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June 19, 2020, 10:46 |
Slow transient simulation
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#1 |
Senior Member
Join Date: Aug 2012
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I am running CFX simulation of a transient problem with 5M elements.
The simulation is too slow even though I am running it with parallel processing. It takes an hour to complete about 30 time steps. Each time step has 5 or 6 iterations. I have just found out that under host memory information out of 32 GB ram only 510 (1.56%) has been allocated at the start of run. Again, there is another memory allocation data which says out of 32 GB ram only 959 (2.94%) has been allocated. At last, there is another host memory information which shows Npart is 20 and 15GB (47.14%) has been allocated. I have a workstation with 2X v3 xeons and am using 1 16 GB ram per cpu. The rest 6 slots are empty. I wonder is there a problem that makes the simulation too slow and would I benefit from upgrading the memory? |
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June 19, 2020, 13:39 |
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#2 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
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It's the memory population. You need 4 identical DIMMs per CPU.
So yes, upgrading memory is the most urgent hardware change you need. |
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June 20, 2020, 10:20 |
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#3 | |
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Quote:
Does it really make huge difference when all slots are occupied? Is there a big difference between 8*8GB and 8*16GB in my case? |
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June 20, 2020, 11:51 |
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#4 | ||
Super Moderator
Alex
Join Date: Jun 2012
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Your seller either does not know what exactly you are doing with your workstation, or he does not know what he is talking about.
For parallel CFD, memory bandwidth is what matters the most. Your CPUs have 4 memory channels each, and you need to populate each channel with at least one DIMM, and avoid using different size DIMMs. So you need 8 identical DIMMs, since you have two CPUs. For performance in your particular case, it almost doesn't matter if you use 8x4GB, 8x8GB, 8x16GB, 8x32GB or even 8x64GB. There are minor differences, but since each of these configurations gives you enough memory to fit your simulation, these differences pale in comparison to bottlenecking the CPUs with only one memory channel. Quote:
Quote:
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June 30, 2020, 01:23 |
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#5 | |
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Quote:
I have populated all the slots with 8x16GB, but the simulation time did not change too much. Before I use additional memory, it took about an hour to complete 12-15 time steps (each time step has 5-6 iterations). Now, it completes 25 time steps in an hour, but I was expecting a better performance. Is it normal for a transient problem with 5M elements to take too much time to solve? Task manager shows that only 21% of memory is used. I tried to increase memory allocation factor but it did not change the percentage of the memory usage. Shouldn't the memory usage be more than this percent? |
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June 30, 2020, 03:17 |
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#6 | |||
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,399
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Quote:
Quote:
For a bog-standard single-phase simulation, solution times would be on the slow side. For a multiphase fluid-structure coupling on the other hand... Quote:
Also, increased memory allocation does not show up as used memory in windows task manager. Glossing over pretty much all the details: task manager shows used memory (data that actually resides in RAM), not allocated memory. At least with default settings. In order to make people with a background in CS completely flip their shit, an over-simplified example: You could make a program allocate 1TB of memory. If all the data of the simulation fits into 20GB of memory, task manager will show 20GB as "memory". There could still be other factors holding back your simulation speed. Hardware problems, software/driver issues, unnecessary disk-I/O... |
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