- Hardware scout
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 torunahead 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.Rock Processor Scout
Sun's upcoming
Rock processor uses a form of hardware scout. However, any computations in run-ahead mode that do not depend on the cache miss may be retired immediately. This allows both prefetching and traditional instruction-level parallelism.couting vs. SMT
Scouting and
Simultaneous multithreading (SMT) both use hardware threads to fight the memory wall. With scouting, the scout thread runs the instructions from the same instruction stream as the instruction that causes the pipeline stall. In the case of SMT, the SMT thread executes instruction in another context.Thus, SMT increases the throughput of the processor while scouting increases the performance by lowering the number of cache misses.
See also
*
Rock processor
*Runahead References
* [http://portal.acm.org/citation.cfm?id=263597&coll=portal&dl=ACM Improving data cache performance by pre-executing instructions under a cache miss]
* [http://citeseer.ist.psu.edu/465945.html Improving processor performance by dynamically preprocessing the instruction stream]
* [http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/mags/mi/&toc=comp/mags/mi/2005/03/m3toc.xml&DOI=10.1109/MM.2005.49 High Performance Throughput Computing]
* [http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?isnumber=26557&arnumber=1183532 Runahead execution: an alternative to very large instruction windows for out-of-order processors]
* [http://arstechnica.com/news.ars/post/20080204-sun-can-you-smell-what-the-rock-is-cookin.html Sun: Can you smell what the Rock is Cookin']
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