I'm testing the performance of my Java code, but I noticed that the first new operation is slower then the others. Why?
Time of first new and other 4
Foo g; // The first new in the first iteration
for (int i = 0; i < 5; i++) {
t1 = System.nanoTime();
g = new Foo(a[i], b[i]);
t2 = System.nanoTime();
v[i] = t2 - t1;
}
Elements of v (time in ns):
3669398
230476
230611
234191
181668
Time of other 5 new after the first
Foo g = new Foo(a[0], b[0]);
for (int i = 1; i < 6; i++) {
t1 = System.nanoTime();
g = new Foo(a[i], b[i]);
t2 = System.nanoTime();
v[i - 1] = t2 - t1;
}
Elements of v (time in ns):
254028
222352
222488
228581
219776
This can be due to multiple issues.
The simplest one is that perhaps the class is not loaded yet when you execute your first new, so the class loader needs to load the class first. That depends on the actual code and environment.
A more complicated but just as feasible reason is due to the way that Java optimizes. Java is not done optimizing after compilation. At runtime, Java continues to optimize all the time.
For example, the first few times through an if statement might be slower, but if the runtime optimizer realizes that you keep branching the same way every time it might optimize that and suddenly it starts running faster for the rest of the life of that runtime.
How much this affects you depends on what Foo does. However, I would be surprised if this accounted for the drastic difference you are reporting, which is about 15 times higher. I am not sure what the actual differences would be though, as I have not measured them, so maybe I am underestimating this possibility.
Also, doing an action a few times is not a good benchmark. If your test case is really as simple as your example, there could be other issues too, and they could be wide ranging. Perhaps the runtime is still under a heavy load from having just started up, so the thread keeps yielding, including during the first iteration. I could run wild with speculations, but that is part of why we do a lot of tests if we really need to know why one thing runs different than another.
Collected from the Internet
Please contact [email protected] to delete if infringement.
Comments