Sunday 22 September 2013

Linked List Revisited for In Memory computing

Linked list is one of the first data structure that you learn when you start reading about data structure.
Linked List has its own advantage over other data structure like array, just to recap few of them

Some benefit of Linked List
 - Insert cost is low, just update address of new node
 - Memory Allocation cost is very low because it has to just allocate one node and attach it to last node
 - Very good data structure when you don't know how much data you will be storing, it grow on need basis, no wastage of space
 - Very low overhead when element is removed.

Things that are not good about Linked List
 - Access by Index is bad and not recommended .

  - Creates lot of work for Garbage Collector, can result in frequent Minor GC due which young objects will be promoted early to old generation

  - Extra Node Object is required for each element , so total memory required is (Node+Value) * No Of Objects. This can become problem if there is lot of objects.

 - Random memory access because of the way nodes are linked, this is major draw back because random memory walk is slow as compared to linear walk, CPU does lot of hard work to optimized memory access but Linked List or Tree based Data Structure can't use all those optimization due to random memory walk.
On NUMA based machine it become worse.

Do we need Linked List now ?
That is really tough question to answer, but the way Linked List performs it looks like it has no space in Data Structure Library.

With the current In-Memory trend, where application is keeping all the data in memory and data structure like Linked List are not friendly for that.

Lets Tweak Linked List for today requirement.
So today we need Linked List that has advantage of both Linked List & Array, that way we can get best of both world.

For testing i will create MultiValueLinkedList that has feature of both array & linked list. So it is like each Node contains small Array List and all these nodes are linked.
So you can say Array List linked via Linked List.

For performance test i add some millions of items in Linked List/MultiValueLinkedList/Array List and iterate over it. I start with 10 Million element and increase it by 5 Million.

Add Performance 
Lets look at some numbers for add operation in different types of list

X Axis - Time taken for add in Ms
Y Axis - No of Element added in Millions

MultiValueLinkedList is clear winner in this test, it is around 22X times faster than Linked List.

Linked List performance is so bad that graph scale is screwed up and it is difficult to make out difference between MultiValueLinkedList & Array List, so i will put another graph for MultiValueLinkedList & Array List

ArrayList Vs MultiValueLinkedList

This graph gives better understanding of performance of Array List & MultiValueLinkedList, new linked list is 2.7X times faster than Array List.

 Iterate Performance
Look at iterate performance before i try to explain reason of performance boost

X Axis - Time taken for iterate/access in Ms
Y Axis - No of Element in Millions

As i said earlier that new Linked List will be hybrid one , it will take best from Linked List & Array List and this graph confirms that , it is somewhere in between Linked List & Array List.
MultiValueLinkedList is around 1X times faster than linked list

Garbage Collections
This test is all about allocation and will not be completed without GC activity graph, i want to put more GC observation but that will make this blog big, so i will put just one Visual GC graph

Visual GC Graph for Linked List

Visual GC Graph for MultiValueLinkedList

I don't have to explain much in these GC behavior, for Linked List minor GC activity is so much that object are promoted to old generation, all though it is not required.
GC behaves as expected for MultiValueLinkedList.

 - Performance of add is best for MultiValueLinkedList & it is due to less number of allocation & almost no need to copy the Array as it is required for Array List whenever it grows

 - Garbage produced by MultiValueLinkedList is very less. Memory usage of MultiValueLinkedList is same as ArrayList & for LinkedList it is double, that explains why we see object promotion to old generation.

- Iteration of MultiValueArrayList is slow as compared to Array List because memory access pattern is like Page by Page, it is possible to improve it further but adjusting the size of element in nodes.

Code is available @ github

Saturday 14 September 2013

Concurrent Counter With No False Sharing

This blog is continuation of Scalable Counter post.

One of the reader shared result from his system. He ran test on XEON intel processor with 16 core and total time taken for each type of counter is almost same, although Atomic counter has CAS failure & other type does't has any CAS failure but it does't make any difference in execution time.
Very strange result , needs further investigation.

