The new ergonomics code calculates the time spent in gc vs time
spent in mutator, and uses this value to make decisions.

Unfortunately, we aren't calculating the value correctly.

One of the things that can be done with the value is to cause
an OOM to be thrown if an app spends more than a certain percentage of
its time in GC.

I have found in practice, though, that the calculated value
can easily exceed 150%, and for some apps I need to set it to over
1000% to get the app to pass. As 100% is an upper bound for this value,
it would seem we are doing something wrong.

For examples of tests hitting this problem, see bug 4976274 & 4975813