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

[Nick Gasson]

On Thu, 22 Oct 2020 04:57:18 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Nick Gasson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update for review comments
>
> src/hotspot/share/code/codeCache.hpp line 194:
> 
>> 192:   static void print_summary(outputStream* st, bool detailed = true); // Prints a summary of the code cache usage
>> 193:   static void log_state(outputStream* st);
>> 194:   static void write_perf_map(outputStream* st);
> 
> Seems weird for this function to have an outputStream parameter only to write the dump to an unrelated file and ignore the stream for everything but the final message.
> 
> I would either pass in the file name as an option - preferably configurable - and write the last message out here; or just write the whole perf dump to the outputstream itself, piping it to jcmd and let the caller do what he wants with it (e.g. just redirecting). The latter is what most commands do. Not sure how large the perf dump gets though, may be impractical.

OK. I think I'll change it so `write_perf_map()` writes to the `outputStream` and then `PerfMapDCmd::execute()` handles redirecting it to a file. I don't think it makes sense to write it directly to the jcmd output though.

> src/hotspot/share/code/codeCache.cpp line 1562:
> 
>> 1560: }
>> 1561: 
>> 1562: void CodeCache::write_perf_map(outputStream* st) {
> 
> Could this whole function possibly live inside os/linux in an own file? Or would that expose too many code heap internals?

Probably creates too much dependency between the os layer and the codeCache internals? I'll put it all in `#ifdef LINUX` though.

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