RFR: 8255047: Add HotSpot flag to use with debuggers that restrict the CPU… [v2]

[David Holmes]

On Wed, 21 Oct 2020 11:36:25 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.
>
> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8255047: Add HotSpot flag to use with debuggers that restrict the CPU count

LGTM!

Thanks,
David

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Marked as reviewed by dholmes (Reviewer).

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