Integrated: 8255047: Add HotSpot UseDebuggerErgo flags

[Stefan Karlsson]

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.

This pull request has now been integrated.

Changeset: ae72b528
Author:    Stefan Karlsson <stefank at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/ae72b528
Stats:     51 lines in 4 files changed: 48 ins; 0 del; 3 mod

8255047: Add HotSpot UseDebuggerErgo flags

Reviewed-by: dcubed, dholmes

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