HiGHS

HiGHS is an optimization package for solving continuous and mixed-integer linear programming problems (LPs and MIPs) using simplex, interior-point, and branch-and-cut algorithms. HiGHS is developed by the Edinburgh Research Group in Optimization.

For more detailed information on the implemented simplex method, we refer to [98].

# Usage

The following statement can be used inside your GAMS program to specify using HiGHS

Option MIP = HIGHS;     { or LP or RMIP }


The above statement should appear before the Solve statement. If HiGHS was specified as the default solver during GAMS installation, the above statement is not necessary.

## Specification of HiGHS Options

GAMS/HiGHS supports the GAMS parameters reslim, iterlim, nodlim. optca, optcr, cutoff, and threads.

Options can be specified by a HiGHS options file. A HiGHS options file consists of one option or comment per line. A pound sign (#) at the beginning of a line causes the entire line to be ignored. Otherwise, the line will be interpreted as an option name and value separated by an equal sign (=) and any amount of white space (blanks or tabs).

A small example for a highs.opt file is:

solver = ipm
ipm_optimality_tolerance = 1e-6
run_crossover = false


It causes HiGHS to use an interior point solver for an LP solve, increases the optimality tolerance to $$10^{-6}$$, and turns off crossover to a basis solution.

# List of HiGHS Options

In the following, we give a detailed list of all HiGHS options.

