Non-blocking hash map in Rust
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Highly Scalable Non-blocking Hash Map in Rust

The design of this hash map is taken and modified from Dr. Cliff Click's design, originally implemented in Java. The hash map is thread-safe, thus can be safely used as a shared resource among multiple threads, without requiring mutual exclusion, and typesafe, because it is implemented in Rust. It is also scalable and can be shared among a large number of threads without experincing a major bottleneck. The work is currently under development.


The design of this hash map is based in the reasoning of a state machine, where a key-value pair is allowed to be in many valid states. To transfer from one state to another, the table requires an atomic compare-and-swap function to compare-and-swap pointers.

State Machine

Our design bases on the assumption that a key, once inserted, is never deleted directly from the table, but it can be cleaned up in a certian way.

To simplify things and make them easier to understand, let us start by assumming that we never need to resize the table. If it is the case, then only the put_if_match function can alter the states. If a key-value is inserted to the table, then the transition {Empty, Empty} -> {Key, Empty} -> {Key, Value} atomically occurs in the strict order, and only one thread can switch the state at a time (since we are using an atomic compare-and-swap.) If two threads are competing to update the table, say Thread 1 just finishes the transition {Empty, Empty} -> {Key, Empty} and Thread 2 suddenly shows up at {Key, Empty} and wants to update the same key, what would happen? This is quite simple: Assume that Thread 1 wants to update the key-value slot to {Key, Value_1} and Thread 2 wants to update it to {Key, Value_2}, they will both be competing to swap in their value. Suppose that Thread 1 wins, then the transition {Key, Empty} -> {Key, Value_1} occurs. However, Thread 2 will still be trying to update and finally do the transition {Key, Value_1} -> {Key, Value2}, which is a valid transition. For the get function, it will read at any current state and return the found value immediately.

[...more explanation on the way...]

Current State of Development

As for now, the put_if_match and copy_check_and_promote function work fine with multiple threads, but there could be some hidden bugs and possibly memory leak. However, I did not see any bugs while testing it.

Setup & Run

To compile the library using rustc 0.10:

$ rustc --crate-type=lib

To run a test in the main() function:

$ rustc -L . && ./main