Threads Are a Bad Idea for Most Purposes (1995) [pdf]
web.stanford.edu
web.stanford.edu
This paper came at a time when threads were really painful to work with. POSIX threads were still new and mostly unsupported, so you were stuck with whatever your OS exposed. On IRIX, you would do threads yourself by forking and setting up a shared memory pool, on Solaris, you had the best early pthread support, in Java, you used the native Thread classes which only really worked on Solaris at the time. It was a mess!
This mess is now solved. Pthreads are everywhere, C++ has std::thread, Java threads work everywhere, and we've had many new language some out which handle parallelism beautifully - for example, the consumer/producer model built into channels in Go is very elegant. The odd one out is Win32, but it's close enough to pthreads for the same concepts to apply.
Event driven programming has also become threaded, this is how the whole reactive family of frameworks, node.js, etc, handle parallelism.
As someone who's been doing this for a long time, what I find confusing is higher level constructs that try to hide the notion of asynchronous operations, such as futures, promises. As a concept, they're fine, but they're difficult to debug because most developer tools seem not to care about making debugging threads easier.
It was great fun to write code which drove 8 displays using 8 graphics pipes, with roughly 8 cores working in concert with each pipe. All this work for something that runs faster on the latest iphone using a single thread...
I went on to work at SGI for a few years, and it was still my favorite job ever. It was pure R&D, graphics and realtime systems for their own sake. Today, this doesn't exist. 3D graphics are an applied technology that's part of an app, but not a product research area of its own.
I'm quite certain there are teams here at Microsoft that do just this, and their counterparts in the GPU industry (AMD/Nvidia/Intel - although Nvidia seems to be the dominant one in research)
c.f.: Data Scientists
Working with epoll or select/poll is really nasty when dealing with "some data" that's coming on a socket, where as IOCP allows you to tell the kernel "as soon as you see some data destined for my thread, just populate it in the memory here, tell me when something is there and then tell me how much is there"; for high performance C++ programmers this is basically invaluable and as far as I understand there's no direct competitor in UNIX and unix-like land. (although; according to the C++ devs where I work, "kqueues are less braindead than epoll")
If the thread is waiting on a specific set of objects (e.g. file descriptors, kqueues), then this can be done easily, but it's still not quite as simple as it has been under Windows (and by the way, IOCP is built on top of the underlying mechanism that makes this possible on Windows; they are not the mechanism itself)
io_uring is "different" and definitely has some advantages due to its shared ring buffer. but NT has a core/generic set of async completion/wait/etc functions that can be applied to basically the entire API surface.
I wouldn't assume it does until I've verified and read the code, I know last year at plumbers there was a fuss about the fact that a lot of operations which should have been working wernt. Frequently in odd ways, for example direct read worked but not direct write IIRC.
And yes, you correctly point out that on nix and Win32, interaction with the kernel is often different. Win32 does a lot right, I won't ever speak negatively of it, it is the desktop OS of choice by a huge margin. It's just that nix and Win32 are so different from each other, that you can't generally write a single implementation of your low level thread interaction for both classes of platforms.
There is no equivalent in UNIX land to the thread-agnostic asynchronous I/O primitives available on NT. When paired with a robust threadpool API (Vista+), it is an unbeatable platform.
That depends how you define "work". Java has a decent concurrency library such that if you have a limited-scope project implemented by 1 or 2 highly-skilled programmers, you stand a good chance of avoiding thread safety issues. But as the project grows, the probability of thread safety issues approaches 1.
> Event driven programming has also become threaded, this is how the whole reactive family of frameworks, node.js, etc, handle parallelism.
Event-driven programming is way less painful than Java threads. With threads, in every place you mutate anything you have to ask yourself, "what happens if my thread is preempted here?" With event-driven programming you only do that sort of thinking in places where you see the `await` keyword.
in 23 years programming Java (and in the all places where i've worked having been recognized as local expert on threads and synchronization, incl. in the current very large C++ platform project) i have never asked myself that question in the context of the mutation of data.
I don't think I've actually thought about preemption explicitly since the Nintendo64 days when thread priorities figured into our locking strategy. But then we'd also re-implemented the scheduler and, at times, turned off the official OS to go do something we needed to do without interruption. So, like I said, preemption not high on my radar these days, as it falls under the general category of "concurrency". Even an iPhone has 4 CPUs.
The effect is much the same, whether the race is caused by task scheduling, or just because another core got there first.
Modern kernel people still have to worry about locking as well as preemption, if you look at the NT native API's you will see dispatch levels, which control whether kernel threads can be prempted for higher priority activities, linux is similar with the _irqsave() and preempt_ calls.
Is this for example important when syncing with an interrupt handler (no preempt/rt config), which is why there are both spin_lock() and spin_lock_irqsave() which implicitly blocks irq preemption. AKA if you grab a lock that is needed by say an interrupt handler, then you take said interrupt the machine will deadlock because the scheduler won't deschedule the interrupt handler.
