Concurrent counter are part of almost every system, it is used to collect data, thread synchronization etc.
Java has good support of heap based counter.
There could be use case when you need counter that can be shared between processor.
How to build inter process counters
- Database
This is first option that comes to mind, database sequence is counter that can be used by multiple process. All concurrency is handled by database. It is good option for starter but we know types of overhead(Network,Locks etc) you get with database. Only Larry Elision will be happy about it, not you!
- Some Server
You could develop some server/middleware that provides such type of service. This option will still have network latency,marshal/unmarshal overhead.
- Memory Mapped file
You could use memory mapped file to do this. I got idea from looking at thread-safe-interprocess-shared-memory-in-java presentation from PeterLawrey.
Challenges involved in multi process counter.
- Data visibility
Changes done by one process should be visible to all the process. This problem can be solved by using memory mapped file. Operating System gives guarantee about it and java memory model supports it to make is possible.
- Thread safety
Counters is all about multiple writers , so thread safety becomes big problem. Compare-and-swap is one option to handles multiple writers.
Is it possible to use CAS for off heap operation ? yes it is possible to do that , welcome to Unsafe.
By using using Memorymapped & Unsafe it is possible to use CAS for Off heap operation.
In this blog i will share my experiment of Memory mapped using CAS.
How ?
- How to get memory address
MappedByteBuffer is using DirectByteBuffer, which is off heap memory. So it is possible to get virtual address of memory and use unsafe to perform CAS operation. Lets look at the code.
MappedByteBuffer is using DirectByteBuffer, which is off heap memory. So it is possible to get virtual address of memory and use unsafe to perform CAS operation. Lets look at the code.
Above code create memory mapped file of 8 bytes and get the virtual address. This address can be be used to read/write content of memory mapped file.
- How to Write/read in thread safe manner
Important function to look are readVolatile and increment
readVolatile reads directly from memory and increment is using unsafe to perform CAS on the address obtained from MemoryByteBuffer.
-Performance
Some performance numbers from my system. Each thread increments counter 1 Million times.
Performance of counter is decent , as number of threads are increased CAS failures starts to happen and performance starts to degrade.
Performance of these counter can be improved by having multiple segment to reduce write contention.
I will write about it in next blog.
Conclusion
- Memory mapped file is very powerful, it can be used to developed lot of things like off heap collections, IPC, Off heap thread coordination etc.
- Memory mapped file opens gates for GC less programming.
All the code used in this blog is available on github.
