RFR (M): 8235741: Inappropriate uses of os::javaTimeMillis()

[David Holmes]

Hi Robbin,

Thanks for looking at this.

I'm going to wait to get more eyes on this as it does cover various 
parts of the VM.

David

On 13/01/2020 11:08 pm, Robbin Ehn wrote:
> Hi David, looks good, thanks!
> 
> /Robbin
> 
> On 1/13/20 8:13 AM, David Holmes wrote:
>> webrev: http://cr.openjdk.java.net/~dholmes/8235741/webrev/
>> bug: https://bugs.openjdk.java.net/browse/JDK-8235741
>>
>> Full details in the bug report about the existing uses of 
>> javaTimeMillis(), many of which just want an elapsed time in ms and so 
>> should be using javaTimeNanos() and convert to ms. This covers areas 
>> all across the VM.
>>
>> Only non-simple change is in os_perf_linux.cpp (and the same code will 
>> be in os_perf_aix.cpp once it has been validated). There we are 
>> tracking an elapsed time in ms but relative to the boot time, which is 
>> seconds since the epoch. Consequently the first interval has to be 
>> calculated using javaTimeMillis, but after that we can use 
>> javaTimeNanos (using a new 'first time' captured at the same time we 
>> used javaTimeMillis). I think I have the logic right but other than 
>> through JFR this code seems unused and I have limited means of testing 
>> it. The JFR test jdk/jfr/event/os/TestThreadContextSwitches.java 
>> exercises the code but the results of running that test seems to 
>> exhibit arbitrary randomness in the rates reported - e.g. 0 to 16000Hz 
>> - both with and without my change, so not really that useful. Stefan 
>> K. suggested a gtest which I may look into - though it is frustrating 
>> to have to expend such effort to validate this.
>>
>> Other testing tiers 1-3.
>>
>> Thanks,
>> David