This is exactly why I run unit tests concurrently (other than the speed boost), and try to write the tests (and the code) so that it won't break when run concurrently (like inserting something into a DB and then assuming success only if the count has gone up by exactly 1).
I would recommend using specialized tools for the task like Intel's Thread Inspector or Coverity's Helgrind.
The one being a good idea doesn't preclude the other from being a good idea as well. Do them both.
(And also avoid concurrency wherever you can. And of the available forms of concurrency, threads are one of the worse ones.)
Thinking about asynchronous behavior is always tricky, and code designed to run on threaded systems isn't generally arbitrary code with locks thrown into it the mix, you strive to write the largest possible reentrant sections, and only lock where you have to, and as infrequently as possible. With reentrant code, it doesn't matter where you get preempted.
Not if you do it right. Interacting with sync-only systems from async systems is a problem, but it's not fair to blame that on async - if you make the whole system work async, you don't have that problem.
Event-driven programming is a different pain in the ass but still a pain in the ass.
The current status of the Web speaks for itself: it's event-driven based JavaScript, and it's full of state/transitions bugs everywhere.
In event driven you trade concurrent r/w access issues for a loosing execution contexts issues and out of order issues. The result is often a mess of spaghetti callback close to unreadable.
The less worst of the world might be simply a proper coroutine system, which seems to come to almost every programming language after 30 years of existence
I don't disagree with what you've said, I just wanted to chime in that in the context of the conversation to this point the definition is something in the ballpark of "are available cross-platform without jumping through hoops".
Might as well forget debugging in promise-land. I think this is partly because they are conceptually problematic as well, apparent once beyond the simplest use-cases, given the number of bloggers that still seek to explain it - and then are forced to edit their post because of errors. And the need to list common mistakes on MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guid...
I don't do intense concurrency in the browser, but I'll do non trivial stuff, like collating responses from a server then sending a request once I have all that information. I think promises are fine for this and it is possible for most programmers to write clean, mostly mistake-free promise based code.
As for difficult to debug? I've never had that issue, neither in JS (browser/node) or C# with the similar but different Task<>.
For example in JS, I can put breakpoints at any point of the promise chain and see what is going on. If they are network requests I'll also look at the network tab. If everything is happening real fast I might use console.log statements, but that is rare.
I think promises are OK for a lot of situations that most of us will encounter developing business software - this might be a reflection on the simplicity of the problems I end up solving. Obviously if you are creating a multi-threaded high-frequency market making trading thing then this might not apply.
If you say so. I can't count the dollars I've made fixing other people's poorly written multithreaded code in my entire career, including in the last 3 years.
Thread support is [more or less] standardised in all major OS-es now, sure. Doesn't change the fact that it's an extremely bad fit for the human brain to think about parallelism.
Stuff like actors with a message inbox (Erlang/Elixir's preemptive green threads) or parallel iterators transparently multiplexing work on all CPU cores (Rust's `rayon` comes to mind) are much better abstractions for us the poor humans to think in terms with. Golang's goroutines are... okay. Far from amazing. Still a big improvement over multithreaded code as you pointed out though, I fully agree with that.
I might be projecting here and please accept my sincere apologies if so, but it seems to me you are a bit elitistic in your comment. Multithreaded programming is still one of the most problematic activities even for senior programmers, to this day. Multithreading bugs get written and fixed every day.
At this point I believe we should just move to hardware-enabled preemptive actors where message passing is optimised on the hardware level, and just end all parallelism disputes forever since they are an eternal distraction. (The overhead when utilising message passing today is of course not acceptable in no small amount of projects. Hence the hardware suggestion.)
I'm curious why you feel this way? Certainly anything with multiple (more than one) thread is a lot harder than single threaded. Given.
But I think the human experience is rich with real world counterparts that can make multithreaded programming "natural" (not necessarily easy, there's a difference). Basically any process in real life you do that involves collaborating with multiple people is a collaborative multi-threaded process. When forced to grapple with multi-threaded solutions, I often ask myself, "if I had a room full of people working on this problem, what would have to be in place to make the operation flow smoothly?" For me, this anthropomorophisation of the process makes it a "good fit" for how my brain is used to solving lots of real world problems.
On the flip side, I haven't had a lot of luck finding real world experiences that I can model coroutines/async/etc on. So they may be ultimately easier, but if our rubrik for "fit for the human brain to think about" is what the wealth of human experience has evolved our brain to handle well, I'm less convinced.
But I like learning new things. Maybe I just haven't seen the lightbulb yet. Help me see your point of view?
I am not arguing in favor (or against) threads. I am not contesting that they lead to more bugs or not. What I was contesting was the assertion was that "threads are not a natural fit." My point is that procedural programming (which nearly all programming is at least small level, minus languages like Prolog) is something human beings have been doing for thousands of years. Look at "recipe" for doing something, and you have procedural program. Look at any recipe for lots of people working together to do something, and you have a pool of cooperating procedures. It's something we get schooled in our whole lives.
My argument at least is that, given the choice, there exist much more productive ways to make your employer money than to try and learn nuclear phys... I mean multithreaded programming. :-)
But stopping to use C is a good start (for whoever has the choice)!
