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

[Daniel D.Daugherty]

On Tue, 20 Oct 2020 10:10:12 GMT, Stefan Karlsson <stefank 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.

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

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