concurrency: part 1

Posted by anton
on Friday, September 12, 2008

true to the purpose of this blog, below is an attempt to organize my (admittedly very superficial) experience with concurrency.

my 10GHz CPU

you probably noticed that moore’s law does not really apply anymore when it comes to CPU speed. if it were holding up, we would have had 10GHz CPUs by now, but for half a decade we haven’t really moved past 3GHz.

that is to be expected for the current generation of hardware – the gains have to happen elsewhere. for a little while we’ll get performance boost due to increase in size and speed of the caches that would improve locality, but in the long run it seems that multiple CPUs is where the improvements are to be mined from (this also includes specialized CPUs like Cell and GPUs in general).

this means that more and more people will have to think about their applications in terms of parallel processing. this also means that optimizations will become more and more important for those workloads that cannot be parallelized and therefore will be stuck on a single CPU (for a good introduction see The Free Lunch Is Over at Dr. Dobb’s Journal).

the bottom line is that as an app developer you cannot ignore the problem any longer; to make matter worse, there is no automagical solution in the nearest future that would make your application take advantage of multiple processors.

my concurrency story

in past decade most of the stuff i’ve worked with had some sort of coarse-grained parallelism; the rest was taken care of by the underlying framework.

i started with a unix philosophy of small programs connected via pipes, each performing a simple task. a little later came in fork and signals. things were simple, and OS took care of everything.

then came the web – it was mostly stateless with the database doing all the heavy lifting when it came to shared state. we just added boxes if we needed to grow. in ETL multi-box, multi-cpu setup was also natural, and the tools were designed to conceal concurrency; same goes for integration, where concurrency was at the level of data flows, which made things rather simple.

it is only in the past year or so when i had to really dive in deeper into relatively low-level concurrent development with java.

my dog-eared copy of Java Concurrency in Practice has proved to be quite an indispensable reference. the book is a bit uneven, and the editor should have spent more time on it, but you get used to it. it is a great practical resource, especially in the presence of so much confusing and incomplete information online.

jsr-166 introduced in java5 (and the primary subject of the book) is such a productivity boost; being a part of JDK, it is a big step forward towards letting mere mortals like me really embrace concurrent programming.

i find myself using Executors convenience methods all the time: it is so easy to create a pool, and then just feed it Callable instances, getting a Future instance as a handle in return. if more flexibility is needed, i use ThreadPoolExecutor. Queues are great as a communication channel for any sort of producer/consumer scenario, anything that requires message-passing or any sort of other work hand-off. Atomics are also great – i do not have to think twice when implementing counters or any other simple data structures.

most of the time i do not even have to work with threads or low-level synchronization primitives directly – they are buried deep within the libraries. i have less nightmares, since i do not have to touch volatile as often.

at some point i’ve read both editions of doug lea’s book, but i was always hesitant to recommend it; i’d rather rely on libraries that abstracted all of this away. now that java.util.concurrent has been out for 4 years, and Java Concurrency in Practice has become a bestseller, there are no more excuses.

one thing i’ve learned though – when you think you got this stuff, you discover a whole new class of problems that make you realize how complicated all of this really is, and how truly difficult it is to write larger concurrent programs.

you really, really have to think hard about how you share your objects, how you compose them and operate on them. you need to really understand how the language and the runtime work (i find myself checking JLS quite often). this is where good OO practices like encapsulation become even more important, since you are not just risking maintenance overhead, but you are risking the correctness of your program.

now, i always told myself that programming is not an exercise in manliness. i am just an app developer; i want to ship a working code that solves customer’s problems, not spend countless hours trying to reason through non-blocking algorithms just because i decided to do something non-trivial with ConcurrentHashMap. at the same time i do not want to waste my precious CPUs, so what am i to do? shouldn’t this stuff be easier? is there something I am missing?

threads considered harmful

actually, there is no problem with threads per se; the problem is with shared state.

in a normal sequential program you only worry about the logic as it is unfolding before you – one statement after another, in order. in a concurrent program that uses threads and shared state in addition to all your usual problems you also have problem of the non-deterministic state: since at any point in time any thread can come in and mess with your data, even between the operations you considered atomic before (like counter++), the number of states that your program can be in suffers a combinatorial explosion. this makes it really hard to reason about its correctness.

your code becomes brittle, sacrificing failure isolation – one misbehaving thread can potentially harm the whole runtime (a good analogy is BSOD caused by a device driver).