Not as toy, proof-of-concepts. Fully fledged replacements for all that stuff that can be used in our our daily work. Build distros of the stuff so that we don't have to pick and choose this stuff and replace the bits of our systems in a piecemeal fashion.
They could show us how it's done instead of talking endlessly about it in online forums. So let's face it, it's never going to happen.
As for managed code, it's being used with huge success in a lot of places (not in OS-es or drivers, yes) but that's quite the huge topic by itself.
I'm not defending C, I'm merely sneering at its noisy detractors who spend more time complaining about it than supplanting it.
Perhaps as the Rust community grows in size and momentum this will happen more. Right now it seems like a bit of a manpower problem.
Note the section 1.7 in your own link ("End of the Lisp machines"). The Lisp software ecosystem was never as rich, broad and varied as the C/C++ software ecosystem is today, before it died.
BTW, I only stumbled across your comment by accident, since you replied to the parent poster instead of me. Parenthesis mismatch?
Cheers.
It's perfectly fine to continue using an existing C codebase for a program, not exposed to the public internet, that's maintained by a focused group of maintainers. But on the other side of this spectrum, for large exposed projects like OpenSSL, Chromium, or even Linux, C/C++ has become risky.
Threads in C are hard if you have a sloppy codebase with global variables everywhere and a general lack of structure and abstraction. Unfortunately, this describes many many C codebases.
That would be actors or processes communicating with each other, not a collaborative multi-threaded process.
Real world has pretty much no natural notion of threads, only of actors.
The closest s/w equivalent I could think of is Unix pipes. When I do “cat foo.csv | sort | uniq -c”, cat/sort/unique aren’t coordinating with each other, each takes the content from stdin, transforms and place it in stdout.
The assembly line principle can also be applied to the concept of threads. Workers collaborate by handing items off to one or more workers(threads) down the line. Thread pools are just micromanagers constantly reassigning the poor workers, but at least the make a lot of friens collaborating.
In short if it's not a good fit for your brain, change your brain!
Not sure what you mean by your statement that hardware engineers are trained to think differently about it. I am not sure there's much overlap between HW and SW engineers in terms of parallelism modelling. Do you have a few examples?
However having said that I find myself designing silicon again, change is good for the brain I guess
Hardware people tend to think in terms of netlists and pipelining so they have to worry about parallism on every clock
I am curious as to how your h/w experience makes it easier hacking on the linux kernel....
Go is sort of like saying "we're not going to deal with parallelism directly, just throw everything into fifos to talk to other places" mostly you can't afford to waste gates like that in real hardware (there are places where it makes sense - I've built a lot of display controllers).
I think the big difference is the way you end up thinking about time and timing holes in code (dealing with interrupts), after building gates for a while i think you tend to see the timing holes more easily
I would argue that the code equivalent of people collaborating in the real world is multiple processes with IPC (or even further afield, multiple nodes in a distributed system), not threads. You don’t share memory with other human beings. Imagine how productive you could be if you did!
(Technically these aren’t best expressed as SHM but rather as a tuplespace or greedy worker-pool abstraction, but they can be achieved starting off with “just” an SHM threading primitive, so they count, I think.)
This really depends on where you draw the boundaries. In a computer everything is "memory" of one kind or another, but that doesn't necessarily correspond to human memory. The analogy works if you think of the "actor" as merely the thread's context and dedicated working space (stack), with everything else being part of the surrounding environment. In the real world we don't have anything akin to process separation, except by convention. Even the body itself can be directly manipulated by others. The convention we use for communication between individuals looks more like a message-passing system, but physical manipulation of matter is more like operating on data in shared memory. There is no physical law preventing someone else from coming along and messing with an object I happen to be holding any more than the rules of a computer prevent one thread from messing with a data object held by another thread.
Not the OP, but in my experience most devs are bad enough dealing with state and flow in single-threaded code already. Including myself. But I've at least accepted that and try to mitigate it by avoiding things that make it worse (like threads). Meanwhile, I watch people around me doing code by spaghetti (throwing it on a wall and seeing if it sticks). The better devs don't apply that to their code, but still end up applying it to their "reasoning" about a problem domain, especially a business one.
E.g.: "Screw it, if this variable that I need is null I'll just return a False in this function I'm writing" ... without ever thinking "why" this variable might be null and how that explanation impacts the problem they're solving. Nevermind the fact that they're fully OK with propagating such a broken state further into the program with their False response. Now some poor other dev has to figure out why this function is returning a false result. Odds are they'll do the same thing and now we've doubled the amount of bad states.
Adding in even more complexity in terms of being able to "predict" code state by adding threads is a nightmare. I would most certainly protest and heavily advise against anyone using threads unless it's really really the thing that's needed to solve the problem. Otherwise, it's just adding more a lot more complexity for very little benefit if any at all.
I feel we might be having too incompatible perceptions of the world but I'll try.
