Why does the JVM require warmup?

Question

I understand that in the Java virtual machine (JVM), warmup is potentially required as Java loads classes using a lazy loading process and as such you want to ensure that th

Answer 1

I always pictured it like the following:

You as (a C++ developer) could imagine an automated iterative approach by the jvm compiling/hotloading/replacing various bits an pieces with (the imaginary analog of) gcc -O0,-O1,-O2,-O3 variants (and sometimes reverting them if it deems it neccessary)

I'm sure this it not strictly what happens but might be an useful analogy for a C++ dev.

On a standard jvm the times it takes for a snippet to be considered for jit is set by -XX:CompileThreshold which is 1500 by default. (Sources and jvm versions vary - but I think thats for jvm8)

Further a book which I have at hand states under Host Performace JIT Chapter (p59) that the following optimizations are done during JIT:

• Inlining
• Lock elimination
• Virtual call elimination
• Non-volatile memory write elimination
• Native code generation

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EDIT:

regarding comments

I think 1500 may be just enough to hint to JIT that it should compile the code into native and stop interpreting. would you agree?

I don't know if its just a hint, but since openjdk is open-source lets look at the various limits and numbers in globals.hpp#l3559@ver-a801bc33b08c (for jdk8u)

(I'm not a jvm dev this might be the completly wrong place to look)

Compiling a code into native does not necessarily mean it is also optimized.

To my understanding - true; especially if you mean -Xcomp (force compile) - this blog even states that it prevents the jvm from doing any profiling - hence optimizing - if you do not run -Xmixed (the default).

So a timer kicks in to sample frequently accessed native code and optimize the same. Do you know how we can control this timer interval?

I really don't know the details, but the gobals.hpp I linked indeed defines some frequency intervals.