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. 

Example


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:

90
    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

159
    167  176       3       com.ojha.Rawr::main (131 bytes)

231
    169  172       3       java.io.PrintStream::ensureOpen (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 java.io.PrintStream.ensureOpen has been called enough times to be compiled at C1 level 3.

1818
    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.

1999
    193  234   !   3       java.io.PrintStream::println (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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