The Algorithm

As I’ve said before, I think of the vector as two connected systems: memory and synchronization. By “The Algorithm”, I mean the synchronization aspect. To recap, synchronization is controlled by two little data structures, the Descriptor and the WriteDescriptor. These data structures describe the vector itself and a write operation, respectively.

I think the best way to explain the algorithm is to dive right in.


First, I want to explain a little routine called complete_write. This function is true to its name and completes a write.

Write means "write operation", in this context, a push or pop. In my experience, "write" has been a more colloquial term used in CS for whenever we make a modification to something. Really anything can technically be a "write", but I would say things that are more final are "writes". For example, incrementing a loop variable is pretty insignificant in the grand scheme of things, so it's not really a "write", but increasing the size of the vector is an important "write". This usage might also be particular to concurrent programming, where balancing reads/writes is an important consideration for designing a data structure. Concurrent data structures are often designed for infrequent writes and frequent reads. Modifications to databases (which are heavily concurrent) can also be called writes. tl;dr a "write" in this case means the details describing a particular instance of "writing"

complete_write takes two arguments, a WriteDescriptor, and the vector itself. complete_write applies the write operation described in the WriteDescriptor on the vector. Recall that a WriteDescriptor contains three things: a reference/pointer to the location where the write will take place, a new value to write, and an old value that we loaded in from the location.

First we perform a compare_exchange using the data in the WriteDescriptor. We only swap in the new data if the data at the location of the swap matches the old data we have. If the compare_exchange succeeds, this means that we swapped in the value we want to write. If it fails, it means someone else beat us to it and performed the write. Remember, many threads can access the vector's Descriptor and WriteDescriptor at once, so many threads will be trying to complete the same write. Only one of them can succeed. It's a fight to the death! Arrghhh!!!

I'm kidding. After performing the compare_exchange, successful for not, we modify the vector to indicate that there is no pending write operation. If all threads do this, at least once will succeed, and all will indicate that there is no pending write operations. Though some of the threads may be sad because their compare_exchange failed, the vector is happy because it's in a consistent and correct state.


Now that we know writes are actually performed, let’s get into how a push operation works. Here are the steps:

  1. Load in the current Descriptor.

  2. If the Descriptor contains a write operation, complete it . This is important because it ensures that before any new write operation happens, the previous one is completed. We cannot do anything before completing the previous write operation, so all operations will eventually get executed.

  3. Calculate which bucket our new element will go into.

  4. If that bucket has not been allocated memory yet, do so.

  5. Make a new WriteDescriptor. The new value in the WriteDescriptor will be the data passed into the push function.

  6. Make a new Descriptor which contains the following data: the size held in the current Descriptor + 1, and the new WriteDescriptor.

  7. Now, here comes the part that makes this a compare-and-swap or compare_exchange algorithm. We compare_exchange the new Descriptor we made with the old one. If the Descriptor held in the vector didn't change, our new Descriptor will replace it. If it did change, we will fail to swap in our new Descriptor, and we go back to Step 1.

    Note: I think it's important to consider why this routine (particularly step 6) ensures correctness. If the compare_exchange succeeds, this means that the vector did not change in the time it took us to prepare a new Descriptor. Why is this important? It means our assumptions about the vector's state did not change. In our new Descriptor, we used the size from the Descriptor we loaded in, and incremented that. So, if the size we loaded in was 4, our new Descriptor would say the size of the vector is 5. Now, imagine that we could just swap in our fresh Descriptor without comparing it with the current one. If someone else was also trying to push, their Descriptor might get swapped in before ours. It would say the size of the vector is 5, because it made the same assumptions we did. Then we swap in our Descriptor, our Descriptor would maintain that the size of the vector is 5, even though it should be 6 because there were two push operations. Furthermore, we would overwrite the element that was pushed on by the first call to push, because both our WriteDescriptors would be referencing the same location in memory. This is terrible! compare_exchange is our friend.

  8. Now that we have swapped in our Descriptor, we execute the WriteDescriptor we made using complete_write, finalizing the changes we want to make to the vector.

And that's a push!

Pop pretty much works the same except for some small variations, so we'll get into that when we implement push/pop. However, the way we make sure changes are valid using compare_exchange is identical for both operations.

I think it's finally time to start looking at some code. When I was writing code, it felt very different from reasoning about the theory. I really felt like I had to consider every line I wrote and every decision I made. I'll walk you through what I came up with now.

Note: we're going to first write a version of the vector that doesn't reclaim the memory it uses; it leaks.