Hardware scout is a technique that uses otherwise idle
processor
Processor may refer to:
Computing Hardware
* Processor (computing)
**Central processing unit (CPU), the hardware within a computer that executes a program
*** Microprocessor, a central processing unit contained on a single integrated circuit (I ...
execution resources to perform
prefetching during
cache
Cache, caching, or caché may refer to:
Places United States
* Cache, Idaho, an unincorporated community
* Cache, Illinois, an unincorporated community
* Cache, Oklahoma, a city in Comanche County
* Cache, Utah, Cache County, Utah
* Cache County ...
misses. When a thread is stalled by a cache miss, the processor pipeline checkpoints the
register file, switches to
runahead
{{No footnotes, date=November 2010
Runahead is a technique that allows a microprocessor to pre-process instructions during cache miss cycles instead of stalling. The pre-processed instructions are used to generate instruction and data stream pre ...
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
Rock processor (later canceled) used 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
Instruction-level parallelism (ILP) is the parallel or simultaneous execution of a sequence of instructions in a computer program. More specifically ILP refers to the average number of instructions run per step of this parallel execution.
Disc ...
.
Scouting vs. SMT
Scouting and
simultaneous multithreading (SMT) both use hardware threads to fight the
memory wall
Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the ...
. 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
{{No footnotes, date=November 2010
Runahead is a technique that allows a microprocessor to pre-process instructions during cache miss cycles instead of stalling. The pre-processed instructions are used to generate instruction and data stream pre ...
References
Improving data cache performance by pre-executing instructions under a cache miss*
ttp://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 ComputingRunahead execution: an alternative to very large instruction windows for out-of-order processors Instruction processing