in addition, things don’t compose – a transfer operation performed by a thread-safe customer between two thread-safe accounts is not going to be automatically thread-safe.

to make matter worse, some of the errors remain hidden when run on commodity 1-2 CPU IA32 hardware, but as the number of CPUs grow, or their architecture becomes less restrictive to help with concurrency, things start to break down.

for more thorough discussion see The Problem With Threads by Edward A. Lee and Cliff Click’s We Don’t Know How To Program…

now what?

a natural reaction is to forget about fine-grained parallelism and offload the hard stuff onto someone else. after all, i am an app programmer, i care about business problems, what’s all of this yak shaving about?!

in some cases we can get away with firing up individual processes to take advantage of multiple CPUs. most of the time though it means that the problem has been pushed further down the stack, which often turns out to be the database. this is the route that rails folks went, and it certainly was pragmatic approach at the time (now that they are forced to deal with efficiency, threading is back in the picture. for discussion of issues see Q/A: What Thread-safe Rails Means).

if you can get away with using individual processes, go for it (see google chrome) – you get failure isolation, you get immutability in respect to other processes (it won’t be as easy for another process to mess with your data), and as an additional benefit, you get to use all the standard tools that the OS has when it comes to managing and troubleshooting processes (as opposed to using often incomplete and idiosyncratic tools for thread management that your runtime platform of choice offers – if any).

still, as we need more and more fine-grained concurrency and as the level of concurrency increases (it is not just a handful of CPUs now, but dozens, and even hundreds), one process per task becomes too expensive (context switching, high costs of creating a new process, memory overhead, etc). so we are back to some sort of lightweight thread-like primitives running within the same process, sharing some common resources.

most of the popular languages/platforms these days provide some sort of threading and shared memory support. but as outlined above, they suffer from some fundamental problems. there are some practical things at various levels of abstractions that that can help: low-level constructs within the language/platform itself, tooling, and higher-level libraries/mini-languages

language

  • make immutability easier – take note of functional languages, but also make it practical. in java case, for instance, it could mean extending immutability to some core data structures (see scala collections) or making it easier to tag an instance as immutable (see ruby’s freeze; this reeks of boilerplate though) – this way errors will be caught at compile time
  • consider sharing data only through explicit, ideally checked at compile-time, means. thus by default nothing is shared, and in order to make something shared you have to explicitly tag it as such. ideally, this would also come with some sort of namespace support, thus limiting mutations to a sandbox (see clojure for reference)
  • make language safer to use when it comes to exposing shareable state (this is when something like static becomes a problem – see Shared Data Considered Harmful for an example that applies to concurrency)

tooling

  • static analysis tools might help, but we need to give them a bit more than just an infinite number of states. findbugs for instance, supports concurrency annotations and something like chord could also be promising. this stuff is complex though and there are limits to static analysis (and i do not even want to bring up formal proofs using process calculi)
  • i want more support from the platform to help me troubleshoot lock contention, deadlocks, cpu-intensive threads, and other concurrency-related infrastructure. sun’s hotspot has some rudimentary stuff in place, but i want more things out of the box (azul claims that they have always-on built-in tools in their product, but i have not played with them)
  • speaking of azul, i need to study them more. although perceived as a boutique solution, they are addressing issues that everyone will be facing in just a few years. seems like they ported sun’s hotspot to their hardware which allowed them to achieve scaling by automatically replacing synchronization with optimistic concurrency which scales much better. incidentally, this truism about optimistic concurrency has been obvious to database folks for decades

libraries/mini-languages

one of the approaches is to focus on your problem domain and come up with a library/language that solves your particular problem and abstracts away concurrency. web frameworks (J2EE, rails), or ETL tools, or even databases are all examples of such approaches.

this is where my interest lies as an app developer – how can i make concurrent programming easier for me, the layman.

the bottom line is that if we insist on using low-level synchronization primitives, it would be really hard to paper over the underlying complexities. right now there is no generic universal approach that will simplify concurrent programming. so at this point a pragmatic programmer is left with patterns, supporting libraries, and heuristics.

to be continued

there are some patterns (for the lack of a better word) that i found to be helpful in dealing with concurrency; there is also some stuff on the horizon that promises all sorts of benefits – is there really a silver bullet? but also there is plenty of stuff that has been with us for decades, and i would be the first one to bow my head in shame, acknowledging my ignorance.

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