mtak- / lstm

lstm is a header only C++14 take on Software Transactional Memory (STM)

lstm

WIP: This library was written for fun and could have ubgs :)

lstm is a header only implementation of Software Transactional Memory (STM) designed to be simple to use, and flexible. It's only been tested on Apple Clang so YMMV.

Features

• It's header only! Simply #include <lstm/lstm.hpp> and you're off.
• Custom allocators are fully supported.
• C++14 implementation.
• lstm shares objects not memory. Polymorphic, and non-POD types are safe to use.*
• Type traits are used to create user friendly static_assert error messages.
• If you want the library to be SFINAE friendly, simply #define LSTM_MAKE_SFINAE_FRIENDLY.
• The library heavily uses <atomic> to provide low overhead reads and writes.
• Nested transactions automatically merge into the rootmost transaction.
• Read only transactions are supported, providing a performance boost.
• Aborted transactions unwind the stack, so all of your destructors will be run.
• Lower level operations, while not the default, are exposed if you need some extra performance.
• thread_local means there's no need to register threads manually.
• Per thread caches result in very little overhead in maintaining a read and write set.
• Relativistic programming serves as the backbone for resource reclamation.
• The commit algorithm can be thought of as distributed seqlock which helps to reduce contention on cache lines.

* non-POD types work as long as the following hold. 1) You don't care if an objects destructor is called later than you expect, and 2) it's ok if writing to a variable creates a new instance of that type and the old one is destroyed after the transaction completes. 1) and 2) are true for most types, but not all types.

Usage

lstm is header only:

1. Add lstm/include to your header search paths
2. In your source files #include <lstm/lstm.hpp>
3. All types/functions live in the lstm:: namespace.
4. Done

Building Tests

Debug

$ mkdir -p build/debug && cd build/debug
$ cmake -G"Unix Makefiles" -DCMAKE_BUILD_TYPE=Debug ../..
$ make -j8 && make test

Release

$ mkdir -p build/release && cd build/release
$ cmake -G"Unix Makefiles" -DCMAKE_BUILD_TYPE=Release ../..
$ make -j8 && make test

Getting Started

Simply wrap shared data up in a var, and then start a transaction as follows.

// declare your shared variables, and initialize them as though they weren't wrapped by an lstm::var
static lstm::var<std::vector<int>> buffer{0, 1, 2, 3, 4, 5, 6};
static lstm::var<std::vector<int>> active;

std::size_t publish_buffer() {
    return lstm::atomic([&](const lstm::transaction tx) {
        std::vector<int> tmp = buffer.get(tx);
        const auto amount_published = tmp.size();
        active.set(tx, std::move(tmp));
        
        return amount_published;
    });
}

Transactional variables (shared variables) must be wrapped in the var template. var's detect the constructors of the wrapped type, and have var() constructors to match.

A transaction is started by calling atomic and passing in a callable that takes, by value or const&, either a transaction (read-write) or a read_transaction (read only). If the callable takes both (e.g. [](auto tx) {}) then the transaction overload is preferred.

Getting values out of a var is as simple as calling the method get passing in the transaction object.

Setting values is done by calling some_var.set(tx, value_to_set_to).