Replay system
Encyclopedia
The Replay system is a little known subsystem within the Intel Pentium 4
processor. Its primary function is to catch operations that have been mistakenly sent for execution by the processor's scheduler
. Operations caught by the replay system are then re-executed in a loop until the conditions necessary for their proper execution have been fulfilled.
. These higher clock speeds necessitated very lengthy pipelines (up to 31 stages in the Prescott core). Because of this, there are 6 stages between the scheduler and the execution unit
s in the Prescott core. In an attempt to maintain acceptable performance, Intel engineers had to design the scheduler to be very optimistic.
.) The most common reason execution fails is that the requisite data is not available, which itself is most likely due to a cache miss. When this happens, the replay system springs into action. The replay system signals the scheduler to stop, and then repeatedly executes the failed string of dependent operations until they have completed successfully.
is in use, the replay system will prevent the other thread from utilizing the execution units. This is the true cause of any performance degradation concerning hyper-threading. In Prescott, the Pentium 4 gained a replay queue, which reduces the time the replay system will occupy the execution units.
In other cases, where each thread is processing different types of operations, the replay system will not interfere, and a performance increase can appear. This explains why performance with hyper-threading is application dependent.
Pentium 4
Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...
processor. Its primary function is to catch operations that have been mistakenly sent for execution by the processor's scheduler
Instruction scheduling
In computer science, instruction scheduling is a compiler optimization used to improve instruction-level parallelism, which improves performance on machines with instruction pipelines...
. Operations caught by the replay system are then re-executed in a loop until the conditions necessary for their proper execution have been fulfilled.
Origins
The replay system came about as a result of Intel's now-defunct quest for ever increasing clock speedsMegahertz Myth
The megahertz myth, or less commonly the gigahertz myth, refers to the misconception of only using clock rate to compare the performance of different microprocessors...
. These higher clock speeds necessitated very lengthy pipelines (up to 31 stages in the Prescott core). Because of this, there are 6 stages between the scheduler and the execution unit
Execution unit
In computer engineering, an execution unit is a part of a CPU that performs the operations and calculations called for by the Branch Unit, which receives data from the CPU...
s in the Prescott core. In an attempt to maintain acceptable performance, Intel engineers had to design the scheduler to be very optimistic.
Operation
The scheduler in a Pentium 4 processor is so aggressive that it will send operations for execution without a guarantee that they can be successfully executed. (Among other things, the scheduler assumes all data is in level 1 cacheCache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
.) The most common reason execution fails is that the requisite data is not available, which itself is most likely due to a cache miss. When this happens, the replay system springs into action. The replay system signals the scheduler to stop, and then repeatedly executes the failed string of dependent operations until they have completed successfully.
Performance considerations
Not surprisingly, in some cases the replay system can have a very bad impact on performance. Under normal circumstances, the execution units in the Pentium 4 are in use roughly 33% of the time. When the replay system is invoked, it will occupy execution units nearly every available cycle. This wastes power, which is an increasingly important architectural design metric, but poses no performance penalty because the execution units would be sitting idle anyway. However, if hyper-threadingHyper-threading
Hyper-threading is Intel's term for its simultaneous multithreading implementation in its Atom, Intel Core i3/i5/i7, Itanium, Pentium 4 and Xeon CPUs....
is in use, the replay system will prevent the other thread from utilizing the execution units. This is the true cause of any performance degradation concerning hyper-threading. In Prescott, the Pentium 4 gained a replay queue, which reduces the time the replay system will occupy the execution units.
In other cases, where each thread is processing different types of operations, the replay system will not interfere, and a performance increase can appear. This explains why performance with hyper-threading is application dependent.
See also
- Instruction pipelineInstruction pipelineAn instruction pipeline is a technique used in the design of computers and other digital electronic devices to increase their instruction throughput ....
- Speculative executionSpeculative executionSpeculative execution in computer systems is doing work, the result of which may not be needed. This performance optimization technique is used in pipelined processors and other systems.-Main idea:...
- Out-of-order executionOut-of-order executionIn computer engineering, out-of-order execution is a paradigm used in most high-performance microprocessors to make use of instruction cycles that would otherwise be wasted by a certain type of costly delay...
- Simultaneous multithreadingSimultaneous multithreadingSimultaneous multithreading, often abbreviated as SMT, is a technique for improving the overall efficiency of superscalar CPUs with hardware multithreading...
- Data dependencyData dependencyA data dependency in computer science is a situation in which a program statement refers to the data of a preceding statement. In compiler theory, the technique used to discover data dependencies among statements is called dependence analysis.There are three types of dependencies: data, name, and...