Sunday, 7 July 2013

How To Write Micro benchmark In Java

So many article has been written on how to write micro-bench mark in java, this blog is my attempt to explain the topic.

What is Micro benchmark
Micro benchmark try to measure actual speed of small piece of code, measuring performance is very complex because JIT does magic to make code fast and you have to measure the code after all the JIT optimization is done.

What factors should be considered for benchmark 
Some of the factors that you should consider for correct bench-marking are

  • Warm-up phase - This is to start measuring only after JIT has compiled hot code, JIT will compile functions only after 10K invocation.
  • Client vs Server - Running JVM in client or server mode will give different performance result
  • GC  - Garbage Collector makes measurement really difficult 
  • Re-compilation effect - Code executing first time will be slow
More details are available @ MicroBenchmarks Rules

How to write micro benchmark
This can be very difficult to write but not to worry, now there are some framework available to do that
 - Caliper  - It is Google solution for writing benchmark
 - JMH   - This one is from java guys, in this blog i will explain some of example using JMH

How to use JMH
JMH is maven based project but don't know the reason why it is not hosted on maven central.
You need TortoiseHg to download code, once you have the code then it is cake walk to setup/build the JMH project.
Refer to Setup page for detail instruction on setup.

Lets look at some examples
As every programming language starts with hello world, so does JHM .
JMH has used annotation based approach to specify which function you want to run as part of benchmark,  GenerateMicroBenchmark annotation is used for that.

-Empty function 
JVM is complex piece of software and it has some overhead, having empty function checks that overhead
Numbers that come outs with this test are 

Run result "wellHelloThere": 2893313.782 ▒(95%) 118441.448 ▒(99%) 161902.046 ops
/msec
Run statistics "wellHelloThere": min = 2388762.991, avg = 2893313.782, max = 310
8116.404, stdev = 253075.135
Run confidence intervals "wellHelloThere": 95% [2774872.334, 3011755.229], 99% [
2731411.736, 3055215.827]

This is interesting number, if you have some empty function then it can be called 2.8 million times per mili second, that is very fast.

JHM does all the magic and shows number that we are interested in. JMH is based on code generation, it generate lots of code to measure performance, all the generated code is available under "generated-sources"

Have look at generated file

That is lot of code!

-Benchmark modes
JHM has benchmark modes, very useful, it has most of the mode that you can think of for eg
 - Throughput
- AverageTime
 - SampleTime - Time distribution, percentile estimation
- SingleShotTime

I must say very well thought of by java team.

-States
This is very common use case , this test measure overhead of shared object vs per thread object. Result are interesting.
Run result "measureUnshared": 1264160.208 (95%) 20349.174 (99%)
Run result "measureShared": 965178.728 (95%) 20847.118 (99%)

Unshared test is around 30% fast as compared to shared & it is due to contention on memory .

-DeadCode
This benchmark measure effect of dead code, very common case.
Lets look at sample code
Results are interesting in this case, JVM is very smart enough to remove dead code from call and time take for function as good as there is no call to log function.

-Constant Folding
This is another interesting optimization that is done by compiler,Constant folding techniques will optimize call to functions that takes constant parameter,sometime this can result in tricky defects.
Way to avoid trap is read inputs via some state. Lets have look at sample code

measureWrong function is faster because JVM performs optimization based on constant folding rules, but be very cautious this an cause some defect in you code!

-Multiple Implementation
This is very interesting test, this test has multiple implementation of counter interface, code is same but just 2 implementation and result are random, you can't conclude why one implementation fast as compared to other.

JVM will mix profile of two test and due to which result are random. JMH has option to measure this type of test properly, there is annotation Fork that will execute benchmark in new JVM.

- Compile Control
JMH has support to measure effect of inline, no inline, exclude compile. These things are really useful to find performance number under different scenario. Sample benhmark for compile control

- Asymmetric
JMH has very good support for asymmetric execution for eg producer/consumer, in traditional test we have to write thread related code to make this happen, but JMH has declarative way of specifying such behavior, multiple benchmark methods can be grouped by using Group annotation.
Sample code for grouping.

Conclusion
JHM is very useful tool for micro bench marking, it is worth spending time to move some of the performance test of application to JHM.
Writing benchmark will give very good understanding of how JVM works.

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