Instructions per cycle

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In computer architecture, instructions per cycle (IPC) is one aspect of a processor's performance: the average number of instructions executed for each clock cycle. It is the multiplicative inverse of cycles per instruction.[1]

Explanation[edit]

Calculation of IPC[edit]

The number of instructions per second and floating point operations per second for a processor can be derived by multiplying the number of instructions per cycle with the clock rate (cycles per second given in Hertz) of the processor in question. The number of instructions per second is an approximate indicator of the likely performance of the processor.

The number of instructions executed per clock is not a constant for a given processor; it depends on how the particular software being run interacts with the processor, and indeed the entire machine, particularly the memory hierarchy. However, certain processor features tend to lead to designs that have higher-than-average IPC values; the presence of multiple arithmetic logic units (an ALU is a processor subsystem that can perform elementary arithmetic and logical operations), and short pipelines. When comparing different instruction sets, a simpler instruction set may lead to a higher IPC figure than an implementation of a more complex instruction set using the same chip technology; however, the more complex instruction set may be able to achieve more useful work with fewer instructions.

Factors governing IPC[edit]

A given level of instructions per second can be achieved with a high IPC and a low clock speed (like the AMD Athlon and early Intel's Core Series), or from a low IPC and high clock speed (like the Intel Pentium 4 and to a lesser extent the AMD Bulldozer). Both are valid processor designs, and the choice between the two is often dictated by history, engineering constraints, or marketing pressures.[original research?] However high IPC with high frequency gives the best performance.

Instructions per cycle for various processors[edit]

CPU Family Dual precision Single precision
Intel Core and Intel Nehalem 4 CPI 8 SP CPI
Intel Sandy Bridge and Intel Ivy Bridge 8 DP CPI 16 SP CPI
Intel Haswell, Intel Broadwell and Intel Skylake 16 DP CPI 32 SP CPI
Intel Xeon Skylake (AVX-512) 32 DP CPI 64 SP CPI
AMD K10 6 DP CPI 12 SP CPI
AMD Bulldozer, AMD Piledriver and AMD Steamroller, per module (two cores) 12 DP CPI 24 SP CPI
AMD Ryzen 16 DP CPI 32 SP CPI
Intel Atom (Bonnell, Saltwell, Silvermont and Goldmont) 2 DP CPI 4 SP CPI
AMD Bobcat 2 DP CPI 4 SP CPI
AMD Jaguar 4 DP CPI 8 SP CPI
ARM Cortex-A7 1 CPI 8 SP CPI
ARM Cortex-A9 1 CPI 8 SP CPI
ARM Cortex-A15 1 DP CPI 8 SP CPI
ARM Cortex-A32 2 DP CPI 8 SP CPI
ARM Cortex-A35 2 DP CPI 8 SP CPI
ARM Cortex-A53 2 DP CPI 8 SP CPI
ARM Cortex-A57 2 DP CPI 8 SP CPI
ARM Cortex-A72 2 DP CPI 8 SP CPI
Qualcomm Krait 1 DP CPI 8 SP CPI
Qualcomm Kryo 2 DP CPI 8 SP CPI
IBM PowerPC A2 (Blue Gene/Q), per core 8 DP CPI (SP elements are extended to DP and processed on the same units)
IBM PowerPC A2 (Blue Gene/Q), per thread 4 DP CPI (SP elements are extended to DP and processed on the same units)
Intel Xeon Phi (Knights Corner), per core 16 DP CPI 32 SP CPI
Intel Xeon Phi (Knights Corner), per thread (two per core) 8 DP CPI 16 SP CPI
Standard GPU Different 2 SP CPI

Generally, large of processor register shows how big numbers core of processor can count one time. Number of registers is important too, because they can connect together for a moment with some instructions.

Computer speed[edit]

The useful work that can be done with any computer depends on many factors besides the processor speed. These factors include the instruction set architecture, the processor's microarchitecture, and the computer system organization (such as the design of the disk storage system and the capabilities and performance of other attached devices), the efficiency of the operating system, and most importantly the high-level design of the application software in use.

For users and purchasers of a computer system, instructions per clock is not a particularly useful indication of the performance of their system. For an accurate measure of performance relevant to them, application benchmarks are much more useful. Awareness of its existence is useful, in that it provides an easy-to-grasp example of why clock speed is not the only factor relevant to computer performance.

See also[edit]

References[edit]

  1. ^ John L. Hennessy, David A. Patterson. "Computer architecture: a quantitative approach". 2007.