Every communication (human or otherwise) is exactly actors with message inboxes. We the humans can't react to 3 incoming conversations at the same time. Our brain queues up what it hears or reads and then responds serially, one by one (not necessarily in order of arrival). Basically an actor with a message inbox, like in Erlang/Elixir.
The way you assert multithreading mimics the real world to me is not convincing. One possible example in favour of your point of view I could think of is probably a table full of food and 20 people reaching and grabbing from it at the same time. But even then, two people cannot successfully get the same piece of meat. So not a true multithreading in the sense of N readers/writers competing for the same resource; it's more like a shared memory area where parallel writes are okay as long as all writers reference different parts of it.
Sure, a lot of stuff collides in real time without synchronisation out there, that much is true. But you likely noticed we the people don't cope with the Universe's chaos very well and rarely manage to grasp it properly. So is it really a good way to model deterministic and non-chaotic systems with programming? To me it's not.
I am curious why do you even think multithreading mimics the real world at all. Elaborate, if you are willing? (We might think of different definitions of multithreading in this instance.)
Async, actor-like processes are everywhere in the real world - "something comes into my inbox, I do my piece of work on it, then send it to their inbox". Or "I'll send this off, do something else for now, and then go back to working on that when I get their reply".
As a programming model, threads don't really help you to achieve this. When there's a bug, your only option is pretty much to think very hard and try to figure out what happened. (Or to use gdb with scheduler locking, which is about as bad.)
FP makes the programmer rethink the way that the problem is solved. I think that adding threads on non FP code makes it hard from the get go.
Don't hide it on hardware, rethink the approach instead.
As for FP... I remember a guy saying "imperative/OOP shows you how it does stuff, while FP shows you what is it doing". It was a very eye-opening statement for me.
I clearly remember the feeling of just getting it as well. It was a very binary moment :)
Threads are definitely a valuable tool for performance-oriented work and things like eg; worker pool patterns aren't always a great fit and you end up with more overhead.
I know there exist a group of programmers that really don't have a choice and have to use `pthreads`. But if you do have the choice then IMO sticking to multithreading programming is very backwards and invites a lot of suffering in your future (or that of your future colleagues).
Just use actors with message inboxes. Measure, profile, tweak. If you exhausted all other possibilities and you absolutely positively can't upgrade your server ever then sure, ditch actors and write multithreading code.
Futures and Promises go a long way in making that better. That's not to say that you can always rely on them, but it is far easier to get them right than it is to get threading right with shared mutable state right.
Actors, Channels, Futures, promises, reactive programming, etc. They all have one thing in common. They kill off shared state in favor of message passing.
Yep, exactly.
Everybody can screw up syncing and message passing as well, given enough lack of experience, or schedule pressure, or simply not getting it. But it's much harder to screw up that while conversely, it's extremely easy to screw up multithreaded code with shared state.
More-or-less, but no barriers on OSX (at least I had to hack around that for a user 6 months ago)
That being said, developers often have a bit of learning to get there, but that can greatly be accelerated by good team/mentor code review and discussion.
It's just that there exist ways to shoot yourself in the foot quite easily.
Example: I'd find it much more intuitive if writing to a closed channel returned an error value and didn't issue a panic.
But I'm guessing that the Go programmers get used to the assumptions that must be made when working with the language so such things are likely quite fine with them and cause them no grief.
Better with panic than silent errors and flawed logic.
"Multithreaded programming is still one of the most problematic activities even for senior programmers, to this day. Multithreading bugs get written and fixed every day."
Strongly disagree. I am going to claim that threads / locking are not hard to write if one has at least a bit of common sense and discipline. Problem is that we have hobbyists without any trace of knowledge on how computers work producing commercial software.
Goes like this: I do not want to deal with memory management - it is too hard, I do not want to think about types - my poor brain can not comprehend those, pointers - OMG what are those beasts, threads and locking - gonna commit Seppuku. I will advise those to come to a logical conclusion: programming is frigging hard when one wants to produce a decent product rather then buggy POS. No matter how many concepts the language hides / converts to something else there will be always something else they will not be able to wrap their mind around. So how about trying gardening instead
You don't have to make this about implying that others don't have "at least a bit of common sense and discipline".
Multithreaded programming is still one of the most problematic activities even for senior programmers
The trouble is we need to stop hunting whales if we're going to move the needle in a permanent and significant way...
My second favorite class in college was the elective for distributed computing (which I took instead of compilers, but I wish I'd taken both). It felt good to me and I came out into the job market ready to concurrent all the things.
By the time I'd worked with a disparate group of coworkers, I found out that 1) my enthusiasm was a rare thing, 2) that this was with good reason (many, many burnt fingers), and 3) that just because I love something doesn't mean that it should have a central role in a team project, even if I am the lead.
In a similar way that I 'got dumber' by learning to think like a user (one of the downsides of studying UX) I sort learned to reach for concurrency after I'd tried several other options.
