Memory ordering

Article summary

Memory ordering is the order of accesses to computer memory by a CPU. Memory ordering depends on both the order of the instructions generated by the compiler at compile time and the execution order of the CPU at runtime. However, memory order is of little concern outside of multithreading and memory-mapped I/O, because if the compiler or CPU changes the order of any operations, it must necessarily ensure that the reordering does not change the output of ordinary single-threaded code. The memory order is said to be strong or sequentially consistent when either the order of operations cannot change or when such changes have no visible effect on any thread. Conversely, the memory order is called weak or relaxed when one thread cannot predict the order of operations arising from another thread. Many naïvely written parallel algorithms fail when compiled or executed with a weak memory order. The problem is most often solved by inserting memory barrier instructions into the program. In order to fully utilize the bandwidth of different types of memory such as caches and memory banks, few compilers or CPU architectures ensure perfectly strong ordering. Among the commonly used architectures, x86-64 processors have the strongest memory order, but may still defer memory store instructions until after memory load instructions.…

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