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High RAM use with few degrees of freedom

Posted 1 août 2012, 11:16 UTC−4 Version 4.1 5 Replies

Camillo Giangiordano
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Hi everybody,

the 3D model attached has a swept mesh with 3468 hexahedral elements and the solver solves for 31395 degrees of freedom; while solving, the request for RAM "explodes" and becomes larger than 25 GB (causing a "Out of memory during assembly" error). I used a Heat Transfer module in COMSOL 4.1; solver: Time Dependent, Time Step 0.1 s (respecting the rule dt > rho * Cp / k * dL^2; dt = Time Step, rho = density, Cp = heat capacity, k = thermal conductivity, dL = meshing element size). k is anisotropic (this is the reason for large difference in meshing element size along different directions).
In other models I built in the past, there where much more mesh elements and degrees of freedom (nearly one million) and the request for RAM was much lower (in line with other threads in the Community Forum: www.comsol.eu/community/forums/general/thread/23213/).

Has anyone an idea why happens this? Thank you

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5 Replies Last Post 31 août 2012, 10:44 UTC−4
Pu
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Posted: 1 decade ago 28 août 2012, 06:16 UTC−4
Hi, all

I also have this problem.

Previously I mainly did 2D simulation. When moving to 3D, the memory consumption increases beyond my estimation... For a simple model with a little bit more than 1M degrees of freedom, it consumes even more than 60G memory. Since I don't have experience in 3D simulation with COMSOL, I don't know if this level of memory cost is normal or not.

And is there any methods to reduce the cost? Thanks a lot!
Hi, all I also have this problem. Previously I mainly did 2D simulation. When moving to 3D, the memory consumption increases beyond my estimation... For a simple model with a little bit more than 1M degrees of freedom, it consumes even more than 60G memory. Since I don't have experience in 3D simulation with COMSOL, I don't know if this level of memory cost is normal or not. And is there any methods to reduce the cost? Thanks a lot!
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 28 août 2012, 10:44 UTC−4
Interesting model. The memory usage explodes because of the heat source dPdV. It is based on an integration operator over the whole domain that is also a function of temperature (the independent variable). This leads to a coupling between all the degrees of freedom in the model creating a full matrix instead of the sparse limited-bandwidth matrices typical of finite element models.

This coupling results from COMSOL trying to calculate the most accurate Jacobian/stiffness matrix. To avoid it you should replace the source term dPdV with nojac(dPdV). That way COMSOL will not calculate the Jacobian of the heat source term. It may require a few more iterations but the memory required will drop by orders of magnitude.

Nagi Elabbasi
Veryst Engineering
Interesting model. The memory usage explodes because of the heat source dPdV. It is based on an integration operator over the whole domain that is also a function of temperature (the independent variable). This leads to a coupling between all the degrees of freedom in the model creating a full matrix instead of the sparse limited-bandwidth matrices typical of finite element models. This coupling results from COMSOL trying to calculate the most accurate Jacobian/stiffness matrix. To avoid it you should replace the source term dPdV with nojac(dPdV). That way COMSOL will not calculate the Jacobian of the heat source term. It may require a few more iterations but the memory required will drop by orders of magnitude. Nagi Elabbasi Veryst Engineering
Doyle Baker
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Posted: 1 decade ago 28 août 2012, 12:47 UTC−4
I am a running into this exact same issue with a simple mems design i am modeling. I am only trying to find a stress induced when a load is added to the surface but the computation time is through the roof and tends to crash. Does anyone know a way to reduce the amount of mesh elements?
I am a running into this exact same issue with a simple mems design i am modeling. I am only trying to find a stress induced when a load is added to the surface but the computation time is through the roof and tends to crash. Does anyone know a way to reduce the amount of mesh elements?
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 28 août 2012, 13:53 UTC−4
This is a different issue. You should use a coarser mesh. Check the COMSOL documentation for details.
This is a different issue. You should use a coarser mesh. Check the COMSOL documentation for details.
Camillo Giangiordano
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Posted: 1 decade ago 31 août 2012, 10:44 UTC−4
I replaced dPdV with nojac(dPdV) and the RAM request decreased 20 times! Now it works, thank you Nagi.

Camillo Giangiordano
I replaced dPdV with nojac(dPdV) and the RAM request decreased 20 times! Now it works, thank you Nagi. Camillo Giangiordano

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