Threads Considered Harmful

LBR9 writes "James Reinders compares native threads with the goto statement so famously denounced 40 years ago by Edsger Dijkstra. Paraphrasing Dijkstra, he says they both 'make a mess of a program,' and then argues in favor of a higher level of abstraction. A couple of people commenting on the post question whether or not we should be even be treading into the 'swamp of parallelism,' echoing the view recently espoused by Donald Knuth."

No threads?

Alright, then all responses to this article need to fall under this one post.

How about "C++ threads considered harmful"?

Because really, multithreading doesn't have to be hard [erlang.org]

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The articles' author explicitly mentions Erlang as a potential solution to threading issues in other languages. In fact he's mainly concerned about POSIX pthreads, Boost threads, Java threads (and presumably Windows low level thread libraries). As I point

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The problem with Java concurrency and threading is, all the locks are advisory. The synchronize statement is a nice bit of syntax, and making it apply to whole blocks of code was the Right Thing to do.

The problem simply comes in that a program is not oblig

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I haven't studied Erlang yet, but threads (or more generally concurrency) done securely would require mandatory locking of all data.

I may have misunderstood (I'm not exactly an expert in threading), but I believe that Erlang handles this is a scarily elega

Re:How about "C++ threads considered harmful"?

(in C++ even more, when a String object is declared on stack and freed in its constructor)

Sounds a bit soon :)

Not exactly a new sentiment

See here: Threads are a fertile source of bugs because they can too easily know too much about each others' internal states. There is no automatic encapsulation, as there would be between processes with separate address spaces that must do explicit IPC to communicate. Thus, threaded programs suffer from not just ordinary contention problems, but from entire new categories of timing-dependent bugs that are excruciatingly difficult to even reproduce, let alone fix. [faqs.org]

I use threads a fair amount, because they are there. But I kinda wish path expressions [wikipedia.org] would catch on. Let the compiler sort out the scheduling given the constraints - that's the kind of scut work computers are good for anyway.

I'm All For Getting Rid Of Threads, But...

I'm all for getting rid of threads, but what are you going to replace them with? Traditional functional languages may be the most obvious solution, but they're also among the most impractical of solutions. Is there anything else out there that can replace threading needs, without throwing out the book on programming? It seems like what we need hasn't been invented yet.

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http://en.wikipedia.org/wiki/Tuplespace [wikipedia.org]

Threads only seemed to get really popular with Windows. Unix programming has typically always been multi-process with some form of shared memory. I've heard (and this is unconfirmed hearsay) that CreateProcess() on Wi

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Um, if what you've heard is true, why have threads been in Unix for a very long time?

In the early 90s, threads were the "next big thing". Around that time Java was being designed, so it ended up oozing in threadedness. Threads were tacked into Unix as an afterthought somewhere around that era to keep up with the latest fads.

Windows NT

processes

Well, for starters, there's processes, which were invented in the 1960s. These may not handle every case, but in my experience they'd cover 95+%...

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I'd love to see a web server spawn a separate process for every page access.

Apache 1.x is not multi-threaded.

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That's what select() is for.

Web server w/o processes OR threads...

Unix's select/poll mechanism avoids all that. See, e.g., here [acme.com].

I disagree with both this guy AND Dijkstra

Goto's and global variables are not inherently wrong or evil. They are tools. Granted, they are tools that, if misused, will wreak havoc on your code's stability and maintainability. The same could be said, however, for pointers. Threads are dangerous, and require special care. This is not a reason to avoid them; it is only a reason to be incredibly careful with them.

Use the best tool for the job, regardless of whether your CS professors demonized it or not.

That's not disagreeing with Dijkstra

I know nobody born in the last 30 years has bothered to read his memo, but he doesn't pretend gotos are "evil". Just that people should adopt structured control flow structures instead. Meaning, design and use languages with such advanced features as "if/else" statements, and "while" loops, and "functions". Goto considered harmful was written in a time when most people were not using the fancy new languages that offered these features, and he was suggesting that they do so, in order to improve the quality of their code.

Unless you seriously think people should use gotos instead of loops and if/else statements, then you don't disagree with Dijkstra.

Not really news

Threads have been considered a "bad idea" by the CompSci profession for a little while now. So there is definitely nothing new about the author's statements. That being said, there is a fundamental difference between Dijkstra's paper 40 years ago and this summary: Dijkstra started his paper by holding up examples of better practices. Only after establishing their existence did he go on to suggest that the GOTO keyword was "too primitive" to be of practical use in software development.

