Sunday, 19 May 2013

Lock Less Java Object Pool

It is being while i wrote anything, i has been busy with my new job that involve doing some interesting work in performance tuning. One of the challenge is to reduce object creation during critical part of application.

Garbage Collection hiccups has been main pain point in java for some time, although java has improved over time with GC algorithmic. Azul is market leader developing pause less GC but Azul JVM are not free as speech!

Creating too many temporary/garbage object does't work too well because it create work for GC and it is going to have effect on latency, too much garbage also does't work well with multi core system because it causes cache pollution.

So how should we fix this ?

Garbage less coding
This is only possible if you know how many object you need upfront and pre-allocate them, but in reality that is very difficult to find that , but in-case if you still managed to do that then you have to worry about another issue

  • You might not have enough memory to hold all the object you need
  • You have to handle concurrency
So what is the solution for above problem
There is Object Pool design pattern that can address both of the above issue,it lets you to specify num of object that you need in pool and handles concurrent request to serve object request.

Object Pool has been base of many application that has low latency requirement, flavor of object pool is Flyweight design pattern.

Both of above pattern will help us in avoiding object creation, that is great so now GC work is reduced and in theory our application performance should improve but in practical does't happen that way because Object Pool/Flyweight has to handle concurrency and  whatever advantage you get by avoiding object creation is lost because of concurrency issue.

What are most common way to handle concurrency
Object pool is typical producer/consumer problem and it can be solved by using following techniques 

Synchronized - This was the only way to handle concurrency before JDK 1.5, apache has written wonder full object pool API based on synchronized 

Locks   - Java added excellent support for concurrent programming since JDK 1.5, there has been some work to use Locks to develop Object Pool for eg furious-objectpool

Lock Free - I could not find any implementation that is built using fully lock free technique, but furious-objectpool use mix of ArrayBlocking queue & ConcurrentLinked queue

Lets measure performance
In this test i have created pool of 1 Million object and those object are accessed by different pool implementation, objects are taken from pool and return back to pool.

This test first starts with 1 thread and then number of threads are increased to measure how different pool implementation perform under contention


.
X Axis - No Of Threads
Y Axis - Time in Ms - Lower time is better

This test include pool from Apache, Furious Pool & ArrayBlocking based Pool

Apache one is worst and as number of threads increase performance degrades further and reason for same is Apache pool is based on heavy use of "synchronized" 

Other two Furious & ArrayBlocking based pool performs better but both of them also slows down as contention increase. 

ArrayBlocking queue based pool takes around 1000 ms for 1 Million items when 12 threads are trying to access the pool, Furious pool which internally uses Arrayblocking queue takes around 1975 ms for same thing. 

I have to do some more detail investigation to find out why Furious is taking double time because it is also based on ArrayBlocking queue.

Performance of arrayblocking queue is decent but it is lock based approach, what type of performance we can get if we can implement lock free pool.

Lock free pool.
Implementing lock free pool is not impossible but bit difficult because you have to handle multiple producer & consumer.

I will implement hybrid pool which will use lock on the producer side & non blocking technique on the consumer side.

Lets have look some numbers 

I performed same test with new implementation (FastPool) and it is almost 30% faster than ArayBlocking queue.

30% improvement is not bad, it can definitely help is meeting latency goal.

What makes Fast Pool fast!
I used couple of technique to make it fast
  • Producer are lock based - Multiple producer are managed using locks, this is same as Array Blocking queue, so nothing great about this.

  • Immediate publication of released item - it publishes element before lock is released using cheap memory barrier. This gives some gain

  • Consumer are non blocking - CAS is used to achieve this, consumer are never blocked due to producer. Array Blocking queue blocks consumer because it use same lock for producer & consumer

  • Thread Local to maintain value locality -  Thread Local is used to acquire last value that was used, this reduces contention to great extent. 
If you are interested in having look at code then it is available @ FastObjectPool.java