July 22

On 7/22/21 1:46 AM, seany wrote:

>

Consider :

    int [] ii;
    foreach(i,dummy; parallel(somearray)) {
      ii ~= somefunc(dummy);
    }

This is not safe, because all threads are accessing the same array and trying to add values and leading to collision.

Correct. You must synchronize on ii.

>

But :

    int [] ii;
    ii.length = somearray.length;
    foreach(i,dummy; parallel(somearray)) {
      ii[i] ~= somefunc(dummy);
    }

This is safe. In this case, threads are accessing an unique memory location each.

This isn't valid code, because you can't append to an integer. Though I think I know what you meant. Is it thread-safe (assuming the array elements are appendable)? I think so, but I'd have to see a working example.

>

But what about this :

    int [ string ] ii;
    ii.length = somearray.length;
    foreach(i,dummy; parallel(somearray)) {
      string j = generateUniqueString(i);
      ii[j] ~= somefunc(dummy);
    }

Is this also guaranteed thread safe?

First, this also isn't valid code. You can't set the length of an AA. But I'm assuming that length setting is really a placeholder for initialization (in your real code). Also, again, you cannot append to an integer.

Second, as long as you don't modify the AA structure, you can parallel with it. In this case, you are generating some string, and appending to that. I don't know what your generateUniqueString is doing, nor do I know what's actually stored as keys in the AA as your initialization code is hidden.

If every j is guaranteed to already exist as a key in the AA, and the code is made to be valid, then I think it is thread-safe.

If any access with a key j is inserting a new AA bucket, it is not thread-safe.

However, this is a tall order, and highly depends on your code. The compiler cannot help you here.

>

In my 5 runs, I did not see any problems, but I'd like to confirm. Thank you.

Testing 5 times is not a substitute for proving the thread safety.

I have learned one thing long ago about threads and race conditions. Just don't do it. Ever. Even if you test 10000 times, and it doesn't fail, it will eventually. I've had code that hit a race condition after 2 weeks of running flat-out. Was one of the hardest things I ever had to debug.

-Steve

July 23

On Thursday, 22 July 2021 at 16:39:45 UTC, Steven Schveighoffer wrote:

>

On 7/22/21 1:46 AM, seany wrote:

>

[...]

Correct. You must synchronize on ii.

>

[...]

This isn't valid code, because you can't append to an integer. Though I think I know what you meant. Is it thread-safe (assuming the array elements are appendable)? I think so, but I'd have to see a working example.

[...]

you are right. in the pseudocode, i wanted to say: ii[i] = somefunc(dummy);

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