[12] RFR (XS): 8215548: G1PeriodicGCSystemLoadThreshold needs to be a double

[Thomas Schatzl]

Hi,

On Wed, 2018-12-19 at 10:37 +0100, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for this small change that changes the
> G1PeriodicGCSystemLoadThreshold value type from an uintx to a double?
> 
> (Manual) Testing by Ruslan from jElastic showed that integers are too
> coarse for this value, as typical load values you want to compare
> with are ~0.5.
> 
> Unfortunately this has not been discovered during review; also
> testing against a particular load value provided by the OS is hard,
> so there has not been a regression test for this particular flag
> where this probably would have been found out. :(

  there has been a question about the comparison against 0.0 in
g1YoungRemSetSamplingThread.cpp:75, whether this will catch all cases.

Any negative values will be blocked already by the "range" specifier,
but there might be unintentional issues with -0.0. I changed the
comparison to "> 0.0" to exclude it in any case.

I updated the webrev in place.

> CR:
> https://bugs.openjdk.java.net/browse/JDK-8215548
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8215548/webrev/
> Testing:
> Manual testing with a hacked gc/g1/TestPeriodicCollection.java test,
> and the command line range checking tests.
> hs-tier1-5

Thanks,
  Thomas