... but here I am, years and years later, debugging other people's async/await code for them. Nobody likes to have to have someone else fix their mistake, but it goes a lot better when I start by telling them this shit is hard. Which it really is. Even when (or maybe especially when) I'm confident my stuff is right, I find a few boneheaded mistakes that I have expressly warned others to avoid in stuff I wrote uncomfortably recently.
The promise pipelining technique (using futures to overcome latency) was invented by Barbara Liskov and Liuba Shrira in 1988.
Yes B. Liskov is the L in "SOLID", if you are wondering!
Happy 40th birthday promises! :-)
I wouldn't be surprised if mathematicians were thinking of this before transistors were invented though.
I think it takes time for things to trickle down. Maybe it takes someone from an academic background (or a curious paper reader) to be forced to use C++ or Javascript for something, but along the lines of "progress relies on unreasonable men/women" they get annoyed and write their own library so they can use the nice Lisp/Haskell/etc. feature they are used to, but often in a more limited way but better than nothing. Then from there it might get hyped to the shithouse, die, or just get used by insiders.
Async/await has also mostly replaced bare Promises. The result almost gives you something as usable as threads.
Fwiw I find it amusing the async is promoted as the pinnacle of elegance in concurrent programming when in reality it exists because the JS interpreter wasn't thread safe.
P.S. sorry so late answering your mail!
(I owe you mail too.)
I was terribly remiss not to mention Dojo, a popular JS toolkit which got its promises from Twisted, which of course got them from E, though Twisted modified them a bit. I don't know how it slipped my mind.
I get the impression E promises had a nicer design than JS's for handling errors -- but that's also a vague memory and I never really learned JS's.
My impression at the time was that developers had a flawed impression of threading complexity and bugginess because they were bolting threads onto existing single-process programs, and their existing hygiene was the problem, nothing inherent to threading.
If you look at the single-process C programs of the era, global variables and global state in general were extremely common. When you start adding threads to such programs to try take advantage of SMP, trying to wrap locks around heaps of unnecessarily shared, poorly encapsulated state, of course you produce a lot of buggy programs. Then everyone starts saying "multi-threading is too hard, not worth it", instead of admitting their programs are a complete mess.
Using modern standards of hygiene, even with threads-naive languages like C and good old Pthreads, I don't find it particularly challenging at all to write threaded programs.
Like you mentioned, I also struggle with the higher level abstractions attempting to make threading easier. Pthreads makes sense to me, I find threads, mutexes, condition variables, rwlocks, all very intuitive and ergonomic to use, but it's probably because I spent a lot of time using that API at a young age.
As I started taking swe jobs in silicon valley in the early-mid 2000s, it surprised me how few people had experience with Pthreads. It blew my mind, four different startups doing C programming with experienced C programmers and nobody was ever familiar enough with Pthreads to quickly review my code without having to rtfm. It's like there was such a stigma surrounding threads being "too hard" a lot of people never even attempted it. But my being a kid learning linux and just excited about new features in my unix, when LinuxThreads arrived and then NPTL, I spent years playing with Pthreads in C and getting my hands on SMP systems just to program them with Pthreads. Those years of playing paid off unexpectedly well in silicon valley, SMP was everywhere and C was still heavily in use on Linux.
I still get a bit of happy nostalgia when the opportunity arises to write some code like:
pthread_mutex_lock(&foo->lock);
while (!foo->ready)
pthread_cond_wait(&foo->cond, &foo->lock);
/* consume from foo */
pthread_mutex_unlock(&foo->lock);I like promises/futures because they tell me when resources will take a while to become available without blocking. I generally don't want to need to know that I need to lock the foo queue before consuming from the queue. I would rather have the queue handle that for me and encode the behavior I want in the type signature:
next() -> T sync, blocking
poll() -> Optional<T> sync, non-blocking
next() -> Future<T> asyncThis article is from 1995, and they were out a bit earlier than that. Sun had been shipping 4-core workstations with thread support since 1993.
The SPARCstation 10 came out in 1992 and had two MBus slots. Each slot could take a CPU card with 1 or 2 CPUs on it.
Although SunOS 4.x had very limited multi-CPU support, SunOS 5.x had support for multiple CPUs. SunOS 5.1 (Solaris 2.1) came out in 1992 and supported SMP, and SunOS 5.2 (Solaris 2.2) came out in 1993 and introduced a thread API.
By 1995, they had also introduced the SPARCstation 20 and the 64-bit Ultra 2.
They also had some server systems (like the SPARCcenter 2000) that had a backplane with several boards that could themselves have CPU cards. I don't know the maximum number of CPUs, probably something like 20.
Info from my memory and:
http://mbus.sunhelp.org/index.htm
One thing that i find very convenient in Win32 that is lacking in other platforms (at least using pthreads) is that every thread has its own message queue and you can have threads communicating with each other simply via PostMessage/GetMessage (which also handles sleeping). You can implement something similar over pthreads, but it is nice that the OS provides that out of the box.
Edit: on the other hand Win32 has one really nice feature: you can WaitForMultipleObjects() on essentially anything that has kernel handle, which includes most of IPC primitives. On the other hand this causes the native Win32 IPC to have significant overhead and is the reason why game developers often resort to userspace spin-locks and why Windows 10 had introduced NPTL/Linux-style lightweight futex-based mutexes...
