RFR(S): 7158457: division by zero in adaptiveweightedaverage

[Mikael Vidstedt]

Hi all,

The statistical counters in gcUtil are used to keep track of historical 
information about various key metrics in the garbage collectors. Built 
in to the core AdaptiveWeightedAverage base class is the concept of 
aging the values, essentially treating the first 100 values differently 
and putting more weight on them since there's not yet enough historical 
data built up.

In the class there is a 32-bit counter (_sample_count) that incremented 
for every sample and used to compute scale the weight of the added value 
(see compute_adaptive_average), and the scaling logic divides 100 by the 
count. In the normal case this is not a problem - the counters are reset 
every once in a while and/or grow very slowly. In some pathological 
cases the counter will however continue to increment and eventually 
overflow/wrap, meaning the 32-bit count will go back to zero and the 
division in compute_adaptive_average will lead to a div-by-zero crash.

The test case where this is observed is a test that stress tests 
allocation in combination with the GC locker. Specifically, the test is 
multi-threaded which pounds on java.util.zip.Deflater.deflate, which 
internally uses the GetPrimitiveArrayCritical JNI function to 
temporarily lock out the GC (using the GC locker). The garbage collector 
used is in this case the parallel scavenger and the the counter that 
overflows is _avg_pretenured. _avg_pretenured is incremented/sampled 
every time an allocation is made directly in the old gen, which I 
believe is more likely when the GC locker is active.


The suggested fix is to only perform the division in 
compute_adaptive_average when it is relevant, which currently is for the 
first 100 values. Once there are more than 100 samples there is no 
longer a need to scale the weight.

This problem is tracked in 7158457 (stress: jdk7 u4 core dumps during 
megacart stress test run).

Please review and comment on the webrev below:

http://cr.openjdk.java.net/~mikael/7158457/webrev.00

Thanks,
Mikael