What makes JNI calls slow?

Question

I know that 'crossing boundaries' when making a JNI call in Java is slow.

However I want to know what is it that makes it slow? What does the underlying jvm implementation do when making a JNI call that makes it so slow?

(+1) Nice question. While we're on the subject, I would like to encourage anyone who has done actual benchmarks to post their findings. — NPE
A JNI call needs to convert the Java objects passed in to something C (for example) can understand; same with the return value. Type conversion and call stack marshalling are a good chunk of it. — Dave Newton
Dave, I understand and have heard about that before. But what exactly is the conversion like? what is that 'something'? I am looking for details. — pdeva
Using direct ByteBuffers to pass data between Java and C can result in relatively low overhead. — Peter Lawrey
the call needs a proper C stack frame, pushing all useful CPU registers (and popping them back), the call needs fencing and also it prevents a lot of optimizations like inline. Also the threads has to leave the execution stack lock (for instance to allow biased locks to work while in native code) and then obtain it back. — bestsss

Andy Thomas Andy Thomas · Accepted Answer · 2011-10-18T14:54:37

First, it's worth noting that by "slow," we're talking about something that can take tens of nanoseconds. For trivial native methods, in 2010 I measured calls at an average 40 ns on my Windows desktop, and 11 ns on my Mac desktop. Unless you're making many calls, you're not going to notice.

That said, calling a native method can be slower than making a normal Java method call. Causes include:

Native methods will not be inlined by the JVM. Nor will they be just-in-time compiled for this specific machine -- they're already compiled.
A Java array may be copied for access in native code, and later copied back. The cost can be linear in the size of the array. I measured JNI copying of a 100,000 array to average about 75 microseconds on my Windows desktop, and 82 microseconds on Mac. Fortunately, direct access may be obtained via GetPrimitiveArrayCritical or NewDirectByteBuffer.
If the method is passed an object, or needs to make a callback, then the native method will likely be making its own calls to the JVM. Accessing Java fields, methods and types from the native code requires something similar to reflection. Signatures are specified in strings and queried from the JVM. This is both slow and error-prone.
Java Strings are objects, have length and are encoded. Accessing or creating a string may require an O(n) copy.

Some additional discussion, possibly dated, can be found in "Java(tm) Platform Performance: Strategies and Tactics", 2000, by Steve Wilson and Jeff Kesselman, in section "9.2: Examining JNI costs". It's about a third of the way down this page, provided in the comment by @Philip below.

The 2009 IBM developerWorks paper "Best practices for using the Java Native Interface" provides some suggestions on avoiding performance pitfalls with JNI.

What makes JNI calls slow?

3 Answers