Hardware scout

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Hardware scout is a technique that uses otherwise idle processor execution resources to perform prefetching during cache misses. When a thread is stalled by a cache miss, the processor pipeline checkpoints the register file, switches to runahead mode, and continues to issue instructions from the thread that is waiting for memory. The thread of execution in run-ahead mode is known as a scout thread. When the data returns from memory, the processor restores the register file contents from the checkpoint, and switches back to normal execution mode.

The computation during run-ahead mode is discarded by the processor; nevertheless, scouting provides speedup because memory level parallelism (MLP) is increased. The cache lines brought into the cache hierarchy are often used by the processor again when it switches back to normal mode.