Changing GAMS solver in message-ix Python API

[Alaa-Eltablawy]

Hello,

I'm using message_ix Python API on windows 10 and trying to change the default GAMS solver when running scenario.solve()
I've tried different ways by setting gams_agrs, solve_args and solve_options parameters.
Scenario.solve(gams_args=["lp=cbc"])

or
Scenario.solve(solve_args=["lp=cbc"]

And in both cases, the output in Message_run file shows that LP = CBC is identified, but the GAMS still runs the default solver which is CPLEX

Message_run:
--- Job MESSAGE_run.gms Start ...x86 64bit/MS Windows
--- Applying:
C:\GAMS\44\gmsprmNT.txt
--- GAMS Parameters defined
LP cbc
Input C:..\Lib\site-packages\message_ix\model\MESSAGE_run.gms
ScrDir C:..\Lib\site-packages\message_ix\model\225a
SysDir C:\GAMS\44
LogOption 4
--in C:..\Lib\site-packages\message_ix\model\data\MsgData_MODEL_test_baseline.gdx
--out C:..\Lib\site-packages\message_ix\model\output\MsgOutput_MODEL_test_baseline.gdx
--iter C:..\Lib\site-packages\message_ix\model\output\MsgIterationReport_MODEL_test_baseline.gdx

Is there a way to force the gams_args to overwrite the default gams solver?

[glatterf42]

Hi @Alaa-Eltablawy, thanks for reaching out with this question!
When you call Scenario.solve(gams_args=["lp=cbc"]), you are already using the correct parameter in the function. In the end, these options will make their way here, where both solve_args and gams_args are set for the model. From ixmp's documentation, we can infer that gams_args is probably what you are looking for :)
The documentation also states where you can find the possible options to pass to gams_args: https://www.gams.com/latest/docs/UG_GamsCall.html#UG_GamsCall_ListOfCommandLineParameters
My guess is that if you want to change the solver, you want to look at this subsection: https://www.gams.com/latest/docs/UG_GamsCall.html#GAMS_solvers
Unfortunately, this list does not include an option called cbc. It does include an option called LP, so maybe all you need to do is use all-caps "LP"="cbc" rather than "lp"=.... Please note that there is also a solver option, so if cbc is the name of a particular solver, you might want to try "solver"="cbc", too.

[khaeru]

If we inspect the MESSAGE GAMS source, we see these lines:

message_ix/message_ix/model/MESSAGE/auxiliary_settings.gms

Lines 18 to 23 in 0fb01b4

	* solver comments for QCP and PATH:
	* - GUROBI, CPLEX and MINOS are fast
	* - CONOPT is slower, but (in non-linear problems) usually more helpful to identify the feasibility problems
	* general comment: sometimes, first using one solver and then another (using the previous solution as starting point)
	* helps even if the previous run did not solve to optimality
	option LP = CPLEX ;

@Alaa-Eltablawy, as @glatterf42 says, you do seem to be providing this option to the Python API in a way that reaches the GAMS invocation. However, because this option is hard-coded in the source files, the hard-coded value then overwrites the option you've given. (For some other settings, the MESSAGE GAMS source avoids to overwrite a user setting—but not this one.)

The simple fix is to modify the lines I linked above, inserting your chosen LP solver.

[Alaa-Eltablawy]

Thanks @khaeru and @glatterf42 for the answers!
Does message_ix initially set the parameters inside auxilliary_settings.gms based on the GAMS defaults?
So if I'm installing GAMS and setting the default solver to specific solver, and then install the message_ix package, the LP solver inside auxilliary_settings.gms will still be CPLEX or it will be whatever is default in GAMS?

[glatterf42]

If I understand correctly, it doesn't matter what you set as default on your system-wide GAMS installation. message_ix will call GAMS with certain options, one of which seems to always be LP = CPLEX unless you change the line 23 in auxiliary_settings.gams, as @khaeru suggested.