• Joe bloow.edu (64/64) Dec 18 2023 How does one really use atomics in D?

Joe bloow.edu <Joe bloow.edu> writes:

How does one really use atomics in D?

shared string[int] AA;
AA[s[2]].atomicStore(s[1]);

causes a hangup.

If I just assign it "works":

AA[s[2]] = s[1];

I've had similar issues where:

atomicOp!"+="(X[Y],1);

fails and I have to use

X[Y].atomicStore(1);		

or

atomicOp!"+="(count,1); // fails (does not increment count)

so I have to create a new variable

int rcount = atomicFetchAdd(count,1);

and this will increment count.


In fact, I don't know what works or what doesn't except by trial 
and error. I don't even know how to go about thinking about these 
issues. In my mind, when using multiple threads and shared 
variables I try to use atomics to avoid any race conditions.  
Usually this involves incrementing or storing values. I try to 
use atomics that replace the single threaded concept but they do 
not always seem to work.


Are there any known issues or is it just me? Should it just work? 
is there any good reference that gets ones mind right for 
multithreading? I have some idea but I rarely do multithreading 
until lately(I guess because I now have the HP to care). I think 
the main issue I tend to have is trying to convert single 
threaded code in to multiple threads and missing certain key 
issues.

E.g.,

I was doing a parallel for and it wasn't really working(it seems 
the task pool stalls the entire pool until all tasks are 
complete):

for(s; parallel(arr))
{
    // do stuff with s.
}

So I converted to using threads:

for(s; arr)
{
    new Thread({
    // do stuff with s.
    }).start();
} // slightly more complex as I limited the total number of 
threads

But as I spun up threads I would get different threads working on 
the same s. I imagined that this likely had something to do with 
the variable being on the stack being reused. so I wrapped the 
thread in a function:

for(ss; arr)
{
    void foo(typeof(ss) s)
    {
        new Thread({
        // do stuff with s.
        }).start();
    }
    foo(ss);
}

and this seemed to solve the problem. I imagine this is because 
ss is duped by the function call essentially detaching it from 
the loop code so it is unique across the thread.

But I guess I need to learn exactly how to deal with all these 
problems rather than just hacking away.