Comments on Rational Java: An Unexpected Allocation - JIT Compilation Jitter

Jean-Philippe, the interesting thing about that pr...

2015-09-14T06:38:25.509+01:00

Jean-Philippe, the interesting thing about that property is that it is not exposed in the ThreadMXBean, but you can see it as a property in JConsole (which is where I found it whilst dicking around during a live presentation in one of my courses). So I'm not even sure if our code is going to be fully compatible and work in other JVMs. Anybody care to try it out in IBM?

For the calibration code, I did know that tiered compilation makes things more tricky, but also didn't want to force users to have to turn it off, just to use the code. This is why I increased the repeats to hopefully catch all the JITter up front :-)

Thanks - interesting point about the tiered compil...

2015-09-08T19:10:32.932+01:00

Thanks - interesting point about the tiered compilation.
Goes to prove that compilation jitter does cause allocation :)

Hi, Did you try without TieredCompilation (enable...

2015-09-08T13:26:32.773+01:00

Hi,

Did you try without TieredCompilation (enabled by default on JDK8) ?
I think it will be more predictable (10K) without it.

why not using
(
(com.sun.management.ThreadMXBean) ManagementFactory.getThreadMXBean()).getThreadAllocatedBytes(Thread.currentThread().getId());

less overhead (48 bytes)
But may not be portable because using com.sun.management package

int VALUE_COUNT = 1000*1000;
long[] values = new long[VALUE_COUNT];
for (int i = 0; i < VALUE_COUNT; i++)
{
values[i] = ((ThreadMXBean) ManagementFactory.getThreadMXBean()).getThreadAllocatedBytes(Thread.currentThread().getId());
}
long prevDiff = -1;
for (int i = 1; i < VALUE_COUNT; i++)
{
long diff = values[i] - values[i - 1];
if (prevDiff != diff)
System.out.println("Allocation changed at iteration " + i + "->" + diff);
prevDiff = diff;
}

With TieredCompilation:
Allocation changed at iteration 1->48
Allocation changed at iteration 5631->560
Allocation changed at iteration 5632->48
Allocation changed at iteration 6655->1304
Allocation changed at iteration 6656->48
Allocation changed at iteration 95122->376
Allocation changed at iteration 95123->48

No Tiered compilation:
Allocation changed at iteration 1->48
Allocation changed at iteration 4999->2480
Allocation changed at iteration 5000->424
Allocation changed at iteration 5001->48
Allocation changed at iteration 9912->816
Allocation changed at iteration 9913->48
Allocation changed at iteration 9999->560
Allocation changed at iteration 10000->48
Allocation changed at iteration 14563->200
Allocation changed at iteration 14564->48