Effective Memory Management

Especially in 3D modeling, extensive memory usage requires some extra precautions. First, check that you have selected an iterative linear system solver. Normally you do not need to worry about which solver to use because the physics interface makes an appropriate default choice. In some situations, it might be necessary to make changes to the solver settings and the model. For details about solvers, see the Studies and Solvers chapter.

Estimating the Memory Use for a Model

Out-of-memory messages can occur when COMSOL tries to allocate an array that does not fit sequentially in memory. It is common that the amount of available memory seems large enough for an array, but there might not be a contiguous block of that size due to memory fragmentation.

In estimating how much memory it takes to solve a specific model, the following factors are the most important:

•	The number of node points

•	The number of dependent and independent variables

•	The element order

•	The sparsity pattern of the system matrices. The sparsity pattern, in turn, depends on the shape of the geometry and the mesh but also on the couplings between variables in a model. For example, an extended ellipsoid gives sparser matrices than a sphere.

The MUMPS and PARDISO out-of-core solvers can make use of available disk space to solve large models that do not fit in the available memory.

You can monitor the memory use in the bottom-right corner of the COMSOL Desktop, where the program displays the amount of physical memory and total virtual memory used.

Creating a Memory-Efficient Geometry

A first step when dealing with large models is to try to reduce the model geometry as much as possible. Often you can find symmetry planes and reduce the model to half, a quarter, or even an eighth of the original size. Memory usage does not scale linearly but rather polynomially (Cnk, k > 1), which means that the model needs less than half the memory if you find a symmetry plane and cut the geometry size by half. Other ways to create a more memory-efficient geometry include:

•	Avoiding small geometry objects where not needed and using Bézier curves instead of polygon chains.

•	Using linear elements if possible (this is the default setting in many physics interfaces). See Selecting an Element Type.

•	Making sure that the mesh elements are of a high quality. Mesh quality is important for iterative linear system solvers. Convergence is faster and more robust if the element quality is high.

•	Avoiding geometries with sharp, narrow corners. Mesh elements get thin when they approach sharp corners, leading to poor element quality in the adjacent regions. Sharp corners are also unphysical and can lead to very large (even infinite, in theory) stress concentrations.

Information About Memory Use

In the bottom-right corner of the COMSOL Desktop is information about how much memory COMSOL is currently using. The two numbers in Figure 3-11 displayed as 921 MB | 1006 MB represent the physical memory and the virtual memory, respectively. If you position the cursor above these numbers, the tooltip that appears includes the numbers with the type of memory explicitly stated:

•	The Physical memory number is the subset of the virtual address space used by COMSOL that is physically resident; that is, it is the amount of physical memory (RAM) in “active” use.

•	The Virtual memory number is the current size of the virtual address space that COMSOL uses.

Figure 3-11: An example of memory use displayed in COMSOL.