The author of this "article" (and I use the term loosely) doesn't really present such options. He hand waves a few work-in-progress solutions at the end, compares threads to GOTO statements, then asks the readers to fill in the (rather sizable) blanks.

Long story short, it's a good topic of discussion, but the comparison to Dijkstra's famous paper is just an advertising point. Nothing more, nothing less.

Hmm

The problem is not threads per se, but the way they are generally used in programming languages like C and C++. Although const correctness is understood by some C++ programmers, they appear to be a minority if I judge by the code I regularly review. There is also memory management which is a much bigger issue in threaded C/C++ applications than in applications written in Java. The Java library provides good examples of immutable classes, most prominently the String class, that remove a number of problems often encountered with their mutable cousins like std::string. Unlike std::string, I don't have to remember to make it immutable by constifying it or wrapping it. The presence of immutable classes, and the more adequate coverage given them along with threading in Java textbooks means that I disagree with the articles' author who lumps Java threads in with pthreads as a bad thing. What we need is more coverage of threading issues and how to alleviate them in intermediate level C/C++ textbooks, because despite the fact that threading is not built into those languages or their standard libraries, concurrency has become too important to ignore once you go beyond the basics.

Right. Mod parent up.

The problem is not threads per se, but the way they are generally used in programming languages like C and C++

Right. C and C++ provide zero help in dealing with the isolation issues of threading. The languages have no concept of parallelism (there's "

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Yeah. The main problem I've seen is that many developers apparently don't understand event-driven programming at all, so they end up creating dozens of threads to poll for various conditions, and then usually fail to come up with a thread-safe way of coor

"Threads" are not the problem.

The problem is that programmers are generally untrained in them or trained very poorly.

Writing a safe threaded application is not a difficult task, but it is a different task then writing a single-threaded app. And unfortunately CS programs, books, tutorials, etc, still train people in the single-thread mindset and yes the programs they produce end up being buggy.

And I'm not sure these 'high abstraction' languages are really the 'answer'. I have found that often in higher level solutions the results become even less predictable and tracing what is actually happening when becomes either extremely difficult, extremely inefficient, or just back to the single-thread mentality.

I think the OP talking about how one might be next writing a parrell app shows the real flaw here... the author is going from one mentality, entering another without really thinking it through, and then complaining when old methods don't work well. Take a programmer who STARTED in parrell space and you don't run into these problems.

chickens

Q) Why did the multithreaded chicken cross the road?
A) to To other the side. get the

Fancy programming languages are NOT the solution.

I'm tired of reading replies to this article that evangelize some fancy-schmancy high-level solution. I wonder if these advocates have ever tried writing production code in such an environment.

Let me give you a wonderful example of when theory simply doesn't meet reality.

Recently, I wrote a bunch of multi-threaded code for a next-generation asymmetric-multiprocessing game console that shall remain nameless. Its operating system has a wonderful complement of synchronization features. There's the usual mutex lock/unlock, and the usual condition signal/wait, but there are also event queues (queues of generic events that can be passed between threads running on different types of processors), lightweight mutexes/conditions, spinlocks, semaphores, reader/writer locks, and so on and so on. Truly a rich palette from which one can paint a wonderfully synchronized multi-threaded application! I then proceeded to try to rewrite a key section of our code in a very multi-threaded way.

The problem was, the first version of this code added NINETY milliseconds per frame to our main thread. A profile showed that nearly all of the extra time was spent in the operating system's synchronization features.

After much rewriting and much pain, I stopped using all of the operating system's synchronization features, and used processor-level atomic operations instead, and finally, the extra code accounted for only FOUR milliseconds per frame in our main thread (with the rest of the time successfully farmed out to separate threads).

I challenge anyone with a fancy-schmancy automatic concurrency solution to demonstrate that it doesn't have this problem.

Re:Slowaris caused threading

Complete crap. Threads solve a number of programming problems much more elegantly than forked processes and sharing data through some IPC mechanisms. Anecdote time: a stock price system I worked on. The first generation used separate processes for a single writer and a large number of readers, with shared memory for interprocess communication. This was switched to a threaded implementation for the second generation, which was faster, even though it was using the old LinuxThreads implementation, and more easily maintained as the pthreads API is much richer than IPC ones.

Threads work fine

Going beyond what you state - indeed I agree: Threads have a useful place in the toolbox. Perhaps this will mark me as "old" at some point in the world of programming.

I use them routinely on MS platforms. Background threads for write-behind me