RFR: 8261447: MethodInvocationCounters frequently run into overflow

[Lutz Schmidt]

On Thu, 11 Feb 2021 08:33:40 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> Dear community,
>> may I please request reviews for this fix, improving the usefulness of method invocation counters.
>> - aggregation counters are retyped as uint64_t, shifting the overflow probability way out (185 days in case of a 1 GHz counter update frequency).
>> - counters for individual methods are interpreted as (unsigned int), in contrast to their declaration as int. This gives us a factor of two before the counters overflow.
>> - as a special case, "compiled_invocation_counter" is retyped as long, because it has a higher update frequency than other counters.
>> - before/after sample output is attached to the bug description. 
>> 
>> Thank you!
>> Lutz
>
> src/hotspot/share/oops/method.hpp line 106:
> 
>> 104:     struct {
>> 105:       int     _compiled_invocation_count;  // Number of nmethod invocations so far (for perf. debugging)
>> 106:                                            // Must preserve this as int. Is used outside the jdk by SA.
> 
> Why not update the SA code to access 64 bit?

Two reasons: scope limitation and lack of expertise in SA code. Don't we need the 32bit decl anyway because it is contained in vmstructs.cpp?

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PR: https://git.openjdk.java.net/jdk/pull/2511