PCGOPT

PCGOPT, Lev_Diff, Fallback, ReduceIO, StrmCk, Wrtfull, LM_Key
Controls PCG solver options.

Valid Products: Pro | Premium | Enterprise | PrepPost | Solver | AS add-on

Lev_Diff

Indicates the level of difficulty of the analysis. Valid settings are AUTO or 0 (default), 1, 2, 3, 4, or 5. This option applies to both the PCG solver when used in a static, full transient, or full harmonic analyses and to the PCG Lanczos method in modal analyses.

Specify AUTO to allow Mechanical APDL to select the proper level of difficulty for the model. For most single-field structural analyses, AUTO sets Lev_Diff = 2.
Lower values (1 or 2) generally provide the best performance for well-conditioned problems.
Values of 3 or 4 generally provide the best performance for ill-conditioned problems; however, higher values may increase the solution time for well-conditioned problems. Higher level-of-difficulty values typically require more memory.
The highest value of 5 essentially performs a factorization of the global matrix (similar to the sparse solver) and may require a very large amount of memory. This level is generally recommended for small- to medium-sized problems when using the PCG Lanczos mode-extraction method. Lev_Diff = 5 is not supported for a full harmonic analysis.

For example, models containing elongated elements (that is, elements with high aspect ratios) and models containing contact elements can lead to ill-conditioned problems. To determine if your problem is ill-conditioned, view the Jobname.pcs file to see the number of PCG iterations needed to reach a converged solution. Generally, static, full transient, or full harmonic solutions that require more than 1500 iterations are considered to be ill-conditioned for the PCG solver.

Fallback

Controls whether Mechanical APDL switches to the sparse direct solver automatically (EQSLV,SPARSE) under certain conditions. (The criteria are listed below.) When Mechanical APDL switched the equation solver, the simulation attempts to continue without interruption.

AUTO

 — 

Automatically switch to the sparse solver when one of the following conditions apply (default):

  • The assembled matrix is detected to be indefinite.

  • The PCG solver requires more than 2000 iterations to reach convergence.

  • The PCG solver fails to converge.

ON

 — 

More aggressive fallback criteria. Automatically switch to the sparse solver when one of the following conditions apply:

  • The assembled matrix is detected to be indefinite.

  • The PCG solver requires more than 1500 iterations to reach convergence.

  • The PCG solver fails to converge.

OFF

 — 

Disables the fallback logic so that there is no automatic switching, and only the PCG solver is used to solve the equations for this simulation.

After switching to the sparse solver, the program reverts back to the PCG solver under certain conditions. See the Notes section for details and limitations.

ReduceIO

Controls whether the PCG solver will attempt to reduce I/O performed during equation solution:

AUTO

 — 

Automatically chooses whether to reduce I/O or not (default).

YES

 — 

Reduces I/O performed during equation solution in order to reduce total solver time.

NO

 — 

Does NOT reduce I/O performed during equation solution.

This option applies to both the PCG solver when used in static and full transient analyses and to the PCG Lanczos method in modal analyses.

StrmCk

Controls whether or not a Sturm sequence check is performed:

OFF

 — 

Does NOT perform Sturm sequence check (default).

ON

 — 

Performs Sturm sequence check

This option applies only when using the PCG Lanczos method in modal analyses. When using this option, a factorization must be performed and will require a very large amount of memory for extra computations. This option is generally recommended for small- to medium-sized problems. If the Sturm sequence check takes a large amount of computing time, use the Jobname.ABT file to abort the Sturm check, or press the STOP button if in interactive mode.

Wrtfull

Controls whether or not the .full file is written.

ON

 — 

Write .full file (default)

OFF

 — 

Do not write .full file.

This option applies only when using the PCG Lanczos method in modal analyses because the .full file is never written when using the PCG solver in static, full transient, or harmonic analyses.

If using MSAVE,ON and conditions for the MSAVE command are met, a complete .full file is never written regardless of this option.

If constraint equations are present in the model, a .full file is always written regardless of this option.

This option is useful in a distributed-memory parallel processing analysis because assembling the global stiffness and mass matrices on the head compute node before writing the .full file can take a considerable amount of memory. By setting Wrtfull = OFF, this assembly process is skipped on the head compute node, decreasing the amount of memory required to compute the modes and mode shapes. Wrtfull = OFF does not affect the results for the modes and mode shapes. However, without a .full file, the participation factor table computations do not occur.

To generate the .full file to be used in a downstream harmonic, transient mode-superposition, or spectrum analysis, rerun the modal analysis with Wrtfull = ON, or use the WRFULL command.