And honestly i never had any bugs with that, if anything i've found it the easiest approach to understand when it comes to inter-thread communication.
IRIX also had that weird 'sproc' API that I think was semi-lifted from something in Sequent's DYNIX.
There is an impedence mismatch between how code is written (sequential, top-to-bottom, left-to-right text) and how parallel code behaves. That's why people use idioms like actors or coroutines. I think to actually solve this you need a new format for writing code, like a graphical function call graph instead of a text editor.
More than a year later we were trying to port much of the codebase to solaris and it was a nightmare. Pretty much nothing worked right so we ended up bolting on the fork/mmap abstraction on top of much of it and building our own lock wrapper. While we sorta got it working, the solaris port died for internal political reasons and the NT version took off and we never looked back.
NT was designed out of the gate for heavily threaded/async workloads, that stuff wasn't bolted on like pretty much every unix clone in existence.
I'd rather use a real multithread-based framework, honestly, though I concede that it also opens up different ways of making developers' lives miserable.
Once we de-couple these two things - how we write code and how we run code - the discussions about this become clearer and easier to have.
> Also the default state of matters is that everything is serialized in a single thread and everything waits for their predecessor, even when they are totally unrelated...
I think this is a preferable default since bugs caused by pre-emptive multithreading are much harder to debug.
https://en.wikipedia.org/wiki/John_Ousterhout
Specifically, he created the Tcl programming language, which had a nice event loop way back when.
It should have been written that way on day one but some programmers default to pounding out a bunch of spaghetti instead of learning about the tools at their disposal.
Way back in 2008: https://news.ycombinator.com/item?id=399670
Didn't you read the article?!
Maybe they're a bad idea, but these days you have no choice but to learn how to use threads. AMD's run-of-the-mill processors have 16 cores, Intel's 8. Servers have lots more. Heck even your iPhone has 6.
The clock rate on CPUs isn't getting any better. It's just more cores from here on.
Why would you write a program using threads if you don't require concurrency? The only purpose of threads is to achieve concurrency. Can you give some examples?
Maybe it’s hard to imagine now, but in the 80s and 90s there were people that pushed this sort of architecture with a straight face. Even if not this extreme the idea of using threads for componentization rather than a focus on concurrency..which was possibly a side benefit was very much a thing (think COM/CORBA))
Hence why many articles like this and Ousterhout from the 90s, etc saying it was idiotic.
In embedded systems, our kernels and threads are lightweight enough that we can go very fine-grained without paying a steep context-switching penalty. I'm not convinced that the penalty in Linux is all that high, either. Its only when you're going after the C10K (or C1M?) problem that you start to notice.
Right, this system would run through the entire runlist at several kHz when idle and 0.5-1kHz under load on a PowerPC 405 that ran at about 200MIPS. Our shortest deadline was 10ms so it was plenty fast enough. Context switch was swapping out 12 machine words.
That being said, threads definitely have their place and are the only real way to take advantage of all the power offered by modern multi-core CPUs (well that and multi-process but that’s not really any easier to get right). But for the basic stuff I think running 80 threads when your app is only using 5% cpu is insane (something I see a lot in the C++ code I’m exposed to).
> way to do blocking operations without stalling everything
Switching between multiple tasks doing IO - that's concurrency isn't it?
> various threads running at different intervals (low priority background thread that sleeps for 1 second then wakes up to do stuff
Different tasks ready to run and switching between them as needed - that's concurrency again isn't it?
Not really sure what everyone else in this thread is seeing that I'm not.
... exactly. There is no good reason. That's what I'm saying the point of the article is. However, this doesn't mean that people don't do it. He is saying that the tasks that people typically use threads to solve can actually be solved with an event loop and handlers, thus eliminating all of the messy issues with shared state, race conditions, etc. that true concurrency introduces.
Threads make any single thing on your program mutable without your direct control. Processes keep the mutability scoped into a few hard to extend areas.
No they don't. Threads don't mutate random variables by themselves, you need actual code that does the mutation (whether it's running on a separate thread or not).
I mean, how is that statement different from "calling other functions makes any single thing on your program mutable"?
On those languages where functions mutate things, that's basically true. But it's much more common that functions can only mutate global variables, and people keep those in low numbers, exactly for that reason. Actually, replace "functions" with "methods" and you will get into one of the largest flaws of OOP.
But anyway, mutability is much less of a problem outside of concurrent code.
> Scalable performance on multiple CPUs
the exceptions to 'when to use threads' in 1995 sound like SOP these days
* Standard, easy way to get a backtrace
* Standard, easy way to get a list of active things going on
* In many cases, threads make it easy to follow control flow
I wouldn't personally say that threads make control flow easier to follow: we might gain a little by disentangling separate activities into threads, but we lose a lot when these get interleaved in arbitrary, non-deterministic ways.
