RFR: 8281213: Unsafe uses of long and size_t in MemReporterBase::diff_in_current_scale [v4]

[Thomas Stuefe]

On Fri, 13 Jan 2023 08:23:36 GMT, Afshin Zafari <duke at openjdk.org> wrote:

>> src/hotspot/share/services/memReporter.hpp line 88:
>> 
>>> 86:     size_t scaled = (amount / _scale);
>>> 87:     if ((amount % _scale) > (_scale - 1)/2) {
>>> 88:       scaled += 1;
>> 
>> This correction is not needed I think. 
>> 
>> The old code did not do this, so it may cause NMT numbers to seemingly go up after this patch, suggesting we use more memory. And it's wrong either way.
>> 
>> A better way to deal with this (as a future improvement) would be to print 2-3 significant digits, e.g. "2.33GB". Like we do in the `proper_unit_for_byte_size()` and friends, but with a fixed scale.
>
> In the tests, there are cases where explicit numbers are expected as the result of the difference function, like +256K or -4K. So, if the tests pass with this code, it means than all the results are as NMT expected.

Ah, I misread the old code. This is fine then.

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PR: https://git.openjdk.org/jdk/pull/11514