Threading Questions

[Russel Winder via Digitalmars-d-learn]

I hadn't answered as I do not have answers to the questions you ask. My
reason: people should not be doing their codes using these low-level
shared memory techniques. Data parallel things should be using the
std.parallelism module. Dataflow-style things should be using spawn and
channels – akin to the way you do things in Go.

So to give you an answer I would go back a stage, forget threads,
mutexes, synchronized, etc. and ask what do you want you workers to do?
If they are to do something and return a result then spawn and channel
is exactly the right abstraction to use. Think "farmer–worker", the
farmer spawns the workers and then collects their results. No shared
memory anywyere – at least not mutable.

On Fri, 2015-09-25 at 15:19 +0000, bitwise via Digitalmars-d-learn
wrote:
> Hey, I've got a few questions if anybody's got a minute.
> 
> I'm trying to wrap my head around the threading situation in D. 
> So far, things seem to be working as expected, but I want to 
> verify my solutions.
> 
> 1) Are the following two snippets exactly equivalent(not just in 
> observable behaviour)?
> a)
> 
> Mutex mut;
> mut.lock();
> scope(exit) mut.unlock();
> 
> b)
> Mutex mut;
> synchronized(mut) { }
> 
> Will 'synchronized' call 'lock' on the Mutex, or do something 
> else(possibly related to the interface Object.Monitor)?
> 
> 2) Phobos has 'Condition' which takes a Mutex in the constructor. 
> The documentation doesn't exactly specify this, but should I 
> assume it works the same as std::condition_variable in C++?
> 
> For example, is this correct?
> 
> Mutex mut;
> Condition cond = new Condition(mut);
> 
> // mut must be locked before calling Condition.wait
> synchronized(mut)  // depends on answer to (1)
> {
>      // wait() unlocks the mutex and enters wait state
>      // wait() must re-acquire the mutex before returning when 
> cond is signalled
>      cond.wait();
> }
> 
> 3) Why do I have to pass a "Mutex" to "Condition"? Why can't I 
> just pass an "Object"?
> 
> 4) Will D's Condition ever experience spurious wakeups?
> 
> 5) Why doesn't D's Condition.wait take a predicate? I assume this 
> is because the answer to (4) is no.
> 
> 6) Does 'shared' actually have any effect on non-global variables 
> beside the syntactic regulations?
> 
> I know that all global variables are TLS unless explicitly marked 
> as 'shared', but someone once told me something about 'shared' 
> affecting member variables in that accessing them from a separate 
> thread would return T.init instead of the actual value... or 
> something like that. This seems to be wrong(thankfully).
> 
> For example, I have created this simple Worker class which seems 
> to work fine without a 'shared' keyword in sight(thankfully). I'm 
> wondering though, if there would be any unexpected consequences 
> of doing things this way.
> 
> http://dpaste.com/2ZG2QZV
> 
> 
> 
> 
> Thanks!
>      Bit
-- 
Russel.
=============================================================================
Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder at ekiga.net
41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel at winder.org.uk
London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder

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