[Haskell-cafe] Can you do everything without shared-memory concurrency?

[Bruce Eckel]

So this is the kind of problem I keep running into. There will seem to be
consensus that you can do everything with isolated processes message passing
(and note here that I include Actors in this scenario even if their
mechanism is more complex). And then someone will pipe up and say "well, of
course, you have to have threads" and the argument is usually "for
efficiency."
I make two observations here which I'd like comments on:

1) What good is more efficiency if the majority of programmers can never get
it right? My position: if a programmer has to explicitly synchronize
anywhere in the program, they'll get it wrong. This of course is a point of
contention; I've met a number of people who say "well, I know you don't
believe it, but *I* can write successful threaded programs." I used to think
that, too. But now I think it's just a learning phase, and you aren't a
reliable thread programmer until you say "it's impossible to get right"
(yes, a conundrum).

2) What if you have lots of processors? Does that change the picture any?
That is, if you use isolated processes with message passing and you have as
many processors as you want, do you still think you need shared-memory
threading?

A comment on the issue of serialization -- note that any time you need to
protect shared memory, you use some form of serialization. Even optimistic
methods guarantee serialization, even if it happens after the memory is
corrupted, by backing up to the uncorrupted state. The effect is the same;
only one thread can access the shared state at a time.

On Tue, Sep 9, 2008 at 4:03 AM, Sebastian Sylvan <sebastian.sylvan at gmail.com
> wrote:

>
>
> On Mon, Sep 8, 2008 at 8:33 PM, Bruce Eckel <bruceteckel at gmail.com> wrote:
>
>> As some of you on this list may know, I have struggled to understand
>> concurrency, on and off for many years, but primarily in the C++ and
>> Java domains. As time has passed and experience has stacked up, I have
>> become more convinced that while the world runs in parallel, we think
>> sequentially and so shared-memory concurrency is impossible for
>> programmers to get right -- not only are we unable to think in such a
>> way to solve the problem, the unnatural domain-cutting that happens in
>> shared-memory concurrency always trips you up, especially when the
>> scale increases.
>>
>> I think that the inclusion of threads and locks in Java was just a
>> knee-jerk response to solving the concurrency problem. Indeed, there
>> were subtle threading bugs in the system until Java 5. I personally
>> find the Actor model to be most attractive when talking about
>> threading and objects, but I don't yet know where the limitations of
>> Actors are.
>>
>> However, I keep running across comments where people claim they "must"
>> have shared memory concurrency. It's very hard for me to tell whether
>> this is just because the person knows threads or if there is truth to
>> it.
>
>
> For correctness, maybe not, for efficiency, yes definitely!
>
> Imagine a program where you have a huge set of data that needs to be
> modified (in some sense) over time by thousands of agents. E.g. a game
> simulation.
> Now, also imagine that every agent could *potentially* modify every single
> piece of data, but that every agent *typically* only touches two or three
> varibles here and there. I.e. the collisions between the potential
> read/write sets is 100%, while the collisions for the actual read/write sets
> is very very low.
>
> How would you do this with threads and message passing? Well you could have
> one big thread owning all of your data that takes "update" messages, and
> then "updates" the world for you (immutably if you wish, by just replacing
> its "world" variable with a new one containing your update), but now you've
> effectively serialized all your interactions with the "world", so you're not
> really concurrent anymore!
>
> So you could decompose the world into multiple threads using some
> application-specific logical sudivision, but then you're effectively just
> treating each thread as a mutable variable with an implicit lock (with the
> risks of deadlock that comes with it - remember we don't know the read/write
> set in advance - it could be the entire world - so we can't just order our
> updates in some global way here), so you're really just doing shared mutable
> state again, and gain little from having threads "simulate" your mutable
> cells...
>
> What you really need for this is some way for each agent to update this
> shared state *in parallel*, without having to block all other agents
> pessimistically, but instead only block other agents if there was an
> *actual* conflict. STM seems to be the only real hope for that sort of thing
> right now.
>  IMO my list of preferred methods goes like this:
> 1. Purely functional data parallelism
> 2. Purely functional task parallelism (using e.g. strategies)
> 3. Message passing with no (or very minimal) shared state (simulated using
> threads as "data servers" or otherwise)
> (3.5. Join patterns? Don't have enough experience with this, but seems sort
> of nice?)
> 4. Shared state concurrency using STM
> 5. Shared state concurrency using locks
> 6. Lockless programming.
>
> So while I wouldn't resort to any shared state concurrency unless there are
> good reasons for why the other methods don't work well (performance is a
> good reason!), there are still situations where you need it, and a general
> purpose language had better supply a way of accessing those kinds of
> facilities.
>
> --
> Sebastian Sylvan
> +44(0)7857-300802
> UIN: 44640862
>



-- 
Bruce Eckel
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