Sunday, 26 June 2016

JIT Fun Part 3: -XX:PrintCompilation

Ok enough guessing about these graphs. Let's look at what's really happening.

But first...

Compiler levels

Level Compiler
0 Interpreter
1 C1 and destined to stay in C1 forever
2 C1 but only pays attention to loop/method counters
3 C1 but gathers details for C2 eg counters, percentage of time conditional evaluates to true
4 C2

Paths through these levels:

0 -> 3 -> 4

  • This is the most common case. Method initially sent to C1 level 3 after a fair few calls, then if it is called a lot or contains a loop with lots of iterations it gets promoted to the super fast level 4 (= C2).

0 -> 3 -> 1

  • This happens if the method is really tiny. It gets sent to level 3 where it is analysed and we realise that it will never go to C2 so we stick it into C1 level 1 forever.

0 -> 2 -> 3 -> 4

  • If C2 is busy at the time of promotion from level 0 then we know we won't get promoted for a while so we go hang out in level 2 rather than hopping off to 3 directly. Then we may get promoted to 4 if we deserve it. 


I've got the following code:

And the times (endTime - startTime) look like this:

I ran the code with -XX:PrintCompilation and I'm using Java 8 so tiered complation is enabled by default.

Let's see what is happening to the rawr method:

    164  171 %     3       com.ojha.Rawr::main91 @ 50 (131 bytes)

What does this mean?

i (ith iteration of my for loop which is on the x axis) = 90
timestamp since vm start = 164
this method was 171st in the queue to be compiled
% indicates on stack replacement (method contains a loop)
3: This is the most important bit! This means we are moving into C1, ie doing the first compilation.
50 is the bytecode index of the loop.

More output

    167  176       3       com.ojha.Rawr::main (131 bytes)

    169  172       3 (18 bytes)
    169  182 %     4       com.ojha.Rawr::main @ 50 (131 bytes)

At i = 231, the loop has been called enough times for the Rawr main method to be compiled at C2 (that's what the 4 means). Note that at the same time has been called enough times to be compiled at C1 level 3.

    3       com.ojha.Rawr::main @ -2 (131 bytes)   made not entrant

This is basically saying that the level 3 version of the rawr method shouldn't be used any more (correct because we should now use the level 4 version). We can see this dip in the graph at x = 1818.

    193  234   !   3 (24 bytes)
    193  182 %     4       com.ojha.Rawr::main @ -2 (131 bytes)   made not entrant

At the very end of the method Rawr main is over so the level 4 version of the compiled code is made not entrant (ie nothing should 'enter' or call that compiled code).

Note that the ! indicated that the method contains a try catch loop. 

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