RFR: 8254723: add diagnostic command to write Linux perf map file [v2]

[David Holmes]

On Mon, 26 Oct 2020 06:10:52 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

>>> 
>>>     1. Like Alexey, I would really wish for an print-at-exit switch. The common naming seems to be xxxAtExit (so not, OnExit). "PrintXxx" seems to be printing stuff out to tty, "DumpXxxx" for writing separate files (e.g. CDS map). So I would name it DumpPerfMapAtExit.
>>> 
>> 
>> OK, makes sense.
>> 
>>>     2. Dumping to /tmp is unexpected for me, I would prefer if the default were dumping to the current directory. That seems to be the default for other files too (cds map, hs-err file etc).
>>> 
>>>     3. Not necessary but nice would be a an option to specify location of the dump file.
>>> 
>> 
>> The `/tmp/perf-<pid>.map` is hardcoded into perf though ([see here](https://github.com/torvalds/linux/blob/master/tools/perf/util/map.c#L155)), so I don't think it's useful for the user to be able to change the location.
>
>> 
>> I think we should use this option carefully because nmethod might be unloaded. So we should use this with `-XX:-UseCodeCacheFlushing`.
>> 
> 
> Thanks for the information. `-XX:+DumpPerfMapAtExit` will turn on `UseCodeCacheFlushing` if it's set to default.

I don't see any reason for this to be a product flag, rather than diagnostic.

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PR: https://git.openjdk.java.net/jdk/pull/760