为什么最大递归深度我可以达到非确定性？

Question

I decided to try a few experiments to see what I could discover about the size of stack frames, and how far through the stack the currently executing code was. There are two interesting questions we might investigate here:

1. How many levels deep into the stack is the current code?
2. How many levels of recursion can the current method reach before it hits a StackOverflowError ?

Stack depth of currently executing code

Here's the best I could come up with for this:

public static int levelsDeep() {
    try {
        throw new SomeKindOfException();
    } catch (SomeKindOfException e) {
        return e.getStackTrace().length;
    }
}

This seems a bit hacky. It generates and catches an exception, and then looks to see what the length of the stack trace is.

Unfortunately it also seems to have a fatal limitation, which is that the maximum length of the stack trace returned is 1024. Anything beyond that gets axed, so the maximum that this method can return is 1024.

Question:

Is there a better way of doing this that isn't so hacky and doesn't have this limitation?

For what it's worth, my guess is that there isn't: Throwable.getStackTraceDepth() is a native call, which suggests (but doesn't prove) that it can't be done in pure Java.

Determining how much more recursion depth we have left

The number of levels we can reach will be determined by (a) size of stack frame, and (b) amount of stack left. Let's not worry about size of stack frame, and just see how many levels we can reach before we hit a StackOverflowError .

Here's my code for doing this:

public static int stackLeft() {
    try {
        return 1+stackLeft();
    } catch (StackOverflowError e) {
        return 0;
    }
}

It does its job admirably, even if it's linear in the amount of stack remaining. But here is the very, very weird part. On 64-bit Java 7 (OpenJDK 1.7.0_65), the results are perfectly consistent: 9,923, on my machine (Ubuntu 14.04 64-bit). But Oracle's Java 8 (1.8.0_25) gives me non-deterministic results : I get a recorded depth of anywhere between about 18,500 and 20,700.

Now why on earth would it be non-deterministic? There's supposed to be a fixed stack size, isn't there? And all of the code looks deterministic to me.

I wondered whether it was something weird with the error trapping, so I tried this instead:

public static long badSum(int n) {
    if (n==0)
        return 0;
    else
        return 1+badSum(n-1);
}

Clearly this will either return the input it was given, or overflow.

Again, the results I get are non-deterministic on Java 8. If I call badSum(14500) , it will give me a StackOverflowError about half the time, and return 14500 the other half. but on Java 7 OpenJDK, it's consistent: badSum(9160) completes fine, and badSum(9161) overflows.

Question:

Why is the maximum recursion depth non-deterministic on Oracle's Java 8? And why is it deterministic on OpenJDK 7?

Answer 1

The observed behavior is affected by the HotSpot optimizer, however it is not the only cause. When I run the following code

public static void main(String[] argv) {
    System.out.println(System.getProperty("java.version"));
    System.out.println(countDepth());
    System.out.println(countDepth());
    System.out.println(countDepth());
    System.out.println(countDepth());
    System.out.println(countDepth());
    System.out.println(countDepth());
    System.out.println(countDepth());
}
static int countDepth() {
    try { return 1+countDepth(); }
    catch(StackOverflowError err) { return 0; }
}

with JIT enabled, I get results like:

> f:\Software\jdk1.8.0_40beta02\bin\java -Xss68k -server -cp build\classes X
1.8.0_40-ea
2097
4195
4195
4195
12587
12587
12587

> f:\Software\jdk1.8.0_40beta02\bin\java -Xss68k -server -cp build\classes X
1.8.0_40-ea
2095
4193
4193
4193
12579
12579
12579

> f:\Software\jdk1.8.0_40beta02\bin\java -Xss68k -server -cp build\classes X
1.8.0_40-ea
2087
4177
4177
12529
12529
12529
12529

Here, the effect of the JIT is clearly visible, obviously the optimized code needs less stack space, and it's shown that tiered compilation is enabled (indeed, using -XX:-TieredCompilation shows a single jump if the program runs long enough).

In contrast, with disabled JIT I get the following results:

> f:\Software\jdk1.8.0_40beta02\bin\java -Xss68k -server -Xint -cp build\classes X
1.8.0_40-ea
2104
2104
2104
2104
2104
2104
2104

> f:\Software\jdk1.8.0_40beta02\bin\java -Xss68k -server -Xint -cp build\classes X
1.8.0_40-ea
2076
2076
2076
2076
2076
2076
2076

> f:\Software\jdk1.8.0_40beta02\bin\java -Xss68k -server -Xint -cp build\classes X
1.8.0_40-ea
2105
2105
2105
2105
2105
2105
2105

The values still vary, but not within the single runtime thread and with a lesser magnitude.

So, there is a (rather small) difference that becomes much larger if the optimizer can reduce the stack space required per method invocation, eg due to inlining.

What can cause such a difference? I don't know how this JVM does it but one scenario could be that the way a stack limit is enforced requires a certain alignment of the stack end address (eg matching memory page sizes) while the memory allocation returns memory with a start address that has a weaker alignment guaranty. Combine such a scenario with ASLR and there might be always a difference, within the size of the alignment requirement.

Answer 2

Why is the maximum recursion depth non-deterministic on Oracle's Java 8? And why is it deterministic on OpenJDK 7?

About that, possibly relates to changes in garbage collection. Java can choose a different mode for gc each time. http://vaskoz.wordpress.com/2013/08/23/java-8-garbage-collectors/

Answer 3

It's deprecated, but you could try Thread.countStackFrames() like

public static int levelsDeep() {
    return Thread.currentThread().countStackFrames();       
}

Per the Javadoc,

Deprecated . The definition of this call depends on suspend() , which is deprecated. Further, the results of this call were never well-defined.

Counts the number of stack frames in this thread. The thread must be suspended.

As for why you observe non-deterministic behaviour, I can only assume it is some combination of the JIT and garbage collector.

Answer 4

You don't need to catch the exception, just create one like this:

new Throwable().getStackTrace()

Or:

Thread.currentThread().getStackTrace()

It's still hacky as the result is JVM implementation specific. And JVM may decide to trim the result for better performance.