Question to JMH and JIT

[David Holmes]

Folks,

This is not a suitable topic for the discuss list, please take technical 
discussions elsewhere.

Thanks,
David

On 5/02/2019 5:37 am, Ngor wrote:
> In addition I highly recommend reading the paper on the topic:
> 
> Statistically Rigorous Java Performance Evaluation
> https://dri.es/files/oopsla07-georges.pdf
> 
> On Mon, Feb 4, 2019 at 7:44 AM Herr Knack <koehlerkokser at gmail.com> wrote:
> 
>> Hey there,
>>
>> I currently try to find a mathematical model to compute the approximate
>> runtime of a function term out of its basic structure for my machine
>> (Ubuntu 16.04, i7-5500U, 2.40GHz×4). For first microbenchmark measurements
>> I used the JMH and measured the throughput of some simple functions (4
>> different forks, 3 warmup and 5 measurement iterations each fork, 10 sec
>> time each iteration).
>>
>> I found out, that it is really difficult to see the connection between the
>> structure of the function term and its runtime in the microbenchmark. I
>> assume that the JIT compiler does some weird stuff to optimize the
>> functions, but I can‘t see which optimizations in detail are done there.
>>
>> Could JIT be the problem? Does anyone know if the workflow of JIT
>> optimizations is documented anywhere?
>>
>> I hope someone can help, because I‘m getting crazy about it. Here some
>> results I got. I converted the throughput (runs/sec) to a average time per
>> run in ns. The standard deviation is smaller as you might think, so the
>> average time is most likely correct in an area of +/-0.3 ns :
>>
>> - (((0.5 * x) * 0.5) * (((((0.5 * x) * 0.5) / 0.5) + (((0.5 * x) * 0.5) *
>> 0.5)) * (((0.5 * x) * 0.5) + x))) + x, 15 operations (3+, 11*, 1/), 28.09
>> ns
>>
>> - x + ((((0.5 * x) * (0.5 * x)) * (0.5 * x)) + ((0.5 * x) * ((((0.5 * x) *
>> (0.5 * x)) * (0.5 * x)) * (((0.5 * x) * (0.5 * x)) / ((((0.5 * x) * ((0.5 *
>> x) * (0.5 * x))) / (0.5 + ((0.5 * x) * (0.5 * x)))) + (((0.5 * x) * (0.5 *
>> x)) * (0.5 * x))))))), 35 operations (4+,29*,2/), 38.61 ns
>>
>> - (((x / (x / (0.5 * x))) * (x / (x / (0.5 * x)))) * (0.5 * x)) + (x +
>> ((((x / (x / (0.5 * x))) * (x / (x / (0.5 * x)))) * (0.5 * x)) / (((x /
>> (0.5 * x)) + 0.5) - ((((x / (x / (0.5 * x))) * (x / (x / (0.5 * x)))) *
>> (0.5 * x)) * (x / (x / (0.5 * x))))))), 38 operations (4+-,18*,16/), 40.70
>> ns
>>
>> I assume that there is an constant offset for method calls and so on. In
>> addition I suppose that already calculated term pieces don‘t have to be
>> calculated again. But these conclusions did not help me either.
>>
>> Regards,
>>
>> KKokser
>>