javaone 2013: effective foreign function interfaces: from jni to jnr

© 2013 IBM Corporation

Ryan A. Sciampacone – Managed Run4me Architect 25 September 2013

Effec4ve Foreign Func4on Interfaces: From JNI to JNR


Important Disclaimers THE INFORMATION CONTAINED IN THIS PRESENTATION IS PROVIDED FOR INFORMATIONAL PURPOSES ONLY.

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-‐ CREATING ANY WARRANT OR REPRESENTATION FROM IBM, ITS AFFILIATED COMPANIES OR ITS OR THEIR SUPPLIERS AND/OR LICENSORS

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Your Speaker: Ryan A. Sciampacone

• Run4me Architect @ IBM (JTC) •  Interpreters • Garbage Collec4on • Plaborm interfacing / op4miza4on • Hardware exploita4on

(E) [email protected] (T) @rsciampacone

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What should you get from this?

• Why a foreign func4on interface (FFI) is important

• Performance 4ps / common piballs when using JNI

• Next steps in the evolu4on of the FFI for Java

• At the very least, 45 minutes or so of entertainment

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WHY IS AN FFI IMPORTANT? What the Java Na4ve Interface (JNI) does for you…

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Why have a FFI?

• Plaborm interfacing

• Legacy System Support

• Cross Language Interop • Speed

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Mo4va4ng Example: Missed some API

• New things come up / Some things missed

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JNI Call Example

Java:

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C:


Benefits of the JNI implementa4on

• API based • GC Safety

• Java / JVM based ac4vity not blocked

• Plaborm agnos4c

• Clear lines between Java / plaborm

• Founda4on for JVM Tooling (JVMTI)

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JNI API Building Blocks

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JNIEnv is your lifeline

• JNIEnv is the connec4on back to the “Java world” • Calls through the JNIEnv are effec4vely round trip

• Performance is dependent on keeping round trips down

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PERFORMANCE PITFALLS But it’s already slow, what’s a few more instruc4ons?

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Caching Classes, Method IDs and Field IDs

• Core elements when interac4ng with objects

• Effec4vely your na4ve “resolve” point

• Not as cheap to acquire • So cache them!

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Basic ID use

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24x!

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NoCache Cache

Field ID Caching


Triggering Array Copies

• Remember: No direct manipula4on of Java Objects

• Basically 3 categories of API: •  Some4mes copying the en4re array (Get*ArrayElements) • Always copy a por4on of the array (Get*ArrayRegion) •  (Usually) Direct on-‐heap access (GetPrimi4veArrayCri4cal)

• Be aware of the cost of copying!

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Elements Region

GetElements vs. GetRegion

8.5x!


Reaching Back for Parameters

• We all hate huge func4on prototypes

• Basic teaching is pass the object go from there

• Think – what is the cost of accessing fields in JNI?

• Pass what you need as arguments

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Fetch Parameter

Fetching vs. Parameters

6x!


Local Reference Abuse

• Local references are beau4ful for hiding JVM-‐isms

• And they get blissfully ignored by a lot of na4ve code

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CORRECTNESS PITFALLS Hey it ran didn’t it?

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Using Array API Correctly

• Recap: Basically 3 categories of API •  Some4mes copying the en4re array (Get*ArrayElements) • Always copy a por4on of the array (Get*ArrayRegion) •  (Usually) Direct on-‐heap access (GetPrimi4veArrayCri4cal)

• You need to be certain that you are geqng a copy!

• Affected by VM vendor, GC technology, and version of JDK

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Respec4ng “Cri4cal” Sec4ons

• Cri4cal API: Get/Release for (almost) assured direct access

•  Idea: Direct on-‐heap access which locks out certain JVM events

• General advice: Do not use JNI API between Get and Release

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Respec4ng “Cri4cal” Sec4ons

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Check your excep4ons!

• Java code and IDEs really help you with excep4ons • Na4ve code doesn’t give you this feedback

• JNI API can raise excep4ons -‐ Check and act accordingly • You are the try/catch block now!

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Check your return values!

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Maintain your Global References

• Global references aren’t cleaned up like Local references • Great for caching, but watch out for leaks

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Use the correct JNIEnv

• JNIEnv are unique to your thread • Using the wrong one can cause subtle problems

• GetEnv / GetJavaVM exist from the invoca4on API

• For callbacks, make sure you have the correct one available

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EVOLUTION If it is broken you should probably fix it

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JNI is good, but some4mes its in the way

• Direct func4on calls • Speed

• Marshalling data is expensive

• Unsigned types? • Op4mizing the JNI interface can be tricky

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Packed Objects

• A solu4on to accessing na4ve data •  Session: An Introduc4on to Packed Objects (CON5758)

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int int

Java Na4ve

…

int[] d

anObject

int int

int …

Object header

Object field / data


References

•  Introduc4on to Packed Objects (CON5758) • JVMLS 2013: Packed Objects (Sciampacone) hup://goo.gl/jWv2Jx

• John Duïmovich (IBM Java CTO) hup://goo.gl/OGeDix

• JNI Best Prac4ces (Dawson,Johnson,Low) hup://goo.gl/hvyGDY

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Copyright and Trademarks

© IBM Corpora4on 2013. All Rights Reserved. IBM, the IBM logo, and ibm.com are trademarks or registered trademarks of Interna4onal Business Machines Corp., and registered in many jurisdic4ons worldwide.

Other product and service names might be trademarks of IBM or other companies. A current list of IBM trademarks is available on the Web – see the IBM “Copyright and trademark informa4on” page at URL: www.ibm.com/legal/copytrade.shtml

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javaone 2013: effective foreign function interfaces: from jni to jnr

Technology

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elements region getelements

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