# Quadratically constrained linear maximisation problem: issues with fmincon

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CT on 27 Mar 2020
Commented: Matt J on 31 Mar 2020
I would like to solve the following quadratically constrained linear programming problem.
I have written a Matlab code (R2091b) that solve the problem using Gurobi. Now, I would like to rewrite the code using fmincon instead of Gurobi. This is because the optimisation problem will have to be solve thousands of times and Gurobi's academic license does not allow to parallelise via array jobs in a cluster. However, I'm encountering a huge problem: Gurobi takes 0.23 second to give a solution, fmincon takes 13 sec. I suspect this should be due to my mistakes/inefficiences in providing gradient, hessian, etc. Could you kindly help me to improve below?
Also, Gurobi gives me 0.2 as solution, fmincon gives 0.089. Can the accuracy of fmincon be improved without trying other starting points?
This is my code with Gurobi
clear
rng default
%1) GUROBI
model.A=[Aineq; Aeq];
model.obj=f;
model.modelsense='max';
model.sense=[repmat('<', size(Aineq,1),1); repmat('=', size(Aeq,1),1)];
model.rhs=[bineq; beq];
model.ub=ub;
model.lb=lb;
params.outputflag = 0;
result=gurobi(model, params);
max_problem_Gurobi=result.objval;
This is my code with fmincon
%2) FMINCON
options = optimoptions(@fmincon,'Algorithm','interior-point',...
[~,fval] = fmincon(fun,z0,Aineq,bineq,Aeq,beq,lb,ub,nonlconstr,options);
max_problem_fmincon=-fval;
y= z'*Q*z -d;
yeq = []; %no quadratic inequalities
if nargout > 2
end
end
hess = 2*lambda.ineqnonlin*Q;
end
y = -f'*z;
if nargout > 1
end
Further, if I run the code with fmincon with as starting point the optimal point given by Gurobi, I still get the solution 0.089 (instead of 0.2 as in Gurobi). Why?
%3) FMINCON
z0=result_Gurobi.x;
options = optimoptions(@fmincon,'Algorithm','interior-point',...
[~,fval] = fmincon(fun,z0,Aineq,bineq,Aeq,beq,lb,ub,nonlconstr,options);
max_problem_fmincon=-fval;
y= z'*Q*z -d;
yeq = []; %no quadratic inequalities
if nargout > 2
end
end
hess = 2*lambda.ineqnonlin*Q;
end
y = -f'*z;
if nargout > 1
end
end
Matt J on 31 Mar 2020
There was no expectation of great gains, but I think it has to be marginally more efficient, maybe a reduction from 36.05 sec to 36 sec. You can plainly see that fewer vector arithmetic operations are done in this version.

Matt J on 30 Mar 2020
Well, it would be interesting to know what algorithm Gurobi uses, but the issue of the objective function difference appears to be a matter of the tolerances
options = optimoptions(@fmincon,'Algorithm','interior-point',...
'StepTolerance',1e-30,'OptimalityTolerance',1e-10);
tic;
[~,fval] = fmincon(fun,z0(:),Aineq,bineq,Aeq,beq,lb,ub,nonlconstr,options);
toc
max_problem_fmincon=-fval
max_problem_fmincon =
0.2000
Matt J on 31 Mar 2020
And does the problem data from the thousands of problem instances that you are trying to solve change in a continuous incremental way? If you had the optimal solution for one instance of the problem, would it serve as a good initial estimate for the next instance?