Wednesday, July 7, 2010

A Brief Overview of Concurrency.

Introduction

Over the last few weeks I have had several conversations with people about concurrency, more specifically the ways in which shared information is handled in concurrent languages. I have gotten the impression that there isn't really a good understanding of whats out there in the world of concurrency. That being the case it would be a good idea to just give a quick overview of some of the mechanisms that are gaining mind share in the world of concurrency.

Aside from some engineers that are currently so deep in their rut that they can't see sunlight its been accepted that the current mainstream approach to concurrency just wont work. The idea of using mutexes and locks to try to take advantage of the up and coming massively multi-core chips is really just laughable. We can't ignore this topic. As software engineers we don't really have a choice about supporting large numbers of CPUs, thats the direction that hardware is going its up to us to figure out how to make it work in software. Fortunately a bunch of really smart folks have been thinking about this problem for a really long time. A few of the things they have been working on are slowly making their way into the gestalt of the software world.

We are going to talk about three things. They are Software Transactional Memory (STM), Dataflow Programing specifically Futures and Promises and Message Passing Concurrency. There are others, but these currently have the most mind share and the best implementations. I have limited space and limited time so I am going to stick to these three topics. You may have noticed that up to this point I have only talked about concurrency. More specifically the communication between processes in concurrent languages. Thats intentional, I am not going to talk about parallelism at all. Thats a different subject and only faintly related to concurrency, but often conflated with it. So if your looking for that you are looking in the wrong place. On another note, I am going to use processes and threads pretty much interchangeably. I know that in certain languages the two terms have very different meanings, however, in many other languages they mean the same or one of the terms doesn't apply. What I am getting at is that if you open the scope of the discussion to the languages at large the meanings become ambiguous. When I use the term process or thread I am talking about a single concurrent activity that may or may not communicate with other concurrent activities in same way. Thats about the best I can do for you.

Traditional Shared Memory Communication


Shared Memory Communication is the GOTO of our time. Like GOTO of years past its the current mainstream concurrent communication technique and has been for a long, long time. Just like GOTO, there are so many gotchas and ways to shoot yourself in the head that its scary. It so bad that this approach to concurrency has tainted an entire generation of engineers with an deeply abiding fear of concurrency in general. This great fear has crippled the ability of our industry to adapt to the new reality of multi-core systems. The sooner shared memory dies the horrible death it deserves then the better for us all. Having said that, I must now admit that, just like GOTOs, shared memory has a small niche where it probably can't be replaced. If you work in that niche then you already know you need shared memory and if you don't you don't. Just a hint, implementing business logic in services is not that niche. OK, I am all done with my rant and I feel much better, now on to the show.

Shared memory typically refers to a large block of random access memory that can be accessed by several different concurrently running processes. This block of memory is protected by some type of guard that makes sure that the block of memory isn't being accessed by more then one process at any particular time. These guards usually take the form of Locks, Mutexes, Semaphores, etc. There are a bunch of problems with this shared memory approach. There is complexity in managing the serial access to the block of memory. There is complexity managing lock contention for a heavily used resources. There is a very real possibility of creating deadlocks in your code in a way that isn't easily visible to you as a developer. There is just all kinds of nastiness here. This type of concurrency is found in all the current mainstream programming and scripting languages, C, C++, Java, Perl, Python, etc. For whatever reason its ubiquitous and we have all been exposed to it that doesn't mean we have to accept it as the status quo.

Software Transactional Memory (STM)

The first non-traditional concurrency mechanism we are going to look at is Software Transactional Memory, or STM for short. The most popular embodiment of STM is currently available in the GHC implementation of Haskell. As for the description I will let wikipedia handle it.

Software transactional memory (STM) is a concurrency control mechanism analogous to database transactions for controlling access to shared memory in concurrent computing. It functions as an alternative to lock-based synchronization, and is typically implemented in a lock-free way. A transaction in this context is a piece of code that executes a series of reads and writes to shared memory. These reads and writes logically occur at a single instant in time; intermediate states are not visible to other (successful) transactions.


