[Symphony] Dramatic performance drop since 5.1.10

Fri Sep 11 22:12:22 EDT 2009

One comment about (2): i would not coin it an ugly hack. it is always very
difficult to know before hand whether dual or primal simplex is faster on
some LPs, in fact this is true for any solver (cplex, xpress, ...) What lp
solver does 5.1.10 use initially, is it dual simplex as well ?

as a side comment: does symphony support barrier as well ? on instances with
>=50K constraints and 100K variables, cplex barrier is extremely faster
(2mns) than its primal simplex (20 mns) or dual simplex (10mns). It is the
same with Xpress. just so you know.

best regards,
Claude Ph Medard

On Fri, Sep 11, 2009 at 6:50 PM, Ashutosh Mahajan <asm4 at lehigh.edu> wrote:

> Dear Pierre Le Bodic
>
> i think you can speed up your symphony for your instances by doing these 2
> things.
>
> 1. set your parameter or option file to contain
> fp_enabled -1
> prep_level  0
> this will disable feasibility pump and presolve. presolve takes >20s on
> some
> instances.
>
> 2. you will have to edit SYMPHONY/src/LP/lp_genfunc.c and change line 275
> from
>         termcode = initial_lp_solve(lp_data, &iterd);
>   to
>         termcode = dual_simplex(lp_data, &iterd);
>
> the LP relaxation of your instances seem to get solved much faster by
> dual_simplex than by initial_lp_solve.
>
> im sorry about the fact that 1. is not documented yet and 2. is an ugly
> hack.
> hope this cuts down the time to something acceptable. thanks for sending
> your
> instances. its really helps to see that a solver 'tuned' towards
> benchmark instances can become significantly slower on others.
>
> we will try to fix the above issues in trunk soon.
>
> ashutosh
>
>
>
> On Thu, 10 Sep 2009, Ted Ralphs wrote:
>
> > This is unfortunate, but not actually too surprising. In 5.2, we added
> > features that were focused on solving the more difficult instances and
> > tuned our parameters to improve speed on those instances, but probably
> > neglected to ensure that performance on easier instances was not too
> > degraded. Thanks for the reminder :). What frequently happens when you
> > do parameter tuning on larger instances is that smaller instances
> > actually get slower as an unintended consequence. It's a difficult
> > balance to strike. Let us look at these and see if we can come up with
> > a better balance of improving performance on large instances without
> > degrading performance on smaller instances. In any case, it should be
> > relatively easy to replicate the performance of 5.1 with customized
> > parameter settings.
> >
> > Chers,
> >
> > Ted
> >
> > On Thu, Sep 10, 2009 at 5:11 AM,  <Pierre.Lebodic at lri.fr> wrote:
> > > Hello,
> > >
> > > I've been using the C callbacks of Symphony 5.1.10 for a while and I
> > > recently tried updating to 5.2. On a batch of 80 test instances, I
> > > measured the run time of the updated solver. "Sys time" went from 2sec
> > > with 5.1 to 2m55sec with 5.2. As a comparison, cbc 2.3 takes 23sec. A
> > > little digging made me try with feasibility pump disabled, which
> roughly
> > > cut the sys time in two, but it's still nowhere close the previous
> > > version. I'd like to know if there are parameters that can be changed
> to
> > > get closer to how fast Symphony 5.1 performs. I'm solving 0-1 problems.
> I
> > > can provide MPS files if needed.
> > >
> > > The reason why I'm willing to switch to 5.2 is that I'm experiencing
> what
> > > seems to be a bug: when TM_gap_limit != -1, feasible solutions that
> don't
> > > respect the gap limit are produced. As far as I've been able to see,
> this
> > > bug seems to be fixed in 5.2. If you know about this bug, a quick edit
> in
> > > my 5.1 source would also do the trick.
> > > Thanks in advance
> > >
> > > Pierre Le Bodic
> > >
> > > _______________________________________________
> > > Symphony mailing list
> > > Symphony at list.coin-or.org
> > > http://list.coin-or.org/mailman/listinfo/symphony
> > >
> >
> >
> >
> > --
> > Dr. Ted Ralphs
> > Associate Professor, Lehigh University
> > (610) 628-1280
> > ted 'at' lehigh 'dot' edu
> > coral.ie.lehigh.edu/~ted <http://coral.ie.lehigh.edu/%7Eted>
> >
> > _______________________________________________
> > Symphony mailing list
> > Symphony at list.coin-or.org
> > http://list.coin-or.org/mailman/listinfo/symphony
>
> --
> regards
> Ashutosh Mahajan
> http://coral.ie.lehigh.edu/~asm4 <http://coral.ie.lehigh.edu/%7Easm4>
>
> _______________________________________________
> Symphony mailing list
> Symphony at list.coin-or.org
> http://list.coin-or.org/mailman/listinfo/symphony
>
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