Re: NDSolve is really slow in version 8 but not 7

*To*: mathgroup at smc.vnet.net*Subject*: [mg126067] Re: NDSolve is really slow in version 8 but not 7*From*: A Retey <awnl at gmx-topmail.de>*Date*: Sat, 14 Apr 2012 03:12:12 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <jm8psc$j38$1@smc.vnet.net>

Hi, > I'm using NDSolve to solve a set of roughly 40 first order > differential equations. It takes about 10 seconds in version 7 but > more like 850 in version 8. Does anyone know why this might be and > how I can speed things up? > > Also for one scenario I'm getting the error > > NDSolve::"ndsz" : ""At "\[NoBreak]t\[NoBreak]" == > "\[NoBreak]39.77534338386624`\[NoBreak]", step size is \ effectively > zero; singularity or stiff system suspected." > > I realise its difficult to help without me giving an example but its > a bit tricky to do so. Are there any standard tricks I could try? I have seen things like that in some cases. Usually the reason was that the newer version was choosing a different Method than the older version did. So what you could try would be to find out which Method was chosen with version 7 and then explicitly set that Method in version 8. I found that usually then version 8 is even somewhat faster. Unfortunately it's not a trivial task to determine which Method was used, but there has been a discussion about that either in this group or on stack exchange. I remember that Oliver Ruebenkoenig was making some valuable contributions to that discussion, so you might want to include his name in a search. There is a chance that those choices are changed for good reasons, so you'd have to make sure that the result will in fact be correct when using the faster method... hth, albert