RFR: 8255047: Add HotSpot flag to use with debuggers that restrict the CPU…

[Stefan Karlsson]

On Tue, 20 Oct 2020 14:52:33 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> Some debuggers don't work well with many threads, and/or incompletely restricts the number of used CPUs to one.
>> 
>> This flag is intended as a catch-all for HotSpot developers (not available in product builds) to allow us to more
>> easily use those debuggers.
>> Currently, the proposal is to let the flag fix a few things:
>> 1) Turn down the number of JVM threads
>> 2) Turn off NUMA
>> 3) Force processor_id() to return 0 instead of values above processor_count()
>> 
>> (1) is purely ergonomics: gdb, rr, valgrind is faster and seems to work much better with fewer threads. The values
>> would still be overridable by devs.
>> (2) and (3) deals with the fact that some debuggers change the reported processor count, but don't change the processor
>> ids returned by sched_getcpu. This causes problems for ZGC and NUMA, that both assumes that they can rely on
>> os::processor_id() < os::processor_count().  The current proposed flag name is -XX:+LimitedCPUsDebugging. I'm not
>> entirely happy with that name, but I been able to find a better name.
>> An alternative to having one flag, is to split this into two flags, and maybe that would solve the naming problem.
>> However, the usability aspects will be worse.
>> If we can't find a suitable name, I rather introduce a flag called:
>> -XX:DebuggerWorkarounds or -XX:DebuggerWorkaround1
>> 
>> Any suggestions / opinions? I really do want to at least fix the (2, 3) problem, because I keep having to add this to
>> every single branch I'm working on.
>
> Perhaps: `UseDebuggerErgo` for the main option name with
> `UseDebuggerErgo1` ... `UseDebuggerErgoN` for the suboptions
> where `UseDebuggerErgo` enables all the numbered
> `UseDebuggerErgo<X>` options in one go.
> 
> Update: Yes, this is the way that I have always wished that
> `UseNewCode*` worked.

@dcubed-ojdk Thanks for the suggestion. I like it.

-------------

PR: https://git.openjdk.java.net/jdk/pull/763