountered when I was developing JDK-8215622. At that time the argument number does not actully exceed the limitation (3), and so I made a quick fix to change back arg_count_max to 3. And the reason that I originally change the arg_count_max is I was trying to add more options to jmap, and finally only "file" and "parallel" were accepted, so the arg_count_max never exceed the limitation.

Then it comes to this patch, with "parallel" added to jmap -dump, there can be 4 arguments and we need to handle the arg count limitation.  Here are the experiments I have been tried recently (and also previously when handling JDK-8219721):

- Enlarge arg_count_max from 3 to 4

   This cause the same issue as describe by JDK-8219721, when using an old jcmd tools to communicate with new JDK, it hangs, which is a huge compatibitily issue, so we need to fix it.

- Let jmap pass all arguments as a whole one, just like what jcmd does, and modify attachlistener.cpp to parse arguments from op->arg(0), and also distinguish old/new jmap by testing whether op->arg(1) is null.

   This works well until the new jmap is used to communicate with old JDK. In this case, the old JDK doesn't parse op->arg(0), which is passed by jmap as a string containing all arguments. So all parameters are treated as dump filename incorrectly.  

- Let jmap still passing three arguents, and combine the "gz" and "parallel" together as a string and pass it to JDK, and modify attachlistener.cpp to parse the 3rd arguments from op->arg(2). 

  This works well for old/new jmap targeting on old/new JDK when "parallel" and "gz" options are not used together. However, it has the issue when new jmap targeting to old JDK and both "parallel" and "gz" are used.  because old JDK
can not parse op->arg(3), so the "gz" option will not work. Although I don't think it is reasonable to use new jmap with "parallel" option on an old JDK, but it is possible, and the issue is that old JDK will not prompt any error message in this case, it just dump the heap by ignoring the "gz" value.

- I also worked out a patch using timeout mechanism in socket. 

This solution seems work, so the arg_count_max could be enlarged to 4, and when old jmap is targeting on new JDK, the socket can be timeout on waiting for the 4th arguments while old jmap only provide 3. And then it could work as expected.  But this seems need to change the socket for different platform, and I just verified it on Linux, I am not sure whether it is acceptable. 

- I am also trying to see whether there can be other solution that mentioned in the previous discussion thread. ( links here: https://mail.openjdk.java.net/pipermail/serviceability-dev/2019-February/027240.html, FYI) 

BTW,  Do you think we need to solve this issue seperately? Maybe have a patch for  JDK-8219896?  And when it is fixed, appling the parallel dump patch on top of that? 

Thanks,
Lin

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PR: https://git.openjdk.java.net/jdk/pull/2261