LM_Key

Controls use of the PCG solver for MPC184 elements that involve the mixed u-P (Lagrange multiplier) method.

By default (LM_KEY = OFF), the PCG solver will automatically detect mixed u-P element types (PLANE182, PLANE183, MPC184, SOLID185, SOLID186, and SOLID187) and enable the analysis to use the PCG solver. This default option only supports static and full transient analyses.

The alternative option (LM_KEY = ON) invokes the PCG solver for models with MPC184 as the only elements formulated with Lagrange multipliers. This option applies only to the PCG solver when used in static analyses, full transient analyses, and modal analyses that use the PCG Lanczos mode-extraction method (MODOPT,LANPCG).

ON

 — 

Allow use of the PCG solver with certain MPC184 element types that use the Lagrange multiplier method. This option does not support models with any other mixed u-P element types.

OFF

 — 

Use the PCG solver with any MPC184 element types that use the mixed u-P (Lagrange multiplier) method (default).

The alternative Lagrange multiplier method (LM_KEY = ON) used by MPC184 elements transfers the Lagrange multipliers into multiple point constraints and, hence, can be solved by the PCG solver. The current MPC184 element types supported are: rigid beam, rigid link, slider, revolute joint, universal joint, translational joint, cylindrical joint, weld joint, spherical joint, and general joint. For all other MPC184 element types, the PCG solver cannot be used, and the equation solver automatically switches to the sparse solver regardless of the LM_Key setting on PCGOPT. The MSAVE command does not support the LM_Key = ON option.

Notes

ReduceIO works independently of the MSAVE command in the PCG solver. Setting ReduceIO to YES can significantly increase the memory usage in the PCG solver.

To minimize the memory used by the PCG solver with respect to the Lev_Diff option only, set Lev_Diff = 1 if you do not have sufficient memory to run the PCG solver with Lev_Diff = AUTO.

For unsymmetric structural and harmonic analyses, the PCG solver supports a Lev_Diff equal to 2-4.

For single-field thermal analyses, the PCG solver sets Lev_Diff = 1 by default for most cases.

For single-field structural analyses, the PCG solver sets Lev_Diff = 2 by default for most cases.

The MSAVE,ON command is not valid in these circumstances:

  • when Lev_Diff = 5; in this case, the Lev_Diff value will automatically be reset to 2.

  • with the StrmCk option; in this case, StrmCk will be set to OFF.

  • when the Fallback option is enabled (set to AUTO or ON); in this case, Fallback will automatically be reset to OFF.

  • when the PCG solver is used for a full harmonic analysis.

  • when the PCG solver is used to solve an analysis containing elements that use the mixed u-P method in the model.

For Lagrange-formulation contact methods and mixed u-P formulations, the PCG solver cannot be used, and the sparse solver is required.

Fallback logic is automatically disabled (Fallback = OFF) in these circumstances:

  • for analyses that include MPC184 elements using the alternative Lagrange multiplier method (LM_KEY = ON) to impose kinematic constraints

  • for thermal analyses that use the quasi-static (THOPT,QUASI) option.

Reverting to the PCG Solver After an Automatic Switch to the Sparse Solver

Linear Analysis (symmetric or unsymmetric): When fallback logic is enabled (Fallback = AUTO or ON) and the program switches to the sparse direct solver during a linear analysis, the sparse solver is used for the remainder of the simulation unless the solver choice is changed by issuing the EQSLV command between load steps.

Nonlinear Analysis (symmetric or unsymmetric): If the program switches to the sparse direct solver during a nonlinear analysis, the sparse solver is used for the remaining equilibrium iterations of the current substep. The program reverts back to the PCG solver at the end of the current substep unless one of the following conditions apply:

  • The previous call to the sparse solver involved an indefinite or near-singular matrix.

  • The last equilibrium iteration of the current substep using the sparse solver was faster than the last successful equilibrium iteration using the PCG solver.

  • Three consecutive fallback switches between the sparse and the PCG solvers have occurred.

For both linear and nonlinear analyses, the solver choice can be changed using the EQSLV command between any subsequent load steps.

Full-Harmonic Analysis: If the program switches to the sparse direct solver during a full-harmonic analysis, the sparse solver is used for the current harmonic substep. The program reverts back to the PCG solver at the next continuing substep.

Menu Paths

Main Menu>Preprocessor>Loads>Analysis Type>Analysis Options
Main Menu>Solution>Analysis Type>Analysis Options