This topic has been moved to our Knowledge Base (KB).
Please refer to the KB link above for some general tips and guidelines for users interested in understanding the computer hardware specifications that have the largest effect on FDTD simulations on a single computer.
Amount of RAM
The amount of RAM determines the size of the simulation that can run on the computer. It does not affect the simulation speed (as long as you have enough RAM so the computer is not forced to swap memory to the hard drive). If swapping occurs, the simulation speed will be extremely slow.
Desktop computers typically have 6-12 GB of RAM, which is sufficient for running a large fraction of simulations. Workstations often have 48-96 GB of RAM, which is sufficient for running almost all simulations.
The memory bandwidth between the CPU and RAM is a crucial hardware specification. In most computers, the memory bandwidth is the factor that limits the overall simulation speed. Therefore, when purchasing a computer, having a fast memory bandwidth is very important.
CPU # of cores
Lumerical’s FDTD software will use all of your CPU cores to run as quickly as possible. However, having more cores does not always make the simulation go faster. Typically, the computer memory bus speed is not fast enough to keep all of the CPU cores running simultaneously. For example, on a typical desktop computer with 8 cores, the simulation speed might increase by a factor of 2-3x when using eight cores compared to one core. The explanation for this increase is that the memory bus is only fast enough to keep two or three cores running at full speed. Using additional cores does not increase the overall simulation speed because each core is forced to spend some time waiting for data to be transferred to/from the RAM. Obviously, a faster memory bus allows more of the cores to be efficiently utilized.
Most CPUs’ support hyper-threading, which allows the operating system to treat each physical CPU core as two cores. This feature does not provide any speed increase for FDTD simulations because the overall performance bottleneck is the data transfer rate between the CPU and RAM, not the actual computing capability of the cores.
CPU clock speed
The CPU clock speed is typically not the most important factor for FDTD simulation speeds. While a faster clock speed does allow each core to run more quickly, the overall simulation speed is limited by the memory bandwidth, not the computing capability of the cores.
Workstations with multiple CPU’s
Workstations with multiple CPU’s are a good way to increase the speed the simulation speed. The most important factor is that each CPU has its own memory bus connection to the RAM. As explained above, the data transfer rate between CPU and RAM is the performance bottleneck, so having one memory bus per CPU allows the simulation speed scale very well with the number of CPU’s.
Other computer hardware
Other computer hardware such as hard drives and GPU’s typically don’t have a significant effect on the simulation speed. Similarly, the network speed does not have a significant effect on the simulation speed when the entire simulation is calculated on a single computer. However, for customers running individual simulations that are distributed between multiple computers, the network speed is extremely important. High speed, low latency interconnects such as InfiniBand are recommended in such cases.