Another reader pointed out that it could be due to false sharing, so it worth to take that into account and i created another class that handles FALSE SHARING

Time take by different counter
 Y Axis - Time taken to increment 1 Million times

X Axis - Number of threads

PaddedAtomicCounter is new type of counter that i have added to test & it it outperforms all other counters.
It is using cache line padding to avoid false sharing.
Cacheline is of 64 byte on most of the today processor. PaddedCounter it Integer based counter so it is adding 16 slot per counter, by using this techniques we avoid cache pollution & as result we see 16X times gain as compared to AtomicCounter, without padding the gain was 5X and with cache line padding gain jumps to 16X.
With cacheline padding you need some extra space, so it is trade off of memory for speed, you can choose what you want!

CAS failure rate
Lets look at the CAS failure for different counter & what it means for performance.

Y Axis - CAS Failure in 100Ks

X Axis - Number of threads

PaddedAtomic has some CAS failure as compared to other counters , but it does't make any difference in execution time of the counter.
CAS failure is not the only factory that can determined execution time, false sharing make significant contribution to it, so this gives good explanation of behavior seen in XEON processor

To get better performance you have to take care of few things

 -  Contention  - There are many techniques to avoid it , this blogs shows one of them

 -    False Sharing - You have to avoid false sharing to get best out of processor, padding is required for that. Some of the JDK classes are using padding are ThreadLocalRandom , now we have @Contended annotation from java to achive same thing, it is being used in ForkAndJoinPool

Code is available @ github


Tuesday 10 September 2013

Scalable Counters For Multi Core

Counters are required everywhere , for e.g. to find key KPI of application, load on application, total number of request served, some KPI for finding throughput of application & many more.

With all these requirement complexity of concurrency is also added & that makes this problem interesting.

How to implement concurrent counter

 - Synchronized - This was the only option before JDK 1.5, since now we are waiting for JDK8 release , so definitely this is not an option.

- Lock based  - You should never attempt this for counter , it will perform very badly

- Wait Free - Java does't have support for Fetch-and-add, so bit difficult to implement it.

- Lock free - With very good support of Compare-and-swap, this looks good option to use.

How does Compare-and-Swap based counter performs

I used AtomicInteger for this test and this counter is incremented for 1 Million time by each thread & to increase the contention number of threads are increased gradually.

Test Machine Details
OS : Windows 8
CPU : Intel i7-3632QM , 8 Core
RAM : 8 GB

Y Axis - Time taken to increment 1 Million times

X Axis - Number of threads

As number of threads are increased, time taken to increment counter increases and it is due to contention.
For CAS based counter , it is CAS failure that causes slowdown.

Is this best performance that we can get ? no definitely their are better solution to implement concurrent counter, lets have look at them.

Alternate Concurrent Counter
Lets look at some of the solution to implement counter that handles contention in better way

- Core Based Counter - Maintain counter for each logical core, so that way you will have less contention. Only issue you have this type of counter is that if number of threads are more than logical core then you will start noticing contention.

- Thread Based Counter - Maintain counters for total number of threads that will be using system. This works well when number of threads are more than number of logical cores.

Lets test it

Time taken by different types of counter

Y Axis - Time taken to increment 1 Million times

X Axis - Number of threads

Concurrent Counter performs much better than Atomic based counter, for 16 threads it is around 5X times better, that is huge difference!

CAS Failure Rate

Y Axis - CAS Failure in 100Ks

X Axis - Number of threads

Due to contention, Atomic based counter see lot of failure and it goes up exponential as i add more threads & other counters performs pretty well.

  Multi core machines becoming easily available & we have to change the way we handle concurrency, traditional way of doing concurrency is not going to scale in today's time when having 24 or 48 core server is very common.

 - To reduce the contention you have to use multiple counters and then aggregate them later

 - Core based counter works well if number of threads will be less or same as number of cores

 - Thread based counter is good when number of threads are much more than available core

 - Key to reduce contention is identify counter to which thread will write,i have used simple approach based on thread id, but much better approach are available, look at ThreadLocalRandom of JDK 8 for some ideas.

 - Thread based approach is used in LongAdder of JDK8, which creates many slots to reduce contention.

Code for all the counters used in this test are available @ Github