When utilizing multiple solve statements (e.g. in a loop) the amount of memory used by GAMS may increase even though the indiviual model size (for the particular solve statement) remains the same. This is due to storage of parameters and variables grows, even though they are not necessarily used for each solve statement.

In some case there are ways to keep the amount of memory pretty constant. They include:

1. Use option solveopt=replace which replaces rather than merges all solution values of each GAMS solve.
2. Use option clear=param to clean temporary data which is not used afterwards.
3. Use put files to store solution values instead of storing in memory.

             file fput /rep.txt/;
             put fput (data);

4. Batch several sub-models into fewer but bigger models.

The first two options are simple, the third one might be inconvenient (especially if you want to do post solution analysis) whereas the option to aggregate the LPs into fewer but bigger models can be quite difficult. This option generally is only useful if each sub-model to solve is not dependent on the solution of the previous model. Although there may be instances where the addition of a couple of more constraints can accomplish this, it is not always possible.

We shall give examples using the transportation model (trnsport.gms) where we have introduced a time series component.

 Set  t      time      / t1*t10 /
 tt(t)       dynamic version of t;
 Parameter   c(i,j)   transport cost in thousands of dollars per case
             at(i,t), bt(j,t), ct(i,j,t)  "supply, demand, cost by time";
  c(i,j) = f * d(i,j) / 1000 ;
  Variables
  x(i,j,t)  shipment quantities in cases
 
  Equations
  cost          define objective function
  supply(i,t)   observe supply limit at plant i
  demand(j,t)   satisfy demand at market j ;

  cost ..           z  =e=  sum((i,j,tt), ct(i,j,tt)*x(i,j,tt)) ;
  supply(i,tt) ..   sum(j, x(i,j,tt))  =l=  at(i,tt) ;
  demand(j,tt) ..   sum(i, x(i,j,tt))  =g=  bt(j,tt) ;

  loop(t,
  tt(t) = yes;
 
  ct(i,j,tt(t)) = c(i,j) * ( 1 + 0.25**ord(t));
  at(i,tt(t)) = a(i) * (1 + 0.15**ord(t));
  bt(j,tt(t)) = b(j) * (1 + 0.15**ord(t));
 
  Solve transport using lp minimizing z ;
 
  tt(t) = no;
  );

1. For the solution replace option we add: option solveopt=replace; before the loop(t, statement.
2. For the clear option we can clear the parameters ct, at, and bt, since they are only used for the local solve. After tt(t) = yes;we specify option clear=ct, clear=at, clear=bt;
3. For the put file option to store solutions, we create a put file called frep.put before the loop by: file frep. After the solve statement, we write to the put file by:

     put frep / t.tl 'modstat' transport.modelstat
     / t.tl 'slvstat' transport.solvestat
     / t.tl 'obj    ' z.l;

4. In order to aggregate the models we need to change our objective function. The model becomes:

    Equations
    defobj
    cost(t)       define objective function
    supply(i,t)   observe supply limit at plant i
    demand(j,t)   satisfy demand at market j ;
    
    defobj ..         obj =e= sum(tt, z(tt));
    cost(tt) ..       z(tt) =e=  sum((i,j), ct(i,j,tt)*x(i,j,tt)) ;
    scalar idx /0/, bsize /5/;
    repeat
    tt(t) = no;
    
    *  Batch 5 single LPs into one larger model:
    loop(t$(ord(t)>(idx*bsize) and ((idx+1)*bsize)>=ord(t)), tt(t) = yes);
    
    ct(i,j,tt(t)) = c(i,j) * ( 1 + 0.25**ord(t));
    at(i,tt(t)) = a(i) * (1 + 0.15**ord(t));
    bt(j,tt(t)) = b(j) * (1 + 0.15**ord(t));
    
    if (card(tt), Solve transport using lp minimizing obj);
    idx = idx + 1;
    until card(tt)=0;

The implementation of all four options using the same equations as above is:

 File frep;
 Option solveopt=replace;
 repeat
 tt(t) = no;
 loop(t$(ord(t)>(idx*bsize) and ((idx+1)*bsize)>=ord(t)), tt(t) = yes);

  option clear=ct, clear=at, clear=bt;
  ct(i,j,tt(t)) = c(i,j) * ( 1 + 0.25**ord(t));
  at(i,tt(t)) = a(i) * (1 + 0.15**ord(t));
  bt(j,tt(t)) = b(j) * (1 + 0.15**ord(t));
  if (card(tt), Solve transport using lp minimizing obj);

  loop(tt(t),
    put frep / t.tl 'modstat' transport.modelstat
             / t.tl 'slvstat' transport.solvestat
             / t.tl 'obj    ' z.l(t);
  );
  idx = idx + 1;
until card(tt)=0;