Search found 2843 matches

by tomfid
Thu Feb 13, 2020 9:32 pm
Forum: Vensim
Topic: Vensim PLE MAC v8 unstable on student machine.
Replies: 7
Views: 56

Re: Vensim PLE MAC v8 unstable on student machine.

Also might help to know the OS version and anything unusual about the machine.
by tomfid
Tue Feb 11, 2020 7:10 pm
Forum: Vensim
Topic: Using High Performance Computing systems to run Vensim simulation
Replies: 7
Views: 57

Re: Using High Performance Computing systems to run Vensim simulation

Ah, yes - that's the first thing to try. The speedup is structure-dependent, but I've never seen less than 2x and 5x is common, 10x possible if there's a lot of array structure. Another thing to check is to be sure that any nondynamic computations are designated as INITIAL(). This is especially impo...
by tomfid
Tue Feb 11, 2020 2:30 pm
Forum: Vensim
Topic: Simulating a model with uploaded data and some variables with equations
Replies: 4
Views: 38

Re: Simulating a model with uploaded data and some variables with equations

However, you can put the data in a lookup as a workaround.
by tomfid
Tue Feb 11, 2020 2:46 am
Forum: Vensim
Topic: Using High Performance Computing systems to run Vensim simulation
Replies: 7
Views: 57

Re: Using High Performance Computing systems to run Vensim simulation

Your license permits 1 user x 2 computers or multiple users on one computer, so that's OK.

A full HPC cluster may not be needed when the time comes - many-core workstations are now widely available and not particularly expensive.
by tomfid
Tue Feb 11, 2020 1:53 am
Forum: Vensim
Topic: Using High Performance Computing systems to run Vensim simulation
Replies: 7
Views: 57

Re: Using High Performance Computing systems to run Vensim simulation

We have a solution for this underway, but it's currently offline due to x64 upgrade turbulence. It's coming though.

Unfortunately calibration optimization is the worst case for performance - it doesn't parallelize easily. Sensitivity and MCMC will benefit much more.
by tomfid
Tue Feb 11, 2020 1:35 am
Forum: Vensim
Topic: Using High Performance Computing systems to run Vensim simulation
Replies: 7
Views: 57

Re: Using High Performance Computing systems to run Vensim simulation

What do you want out of the HPC? Sensitivity analysis? Optimization? Faster single runs?
by tomfid
Sat Feb 08, 2020 5:54 pm
Forum: Vensim
Topic: Warning: "Can load at most 16 runs at one time"???
Replies: 4
Views: 47

Re: Warning: "Can load at most 16 runs at one time"???

Model>Check Model will open into the equation editor, with all the structural issues in the error menu at the bottom. Typically most are usage - variables that are unusued. You can suppress those by checking the Supplementary box if non-use is intentional. The rest of the errors appear at runtime. B...
by tomfid
Sat Feb 08, 2020 5:45 pm
Forum: Vensim
Topic: Simultaneous Equation Error
Replies: 8
Views: 61

Re: Simultaneous Equation Error

The version attached above runs for me - not clear how to replicate the error. The issue is likely a feedback loop without a stock in it. I think this model would benefit from a complete rework of the stock-flow usage. Start by going through and getting rid of the boxes on anything that is not a sto...
by tomfid
Sat Feb 08, 2020 1:35 am
Forum: Vensim
Topic: Warning: "Can load at most 16 runs at one time"???
Replies: 4
Views: 47

Re: Warning: "Can load at most 16 runs at one time"???

It means you can only have 16 runs loaded for viewing at once. Go to the Dataset tab of the Control Panel and unload a few, or just ignore the warning.
by tomfid
Fri Feb 07, 2020 10:50 pm
Forum: Vensim
Topic: Simultaneous Equation Error
Replies: 8
Views: 61

Re: Simultaneous Equation Error

I think the .mdl attachment was omitted. Looking at the diagram, it seems like there's an issue with the regression. I'm not sure whether the "load growth" stock has units of energy (additional load above some baseline) or energy/time (rate of change of load). But both seem problematic: additional l...
by tomfid
Fri Feb 07, 2020 10:34 pm
Forum: Vensim
Topic: Creating a constant value
Replies: 9
Views: 70

Re: Creating a constant value

This may be a quirk of the computation of SAMPLE IF TRUE combined with a gaming variable. You can use a level instead. Add: Max budget to date[Menu]= INTEG ( MAX(0,Additional item daily promotion budget[Menu]-Max budget to date[Menu])/TIME STEP, 0) Units: $/Day Then get rid of the SAMPLE: Constant a...
by tomfid
Fri Feb 07, 2020 4:18 pm
Forum: Vensim
Topic: Help with my SIR model
Replies: 12
Views: 99

