Wait-free Atomic Snapshot

Algorithm

In a parallel environment, the element can be modified during collecting snapshot. Hence, we need to scrutinize that the acquired snapshot is clean or not(or dirty).

Here are the basic idea of scan().

oldCopy = collect()
while(true) {
    newCopy = collect()
    if (oldCopy == newCopy) return oldCopy
    // TODO :: return WAIT-FREE snapshot, which means you have to escape this loop in bounded-time
}

It will be done with N repetitions by checking moved[i]. It records which threads have been observed to move in the scan().

To record that the other thread changed the value,

// In update()
a_table[threadId].stamp++

This will make the line 4 of scan(), oldCopy == newCopy not hold.

moved[] = {false, }
oldCopy = collect()
while(true) {
    newCopy = collect()
    if (oldCopy == newCopy) return oldCopy
    for (j) {
        if (oldCopy[j] != newCopy[j]) {
            if (moved[j]) return oldCopy.snap
            else { 
                moved[j] = true
                oldCopy = newCopy
                continue while
            }
        }
    }
} 

We check moved[j] is true and if so, just return the collected snap. This always returns clean snapshot because we can guarantee that the second change means the snap taken was not interleaved.

Implementation

We use naive c pointers to record or process the arrays to avoid memory leaking.

How to check total count

volatile int status;
enum status_t {PENDING,RUNNING,END};
std::mutex lk; // this should be used only for evaluating performance

status is declared as volatile int to prevent the following codes being optimized.

void Manager::run(int tid) {
    int localUpdate = 0;
    while(status==PENDING); // spinlock
    while(status==RUNNING) {
        snapshot.update(tid, localUpdate);
        ++localUpdate;
    }
    {
        std::unique_lock lock(lk);
        totalUpdate += localUpdate;
    }
}

The status will be changed at here.

for (std::size_t i=0; i < threadCount; i++)
    threads[i] = std::thread(&Manager::run, this, i);
status = RUNNING;
__sync_synchronize();

std::this_thread::sleep_for(std::chrono::seconds(RUNNING_TIME));
__sync_synchronize();
status = END;

Performance

CPU - Intel(R) Xeon(R) CPU E5-2697 v2 @ 2.70GHz (24 core)
RAM - 256GB
OS - Ubuntu 16. 04. 3 LTS