I mean, when a program panics, you get a stacktrace from (a consistent snapshot of) all the threads, so what's the problem?
Similar to having a snapshot of network traffic vs a recording of network traffic.
Threads' contexts should be independent from each other. If reading your stack trace relies on the state of other threads, you've got a very brittle design.
What is lost is the history of the state the current thread is working with, but the state should be fully encapsulated within the thread, except in the case of large shared read-only input buffers that are being processed in parallel. But those latter buffers aren't hidden from the current thread nor its debugging. Debug information can also be logged on state objects to show its provenance.
Sure. The trick is that the thread design does not ENFORCE that, threads as an abstraction involve shared memory.
> If reading your stack trace relies on the state of other threads, you've got a very brittle design.
Or a bug. And a bug or a brittle design is exactly when you need a debugger the most, right?
I honestly think this presentation is confused. "Concurrency is fundamentally hard; avoid whenever possible" seems to go against their own argument. Event-driven models (which rely on message passing) are still doing concurrency, except instead of using locks and semaphores to synchronize things, you're using mailboxes and channels.
Even CPU interrupts are a form of concurrency that is similar to event-driven models. Just because you're not spawning a thread and acquiring a lock, doesn't mean you're not doing concurrency.
Most platforms these days provide such things as part of the language, or in the standard library, or as a freely-available package. Writing one's own concurrency infrastructure is usually unnecessary, but when it is needed, it needs to be kept as small and as easily-auditable as possible.
A bit like `unsafe` in Rust, in fact.
Locking all over the place generally indicates that someone's trying to shotgun-debug concurrency bugs. I've had to use libraries which did that, and wished horrible things upon those responsible.
Which is basically any program running on any modern processor.
Now async/await gives us even better options on top of that, but it’s truly what made me enjoy the language so much. This article is what resonated with me and got me to invest so much spare time over the last 5 years in working with Rust: https://blog.rust-lang.org/2015/04/10/Fearless-Concurrency.h...
I think though that rust is one approach of many and that they aren't orthogonal.
I personally think libraries for multi threading on top of rust or C++ are necessary to really nail the problem down. Even async and await are too granular and difficult to get right in my opinion.
I think regardless of the language, queues of data chunks (not just tasks) combined with solid concurrent data structures will go a very long way. Rust may help get those libraries and data structures to be more correct with less effort, but ultimately getting threading right at the lower level just isn't practical for most programmers most of the time, and I say this as someone confident in their parallel programming skills.
Even still, even in languages with built-in runtimes and GC like GO, it is very easy to create deadlocks/livelocks. Programmers intuitions of concurrency are generally wrong, and the correct intuitions are complex. I don't know how we make concurrency easy. I do like Futures however, they provide a nice abstraction over threads.
I also have a lot of respect for Ada's task-based concurrency approach (independent actors, communicating by rendezvous). You don't get the flexibility to roll your own concurrency strategy in Ada, but the language's support for its chosen mechanism is truly excellent. Even if you'll never use Ada, this part of the language is worth studying just as an example of great engineering design.
|
non-shared mutable state | shared mutable state
|
----------------------------+------------------------
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non-shared immutable state | shared immutable state
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Functional languages are great at the bottom two, since they strongly encourage immutable state. Message-passing based concurrency strongly encourages non-shared state, the safe two on the left.
Rust is the only language I've seen which encourages all three safe quadrants, while making the fourth a compile-time error.
Haskell actually has safe & ergonomic shared mutable state via STM
And it has proven-to-be-not-shared mutable state via ST.
No. Your database is one giant shared mutable state.
How is using a database safe then? Transactions.
Haskell has had software transactional memory for fifteen years now. Microsoft tried to copy it in .NET but it's nearly impossible to do right in a language without clear separation of pure and impure code.
No. Your database is one giant shared mutable state.
How is using a database safe then? Transactions."
Transactions are less an occasion where shared mutable state is made 'safe' during concurrency and more a situation where concurrent processes are forced to temporarily interact and operate in a sequential, non-concurrent, manner. They use locks under the covers. Databases manage to be parallel because different processes operate on different sets of data at the same time; they lock when two or more processes attempt to access the same row on a table. Transactional state access is still vulnerable to deadlocks and other difficulties of concurrent programming. This means that transactional state is not 'safe' in the same way that immutable and non-shared state are safe. It's just a much easier way of managing the kind of difficulties you have with shared mutable state than say, raw locks.
The tl;dr is that
> Locks are known to create contention especially between long read transactions and update transactions. MVCC aims at solving the problem by keeping multiple copies of each data item. In this way, each user connected to the database sees a snapshot of the database at a particular instant in time. Any changes made by a writer will not be seen by other users of the database until the changes have been completed (or, in database terms: until the transaction has been committed.)
In other words, the system uses immutable state under the hood plus some atomics/locking to present the abstraction of safe shared mutable state.
That doesn't refute anything from the parent post. In fact you prove the parent's point in the very next sentence:
> How is using a database safe then? Transactions.