Option Description Default
dual_feasibility_tolerance Dual feasibility tolerance
Range: [1e-10, ∞]
1e-07
infinite_bound Limit on |constraint bound|: values larger than this will be treated as infinite
Range: [1e+15, ∞]
1e+20
infinite_cost Limit on cost coefficient: values larger than this will be treated as infinite
Range: [1e+15, ∞]
1e+20
ipm_iteration_limit Iteration limit for IPM solver
Range: {0, ..., ∞}
GAMS iterlim
ipm_optimality_tolerance IPM optimality tolerance
Range: [1e-12, ∞]
1e-08
large_matrix_value Upper limit on |matrix entries|: values larger than this will be treated as infinite
Range: [1, ∞]
1e+15
mip_abs_gap Tolerance on absolute gap of MIP, |ub-lb|, to determine whether optimality has been reached for a MIP instance
Range: [0, ∞]
GAMS optca
mip_detect_symmetry Whether MIP symmetry should be detected
Range: boolean
1
mip_feasibility_tolerance MIP feasibility tolerance
Range: [1e-10, ∞]
1e-06
mip_heuristic_effort Effort spent for MIP heuristics
Range: [0, 1]
0.05
mip_lp_age_limit Maximal age of dynamic LP rows before they are removed from the LP relaxation in the MIP solver
Range: {0, ..., 32767}
10
mip_max_improving_sols Limit on the number of improving solutions found to stop the MIP solver prematurely
Range: {1, ..., ∞}
mip_max_leaves MIP solver max number of leave nodes
Range: {0, ..., ∞}
mip_max_nodes MIP solver max number of nodes
Range: {0, ..., ∞}
GAMS nodlim, if > 0, ∞ otherwise
mip_max_stall_nodes MIP solver max number of nodes where estimate is above cutoff bound
Range: {0, ..., ∞}
mip_min_cliquetable_entries_for_parallelism Minimal number of entries in the MIP solver cliquetable before neighbourhood queries of the conflict graph use parallel processing
Range: {0, ..., ∞}
100000
mip_pool_age_limit Maximal age of rows in the MIP solver cutpool before they are deleted
Range: {0, ..., 1000}
30
mip_pool_soft_limit Soft limit on the number of rows in the MIP solver cutpool for dynamic age adjustment
Range: {1, ..., ∞}
10000
mip_pscost_minreliable Minimal number of observations before MIP solver pseudo costs are considered reliable
Range: {0, ..., ∞}
8
mip_rel_gap Tolerance on relative gap, |ub-lb|/|ub|, to determine whether optimality has been reached for a MIP instance
Range: [0, ∞]
GAMS optcr
mipstart Whether to pass initial level values as starting point to MIP solver
If the solution is not feasible, HiGHS will solve the LP obtained from fixing all discrete variables to their initial level values.
Range: boolean
0
objective_bound Objective bound for termination
Range: real
GAMS cutoff
output_flag Enables or disables solver output
Range: boolean
0, if GAMS logoption = 0, otherwise 1
parallel Parallel option: "off", "choose" or "on"
Range: string
choose
presolve Presolve option: "off", "choose" or "on"
Range: string
choose
primal_feasibility_tolerance Primal feasibility tolerance
Range: [1e-10, ∞]
1e-07
random_seed Random seed used in HiGHS
Range: {0, ..., ∞}
0
run_crossover Run IPM crossover: "off", "choose" or "on"
Range: string
on
sensitivity Whether to run sensitivity analysis after solving an LP with a simplex method
Range: boolean
0
simplex_dual_edge_weight_strategy Strategy for simplex dual edge weights: Choose / Dantzig / Devex / Steepest Edge (-1/0/1/2)
Range: {-1, ..., 2}
-1
simplex_iteration_limit Iteration limit for simplex solver when solving LPs, but not subproblems in the MIP solver
Range: {0, ..., ∞}
GAMS iterlim
simplex_max_concurrency Maximum level of concurrency in parallel simplex
Range: {1, ..., 8}
8
simplex_primal_edge_weight_strategy Strategy for simplex primal edge weights: Choose / Dantzig / Devex / Steepest Edge (-1/0/1/2)
Range: {-1, ..., 2}
-1
simplex_scale_strategy Simplex scaling strategy: off / choose / equilibration / forced equilibration / max value 0 / max value 1 (0/1/2/3/4/5)
Range: {0, ..., 5}
1
simplex_strategy Strategy for simplex solver 0 => Choose; 1 => Dual (serial); 2 => Dual (PAMI); 3 => Dual (SIP); 4 => Primal
Range: {0, ..., 4}
1
simplex_update_limit Limit on the number of simplex UPDATE operations
Range: {0, ..., ∞}
5000
small_matrix_value Lower limit on |matrix entries|: values smaller than this will be treated as zero
Range: [1e-12, ∞]
1e-09
solution_file Solution file
Range: string
<inputname>.sol
solver LP algorithm to run: "simplex", "choose", or "ipm"; ignored for MIP
Range: string
choose
threads Number of threads used by HiGHS (0: automatic)
Range: {0, ..., ∞}
time_limit Time limit (seconds)
Range: [0, ∞]
GAMS reslim
write_model_file Write model file
Range: string
<inputname>.lp
write_model_to_file Write the model to a file
Range: boolean
0
