1 Purpose

The NaNChecker thorn can be used to analyze Cactus grid variables (that is grid functions, arrays or scalars) of real or complex data type for NaN (Not-a-Number) and (on availability of finite(3)) infinite values. Grid variables can be periodically checked, or a call can be inserted into a thorn to check at a specific point.

This thorn is a utility thorn, designed to be used for debugging and testing code for uninitialised variables, or for variables which become corrupted during a simulation, for example following a division by zero or illegal memory usage.

On many architectures, uninitialised variables will be given the value zero, and simulations using such variables will seemingly run perfectly well. However, not only is it dubious programming practise to assume such behaviour, but also moving to a new machine may well cause pathalogical problems (for example, with Alpha processors used in Compaq or Cray machines). It is thus recommended to test codes periodically with the NaNChecker, and to fix any problems as soon as they are seen.

2 Periodic Testing

Periodic testing of variables can easily be achieved by adding NaNChecker to the ActiveThorns parameter, and setting the parameters

NaNChecker::check_every, NaNChecker::check_after, and NaNChecker::check_vars

to the required values. (For most testing purposes these can be set to 1, 0, and ”all” respectively).

The NaNChecker then registers a routine at CCTK_ANALYSIS which checks at every NanChecker::check_every iteration – starting at iteration number NaNChecker::check_after – all the variables listed in NaNChecker::check_vars for NaN or infinite values (depending on NaNChecker::check_for) and — if such a value is found — performs an action as specified in NaNChecker::action_if_found.

Currently these actions can be to

• just_warn (the default)

  just print a level 1 warning message telling you where NaNs/Infs were found and how many (for grid array variables).
  If the keyword parameter verbose is set to "all" then for each grid array it will also print the grid indices (in Fortran notation) and the physical coordinates for all NaN/Inf elements found. You can limit the number of such warnings by setting the NanChecker::report_max parameter.

• terminate

  also set the CCTK termination flag so that Cactus will stop the evolution loop and gracefully terminate at the next time possible (giving you the choice of outputting the data from the last evolution timestep),

• abort

  print the warning message(s) and immediately terminate Cactus after checking all variables from NaNChecker::check_vars by a call to CCTK_Abort()

By default, the current timelevel of the variables given in NaNChecker::check_vars will be checked. This can be overwritten by an optional string [timelevel=<timelevel>] appended to the variable/group name. For example, to apply the NaNChecker to timelevel 0 of the variable grid::x, timelevel 1 of grid::y and timelevel 2 of grid::z you would use the parameter

NaNChecker::check_vars = "grid::x grid::y[timelevel=1] grid::z[timelevel=2]"

3 Tracking and Visualizing NaNs Positions

The NaNChecker thorn can also mark the positions (in grid index points) of all the NaNs found for a given list of CCTK grid functions in a mask array and save this array to an HDF5 file.

The mask array is declared as a grid function NaNChecker::NaNmask with data type INTEGER. Each bit \(i\) in an integer element is used to flag a NaN value found in grid function \(i\) at the corresponding grid position (the counting for \(i\) starts at 0 and is incremented for each grid function as it appears in NaNChecker::check_vars). Thus the NaN locations of up to 32 individual grid functions can be coded in the NaNmask array.

In order to activate the NaNmask output you need to set the parameter NaNChecker::out_NaNmask to "yes" (which is already the default) and have the IOHDF5 thorn activated.

The NaN locations can be visualized with OpenDX. An example DX network VisualizeNaNs.net and a sample NaNmask HDF5 output file NaNmask.h5 are available via anonymous CVS from the NumRel CVS server:

  # this is for (t)csh; use export CVSROOT for bash
  setenv CVSROOT :pserver:cvs_anon@cvs.aei.mpg.de:/numrelcvs

  # CVS pserver password is ’anon’
  cvs login
  cvs checkout AEIPhysics/Visualization/OpenDX/Networks/Miscellaneous

4 NaNChecker API

Thorn NaNChecker also provides a function API which can be used by other code to invoke the NaNChecker routines to test for NaN/Inf values or to set NaN values for a list of variables:

C API

  int NaNChecker_CheckVarsForNaN (const cGH *cctkGH,
                                  int report_max,
                                  const char *vars,
                                  const char *check_for,
                                  const char *action_if_found);

  int NaNChecker_SetVarsToNaN (const cGH *cctkGH,
                               const char *vars);

Fortran API

  call NaNChecker_CheckVarsForNaN (ierror, cctkGH, report_max,
                                   vars, check_for, action_if_found)

                                   integer ierror
                                   CCTK_POINTER cctkGH
                                   integer report_max
                                   character*(*) vars
                                   character*(*) check_for
                                   character*(*) action_if_found

  call NaNChecker_SetVarsToNaN (ierror, cctkGH, vars)

                                integer ierror
                                CCTK_POINTER cctkGH
                                character*(*) vars

The report_max, check_vars, check_for and action_if_found arguments have the same semantics as their parameter counterparts.
If action_if_found is given as a NULL pointer (C API) or as an empty string (Fortran API) the routine will be quiet and just return the number of NaN values found.

The C function NaNChecker_CheckVarsForNaN() returns the total number of NaN/Inf values found, NaNChecker_SetToNaN() returns the total number of variables set to NaN; this return value is stored in the ierror argument for the corresponding fortran wrapper routines.

5 Parameters

[1]

CarpetReduce[:][:]weight

[1]

CarpetEvolutionMask[:][:]evolution\_mask

6 Interfaces

General

Implements:

nanchecker

Inherits:

reduce

Grid Variables

6.0.1 PRIVATE GROUPS

Adds header:

NaNCheck.h to NaNChecker.h

Provides:

CheckVarsForNaN to

SetVarsToNaN to

7 Schedule

This section lists all the variables which are assigned storage by thorn CactusUtils/NaNChecker. Storage can either last for the duration of the run (Always means that if this thorn is activated storage will be assigned, Conditional means that if this thorn is activated storage will be assigned for the duration of the run if some condition is met), or can be turned on for the duration of a schedule function.

Storage

Scheduled Functions

CCTK_BASEGRID

  nanchecker_resetcounter

  reset the nanchecker::nansfound counter

CCTK_PRESTEP

  nanchecker_resetcounter

  reset the nanchecker::nansfound counter

NaNChecker_NaNCheck

  nanchecker_nancheck_prepare

  prepare data structures to check for nans

CCTK_POSTSTEP

  nanchecker_nancheck

  check for nans and count them in nanchecker::nansfound

NaNChecker_NaNCheck

  nanchecker_nancheck_check

  check for nans

NaNChecker_NaNCheck

  nanchecker_nancheck_finish

  count nans in nanchecker::nansfound

CCTK_POSTSTEP

  nanchecker_takeaction

  output nanchecker::nanmask and take action according to nanchecker::action_if_found

CCTK_POST_RECOVER_VARIABLES

  nanchecker_nancheck

  check for nans and count them in nanchecker::nansfound

CCTK_POST_RECOVER_VARIABLES

  nanchecker_takeaction

  output nanchecker::nanmask and take action according to nanchecker::action_if_found

CCTK_INITIAL (conditional)

  nanchecker_setuptest

  set test grid function to nan

Aliased Functions