Which is to say, you can have shared mutable state, but you need some mechanism to protect access to that state. It is not safe to just access it however you like, which is why database have transactions, and why we have primitives like mutexes for dealing with OS-level threads.
Also, anyone working with an OOP should really read Java Concurrency in Practice. That really helps in terms of learning how to think about multiple threads in a OOP world.
Not sure how events by themselves can solve the threading issues as events can be multi-threaded too. I've seen people write far worse event driven code than multi-threaded code. If people want to use events heavily, I think it's better to use a well known design pattern so others can understand what you are trying to do.
I would say that 99% of the time I am dealing with IO (simply awaiting some asynchronous database/network operation), with the other 1% of cases being things that I actually want to explicitly spread across multiple parallel execution units - I.e. Task.Run() or Parallel.ForEach(). In either case, I am working with the sugar-coated TPL experience, and all the horrific threading code is handled automagically. If I still had to work with threads directly, I would probably have found a different career path by this point.
var session = await _connection.QueryFirstAsync<Session>(GetSessionSql, some session token);
1) Make sure your hardware interface is capable of being accessed by the kernel from multiple threads. Network cards generally allow this, while graphics cards still don't, at least without a lot of overhead.
2) Make sure your application profits form parallelism, and specifically joint parallelism; which is my term for computing that allows many threads to work on the same memory.
Bottom line is in my case only the Java server for my MMO will use threads in a "joint parallel" way. In everything else I will avoid them like a jerrycan full of gas tries to avoid fire.
So I'm transitioning to C with arrays!
Whether he was right or not about threads, it's offensive to insult the person just because you don't like their idea. Likewise for this to be the response to a bug report - if you don't want to support threads, then don't support them rather than lashing out at people who notice your support is buggy.
I think in 2020 he's mostly wrong anyway. Sure, there have been many problems with threads, but...
* this presentation's "Threads should be used only when true CPU concurrency is needed" maybe meant "rarely" in "1995" but means "commonly" in 2020 when single-core performance has been mostly stalled for a while and core counts have risen dramatically.
* There are safer/easier alternate concurrency primitives than mutexes (channels) and at least partial solutions to major problems with threading. For example, in safe Rust there are no data races (even when synchronizing via mutexes). Other problems (deadlocks, contention, other types of race conditions) still exist of course.
* "Threads" vs "events (event loops + callbacks)" as described in this 1995 presentation isn't the whole world, especially today. What about communicating sequential processes with no shared mutable state (such as Erlang's actors)? So to some extent I disagree with the framing of the problem altogether.
* callbacks have their own problems beyond what's described in this presentation. Some that were widespread even in 1995: a string of operations written as a string of callbacks is a lot harder to understand than ones written with the "sequential composition operator" (;) and loops. (Callbacks are basically abandoning structured programming in favor of goto at the macro level.) Likewise harder to debug: you can't just get a stack trace and understand its current state. And some that have become more common since then. These days, event loops are usually multithreaded, so for any cross-request state you have the threading problems as well as the event loop problems. Today I'd say callbacks are the advanced, use them if you need the performance but be wary of the dangers option.
That paper is finally becoming dated.
1) it completely breaks the functional programming model that we all learned as toddlers (instead of call A and then, after that's done, call B, Async is call A which just installs B as a callback, returns immediately and then an "event loop" calls B). Note that promises and tasks and futures are just "syntactic sugar". Personally I'm not a fan. I don't use any of that. I just use callbacks.
2) Even though Async it's great for concurrency, it's not great for parallelism. Everything runs with one thread. So if you want parallel processing you need workers.
But I would argue that issue 1 can be overcome. In fact, I find Async to be quite elegant. I think in the long term people are going to realize that maybe we've had it backwards all along.
Issue 2 is actually not that big of a deal for most things. It's actually somewhat unusual that you need to have some CPU intensive operation running in the background. Maybe image processing, data modelling, etc. But most blocking operations are just I/O operations which are not using CPU that much. If I needed to write some kind of network server, I would look at using libuv as a portable runtime.
First: in Windows 3.1, you got exactly one thread. My former company (BBN Software Products, home of the RS/1 statistical program) managed to get a version of RS/1 on Windows by splitting it into two pieces, each of which ran a single thread. On piece (RS/Client) was the UI; it talked to the "server" using TCP/IP (or a shared memory channel if the client and server were on the same machine)
Second: I also got to help port a networking program over to an SGI box. At the time, the SGI GCC-based compiler could either supports threads, or support exceptions, but not both. (And my "unsupported" I mean, "generated code that would crash even if no exception was ever actually generated"). I couldn't convince the company to keep the threads and dump the exceptions, so instead I had to convert the program to spawn new processes with shared memory (!) to emulate the threads.
TL/DR: actually programming with threads at the time was decidedly unsupported.
I know there's so much more out there, and I'm just not sure how to find relevant problems to solve...it feels like a serious case of "I don't know what I don't know."
I guess I should probably just pick some non-web concept I find interesting and start making something.
Edit: Ah, I see that you did, an hour ago. Very good!
— Joe Armstrong