Assists

Metric Description

This metric estimates cycles fraction the CPU retired uops delivered by the Microcode_Sequencer as a result of Assists. Assists are long sequences of uops that are required in certain corner-cases for operations that cannot be handled natively by the execution pipeline. For example, when working with very small floating point values (so-called Denormals), the FP units are not set up to perform these operations natively. Instead, a sequence of instructions to perform the computation on the Denormals is injected into the pipeline. Since these microcode sequences might be hundreds of uops long, Assists can be extremely deleterious to performance and they can be avoided in many cases.

Possible Issues

A significant portion of execution time is spent in microcode assists.

Tips

1. Examine the FP_ASSIST and OTHER_ASSISTS events to determine the specific cause.

2. Add options eliminating x87 code and set the compiler options to enable DAZ (denormals-are-zero) and FTZ (flush-to-zero).