[LLVMdev] Performance of JNI in VMKit

[Nicolas Geoffray]

Hi Bruno,

On Wed, Jun 20, 2012 at 3:04 PM, Bruno Daniel <abml at mailoo.org> wrote:

> Dear developers,
>
> Are there any benchmarks for the performance of Java Native Interface (JNI)
> calls in VMKit? Since VMKit is based on LLVM which can also run C++ code
> (maybe in the same just-in-time compiler?) I guess calls from Java to C++
> and
> back could be much faster than in Sun's JVM which has extremely slow C++ ->
> Java callbacks.
>
> If this was the case, this would be a big advantage of VMKit over all other
> JVMs and mixing existing C/C++ libraries with Java would be as easy as in
> the
> glue language Python.
>

While mixing C/C++/Java with the same compiler seems appealing for
performance, Java has a garbage collector, and a garbage collector does not
cope well with native frames. It requires object indirection when leaving
Java boundaries, as well as being able to interrupt the execution of a
thread for initiating a collection.

Note that it is possible to do it: VMKit is written in C/C++ and objects
are being passed directly, and a C++ frame, since it is compiled by clang
can be interrupted. However, it is very subtle how it works, and a slight
omission of a GC object declaration leads to crashes that are hard to
detect. It's always possible to do better, like defining a convention that
will tell clang which objects are being used by the GC, but it hasn't been
worked on yet.

If you don't care about a GC, then sure, you can compiler C/C++/Java into a
single native executable, where performance of inter-language calls are the
same as intra-language calls.

Also, note that there were some works on trying to optimize JNI calls
through inlining at runtime (http://dl.acm.org/citation.cfm?id=1064997). I
don't know if the optimization is actually being used in existing JVMs.

Hope this helps!
Nicolas


>
> All the best
>   Bruno Daniel
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