STM has a few benefits that aren't immediately obvious. First and foremost STM is optimistic. Every thread does what it needs to do without knowing or caring if another thread is working with the same data. At the end of a manipulation if everything is good and nothing has changed then the changes are committed. If problems or conflicts occur the change can be rolled back and retried. The cool part of this is that there isn't any waiting for resources. Threads can write to different parts of a structure without sharing any lock. The bad part about this is that you have to retry failed commits. There is also some, not insignificant, overhead involved with the transaction subsystem itself that causes a performance hit. Additionally in certain situations there may be a memory hit, ie if n processes are modifying the same amount of memory you would need O(n) of memory to support the transaction. This is a million times better then the mainstream shared memory approach and if its the only alternative available to you you should definitely use it. I still consider it shared memory at its core. Thats an argument that I have had many, many times.

Dataflow - Futures and Promises

Another approach to concurrency is the use of Futures and Promises. Its most visible implementation is in Mozart-Oz. Once again I will let wikipedia the description for me.

In computer science, futures and promises are closely related constructs used for synchronization in some concurrent programming languages. They both refer to an object that acts as a proxy for a result that is initially not known, usually because the computation of its value has not yet completed.
Lets lay down the difference between Futures and Promises before we get started. A future is a contract that a specific thread will, at some point in the future, provide a value to fill that contract. A promise is, well a promise, that at some point some thread will provide the promised value. This is very much a dataflow style of programming and is mostly found in those languages that support that style, like Mozart-Oz and Alice ML.

Futures and Promises are conceptually pretty simple. They make passing around data in concurrent systems pretty intuitive. They also serve as a good foundation on which to build up more complex structures like channels. Those languages that support Futures and Promises usually support advanced ideas like unification and in that context Futures and Promises work really well. However, although Futures and Promises remove the most egregious possibilities for dead-locking it is still possible in some cases.

In the end both of these approaches involve shared memory. They both do a reasonably good job at mitigating the insidious problems of using shared memory, but they just mitigate those problems, they don't eliminate them. The next mechanism takes a completely different approach to the problem. For that reason it does manage to eliminate most of the problems involved with shared memory concurrency. Of course, there is always a trade off and in this case the trade off is in additional memory usage and copying costs. I am getting ahead of myself let me begin at the beginning and then proceed to the end in a linear fashion.

Message Passing Concurrency


The third form of concurrency is built around message passing. Once again I will let wikipedia describe the system as it tends to be better at it then I am.
Concurrent components communicate by exchanging messages (exemplified by Erlang and Occam). The exchange of messages may be carried out asynchronously (sometimes referred to as "send and pray", although it is standard practice to resend messages that are not acknowledged as received), or may use a rendezvous style in which the sender blocks until the message is received. Message-passing concurrency tends to be far easier to reason about than shared-memory concurrency, and is typically considered a more robust, although slower, form of concurrent programming. A wide variety of mathematical theories for understanding and analyzing message-passing systems are available, including the Actor model, and various process calculi.
Message passing concurrency is about processes communicating by sending messages to one another. Semantically these messages are completely separate entities unrelated to whatever data they where built from. This means that when you are writing code that uses this form of concurrency you don't need to worry about shared state at all, you just need to worry about how the messages will flow through your system. Of course, you don't get this for free. In many cases, message passing concurrency is built by doing a deep copy of the message before sending and then sending the copy instead of the actual message. The problem here is that that copy can be quite expensive for sufficiently large structures. This additional memory usage may have negative implications for you system if you are in any way memory constrained or are sending a lot of large messages. In practice, this means that you must be aware of and manage the size and complexity of the messages that you are sending and receiving. Much like Futures and Promises the most egregious 'shoot yourself in the head' possibilities of deadlocking are removed its still possible to do. You must be aware of that in your design and implementation.

Conclusion

In the end any one of these approaches is so much better then the shared memory approach that it almost doesn't matter which one you choose for your next project. However, they each have very different philosophical approaches to concurrency that greatly affect how you go about designing systems. You should explore each one so that you are able to make a logical decision about which one to use for your next project. That said, opinions are like umm, I can't really complete that, but you get my drift. My opinion on the subject is the message passing concurrency is by far the best of the three, where best is defined by most conceptually simple and scalable. In the end the industry will decide which direction is right for that and head in that direction. We are still to early in the multi-core age to get any good impression of which will win out.

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