Re: Help with my SIR model

Right - k being the rate coefficient. I'm guessing that the tutorial is using normalized population to begin with - it's common practice in epidemiology and nonlinear dynamics because the simplification makes it easy to analyze the model (no nuisance parameters). However, it's generally confusing fo...
by tomfid
Fri Feb 07, 2020 3:55 pm
Forum: Vensim
Topic: Temporal weighting in optimization/calibration
Replies: 2
Views: 23

Re: Temporal weighting in optimization/calibration

Yes - you can either:
- calculate your objective function, including the weight, explicitly in the model
- make the weight term in the payoff file (vpd) a variable
by tomfid
Fri Feb 07, 2020 3:53 pm
Forum: Vensim
Topic: Exporting model document
Replies: 1
Views: 24

Re: Exporting model document

You might try SDM/Doc - it does a nice job.
by tomfid
Fri Feb 07, 2020 3:51 pm
Forum: Vensim
Topic: Creating a constant value
Replies: 9
Views: 70

Re: Creating a constant value

SAMPLE IF TRUE can be used to freeze the value of a calculation at some point.
by tomfid
Thu Feb 06, 2020 7:48 pm
Forum: Vensim
Topic: Stop When Reaching to Zero
Replies: 7
Views: 1048

Re: Stop When Reaching to Zero

An equally simple limit is: outflow = MIN( desired outflow, stock/TIME STEP ) The assumption here is that the dynamics of the constraint are not of much interest, so that it's OK to assume that the minimum outflow time is equal to TIME STEP. This is usually fine. This avoids having TIME STEP as an i...
by tomfid
Thu Feb 06, 2020 3:12 pm
Forum: Vensim
Topic: User-defined file setup in Sensitivity Simulation - for multiple parameters
Replies: 14
Views: 237

Re: User-defined file setup in Sensitivity Simulation - for multiple parameters

I would guess that it probably won't get far. It should be possible to optimize over a small sample for a few control parameters. We really need to get parallel simulation released - that's been held up by turbulence from the 64 bit changeover. However, there's a basic problem, which is that optimiz...
by tomfid
Thu Feb 06, 2020 3:07 pm
Forum: Vensim
Topic: Sample WORLD model has stocks modified by variables, without flows
Replies: 2
Views: 30

Re: Sample WORLD model has stocks modified by variables, without flows

If you're talking about World3, it does. This is basically a stylistic difference that shows its origin in DYNAMO. Those variables aren't levels, but they contain macro functions (SMOOTH) that have an internal state. So, they are drawn with a box to make that state visible. You could replace those w...
by tomfid
Thu Feb 06, 2020 3:00 pm
Forum: Vensim
Topic: Help with my SIR model
Replies: 12
Views: 99

Re: Help with my SIR model

It may be easier to debug if you switch to Euler integration (at least temporarily). It's harder to see the behavior internal to the time step with RK4. I think the problem here is with your "get sick" equation. You have k*infected*susceptible. There's immediately a problem here - imagine if half th...
by tomfid
Wed Feb 05, 2020 4:34 pm
Forum: Vensim
Topic: Help with my SIR model
Replies: 12
Views: 99

Re: Help with my SIR model

The units on "get better" (etc.) should be people/day. This model will not be stable as is, because the shortest time constant (recovery time) is 1/4 is shorter than the time step (see Model>Time Bounds). As a rule of thumb, the time step should be less than half the shortest time constant or delay....
by tomfid
Wed Feb 05, 2020 1:35 am
Forum: Vensim
Topic: Help with my SIR model
Replies: 12
Views: 99

Re: Help with my SIR model

Moving to the Vensim section.
by tomfid
Tue Feb 04, 2020 12:53 pm
Forum: Vensim
Topic: User-defined file setup in Sensitivity Simulation - for multiple parameters
Replies: 14
Views: 237

Re: User-defined file setup in Sensitivity Simulation - for multiple parameters

I would just include the things that vary - the rest could be left in a default cin file. Then you include the sample in a sensitivity simulation via a vsc using the File method.
by tomfid
Tue Feb 04, 2020 11:53 am
Forum: Vensim
Topic: User-defined file setup in Sensitivity Simulation - for multiple parameters
Replies: 14
Views: 237

Re: User-defined file setup in Sensitivity Simulation - for multiple parameters

In your case, you'd have to transpose your cin file(s) into a sensitivity sample file. This isn't as hard as it sounds. If you import a cin file into Excel, and use text to columns to break things at the =, you get a column of variable names and a column of values. Do this once per file, then merge ...