ID 683230
Date 11/12/2018
Public

## 3.5.5.12. Fitter Seed

The Fitter seed affects the initial placement configuration of the design. Any change in the initial conditions changes the Fitter results; accordingly, each seed value results in a somewhat different fit. You can experiment with different seeds to attempt to obtain better fitting results and timing performance.

Changes in the design impact performance between compilations. This random variation is inherent in placement and routing algorithms—it is impossible to try all seeds and get the absolute best result.

Note: Any design change that directly or indirectly affects the Fitter has the same type of random effect as changing the seed value. This includes any change in source files, Compiler Settings or Timing Analyzer Settings. The same effect can appear if you use a different computer processor type or different operating system, because different systems can change the way floating point numbers are calculated in the Fitter.

If a change in optimization settings marginally affects the register-to-register timing or number of failing paths, you cannot always be certain that your change caused the improvement or degradation, or whether it is due to random effects in the Fitter. If your design is still changing, running a seed sweep (compiling your design with multiple seeds) determines whether the average result improved after an optimization change, and whether a setting that increases compilation time has benefits worth the increased time, such as with physical synthesis settings. The sweep also shows the amount of random variation to expect for your design.

If your design is finalized you can compile your design with different seeds to obtain one optimal result. However, if you subsequently make any changes to your design, you might need to perform seed sweep again.

Click Assignments > Compiler Settings to control the initial placement with the seed. You can use the DSE II to perform a seed sweep easily.

To specify a Fitter seed use the following Tcl command :

set_global_assignment -name SEED <value>

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