write_solution_style Style of solution file (raw = computer-readable, pretty = human-readable): -1 => HiGHS old raw (deprecated); 0 => HiGHS raw; 1 => HiGHS pretty; 2 => Glpsol raw; 3 => Glpsol pretty; 4 => HiGHS sparse raw
Range: {-1, ..., 4}
0
write_solution_to_file Write the primal and dual solution to a file
Range: boolean
0
Options for expert users
allow_unbounded_or_infeasible whether to spend extra effort to distinguish unboundedness and infeasibility if necessary
Range: boolean
0
allowed_cost_scale_factor Largest power-of-two factor permitted when scaling the costs
Range: {0, ..., 20}
0
allowed_matrix_scale_factor Largest power-of-two factor permitted when scaling the constraint matrix
Range: {0, ..., 30}
20
cost_scale_factor Scaling factor for costs
Range: {-20, ..., 20}
0
dual_simplex_cost_perturbation_multiplier Dual simplex cost perturbation multiplier: 0 => no perturbation
Range: [0, ∞]
1
dual_simplex_pivot_growth_tolerance Dual simplex pivot growth tolerance
Range: [1e-12, ∞]
1e-09
dual_steepest_edge_weight_error_tolerance Tolerance on dual steepest edge weight errors
Range: [0, ∞]
dual_steepest_edge_weight_log_error_threshold Threshold on dual steepest edge weight errors for Devex switch
Range: [1, ∞]
10
factor_pivot_threshold Matrix factorization pivot threshold
Range: [0.0008, 0.5]
0.1
factor_pivot_tolerance Matrix factorization pivot tolerance
Range: [0, 1]
1e-10
highs_analysis_level Analysis level in HiGHS
Range: {0, ..., 63}
0
highs_debug_level Debugging level in HiGHS
Range: {0, ..., 3}
0
icrash Run iCrash
Range: boolean
0
icrash_approx_iter iCrash approximate minimization iterations
Range: {0, ..., 100}
50
icrash_breakpoints Exact subproblem solution for iCrash
Range: boolean
0
icrash_dualize Dualize strategy for iCrash
Range: boolean
0
icrash_exact Exact subproblem solution for iCrash
Range: boolean
0
icrash_iterations iCrash iterations
Range: {0, ..., 200}
30
icrash_starting_weight iCrash starting weight
Range: [1e-10, 1e+50]
0.001
icrash_strategy Strategy for iCrash
Range: string
ICA
ipx_dualize_strategy Strategy for dualizing before IPX
Range: {-1, ..., 3}
2
less_infeasible_DSE_check Check whether LP is candidate for LiDSE
Range: boolean
1
less_infeasible_DSE_choose_row Use LiDSE if LP has right properties
Range: boolean
1
log_dev_level Output development messages: 0 => none; 1 => info; 2 => verbose
Range: {0, ..., 3}
0
lp_presolve_requires_basis_postsolve Prevents LP presolve steps for which postsolve cannot maintain a basis
Range: boolean
1
max_dual_simplex_cleanup_level Max level of dual simplex cleanup
Range: {0, ..., ∞}
1
max_dual_simplex_phase1_cleanup_level Max level of dual simplex phase 1 cleanup
Range: {0, ..., ∞}
2
mip_report_level MIP solver reporting level
Range: {0, ..., 2}
1
no_unnecessary_rebuild_refactor No unnecessary refactorization on simplex rebuild
Range: boolean
1
objective_target Objective target for termination
Range: real
-∞
presolve_pivot_threshold Matrix factorization pivot threshold for substitutions in presolve
Range: [0.0008, 0.5]
0.01
presolve_reduction_limit Limit on number of presolve reductions -1 => no limit
Range: {-1, ..., ∞}
-1
presolve_rule_logging Log effectiveness of presolve rules for LP
Range: boolean
0
presolve_rule_off Bit mask of presolve rules that are not allowed
Range: {0, ..., ∞}
0
presolve_substitution_maxfillin Maximal fillin allowed for substitutions in presolve
Range: {0, ..., ∞}
10
primal_simplex_bound_perturbation_multiplier Primal simplex bound perturbation multiplier: 0 => no perturbation
Range: [0, ∞]
1
rebuild_refactor_solution_error_tolerance Tolerance on solution error when considering refactorization on simplex rebuild
Range: real
1e-08
simplex_crash_strategy Strategy for simplex crash: off / LTSSF / Bixby (0/1/2)
Range: {0, ..., 9}
0
simplex_dualize_strategy Strategy for dualizing before simplex
Range: {-1, ..., 1}
-1
simplex_initial_condition_check Perform initial basis condition check in simplex
Range: boolean
1
simplex_initial_condition_tolerance Tolerance on initial basis condition in simplex
Range: [1, ∞]
1e+14
simplex_min_concurrency Minimum level of concurrency in parallel simplex
Range: {1, ..., 8}
1
simplex_permute_strategy Strategy for permuting before simplex
Range: {-1, ..., 1}
-1
simplex_price_strategy Strategy for PRICE in simplex
Range: {0, ..., 3}
3
simplex_unscaled_solution_strategy Strategy for solving unscaled LP in simplex
Range: {0, ..., 2}
1
start_crossover_tolerance Tolerance to be satisfied before IPM crossover will start
Range: [1e-12, ∞]
1e-08
use_implied_bounds_from_presolve Use relaxed implied bounds from presolve
Range: boolean
0
use_original_HFactor_logic Use original HFactor logic for sparse vs hyper-sparse TRANs
Range: boolean
1
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