Removing statics from codebase

[badrishc]

We have used ThreadStatic until now, for storing thread-local variables. This does not work when you have multiple instances of FASTER operating at the same time. This PR fixes the problem by using instance-specific thread-local variables instead. We have implemented our own version of thread-local for this purpose, called FastThreadLocal. Incidentally, FastThreadLocal is optimized to be around 10x faster than ThreadLocal in our micro-benchmarks. There is a configurable (in code) limit of 128 instances of a particular thread-local variable type. This may be relaxed in future if the requirement comes up. With this PR, there are no shared objects between instances, except for the buffer pool which is shared static for the entire process.

[gunaprsd]

FasterThreadLocal<T>.InitializeThread isn't thread-safe. It is being called in LightEpoch.Acquire and can lead to concurrency issues.

[gunaprsd]

Can you explain how the current implementation of FastThreadLocal<T> is instance specific? I am confused because the values array is thread static which I suppose means that two objects of FastThreadLocal<int> access the same values array if done from the same thread.

[badrishc]

FasterThreadLocal<T>.InitializeThread isn't thread-safe. It is being called in LightEpoch.Acquire and can lead to concurrency issues.

Fixed as follows:

            if (values == null)
            {
                Interlocked.CompareExchange(ref values, new T[kMaxInstances], null);
            }

Can you explain how the current implementation of FastThreadLocal<T> is instance specific? I am confused because the values array is thread static which I suppose means that two objects of FastThreadLocal<int> access the same values array if done from the same thread.

FastThreadLocal has a thread-static array, i.e., each thread sees its own copy of the array. Each instance of the object (regardless of threads) will correspond to a unique entry offset (id) in the array. You can visualize this as a 2-d matrix of threads and instances, with one array per thread.

[peterfreiling-zz]

FasterThreadLocal<T>.InitializeThread isn't thread-safe. It is being called in LightEpoch.Acquire and can lead to concurrency issues.

Fixed as follows:

            if (values == null)
            {
                Interlocked.CompareExchange(ref values, new T[kMaxInstances], null);
            }

Why is this necessary for a thread static variable? Won't each thread sees a different copy of "values", making the Interlocked operation unnecessary?

[badrishc]

FasterThreadLocal<T>.InitializeThread isn't thread-safe. It is being called in LightEpoch.Acquire and can lead to concurrency issues.

Fixed as follows:

            if (values == null)
            {
                Interlocked.CompareExchange(ref values, new T[kMaxInstances], null);
            }

Why is this necessary for a thread static variable? Won't each thread sees a different copy of "values", making the Interlocked operation unnecessary?

Correct, it's not needed. My original code is fine, I think.

[peterfreiling-zz]

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