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

[Nick Gasson]

On Thu, 22 Oct 2020 07:09:47 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

>> Hi Nick,
>> 
>> this is a very useful idea! I missed this in the past.
>> 
>> Some remarks. I'll try to keep bikeshedding to a minimum since you already have enough input. Mostly ergonomics.
>> 
>> 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.
>> 
>> 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.
>> 
>> 4) I think it would be nice to have these switches always available, so real product switches. Which would require you to write up a small CSR but I still think it would make sense.
>> 
>> Cheers, Thomas
>
>> 
>>     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.

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