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Mini cluster Information: would 1x i7 7700 + 1x i7 8700 be OK? |
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December 23, 2018, 15:44 |
Mini cluster Information: would 1x i7 7700 + 1x i7 8700 be OK?
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#1 |
Member
Andrew
Join Date: Mar 2018
Posts: 82
Rep Power: 8 |
Hello everyone, I kindly wanted to ask you for an opinion.
I'm doing a direct RANS simulation on a (simplified) car, mesh of about 30 or 40 million cells, with OpenFOAM Currently I am using a workstation with an i7 7700 processor clocked 3.60 - 4.20 Ghz and quad-core and 40GB ram ddr4 2400 mhz. I thought of building a simple 2-node cluster by purchasing a second workstation with an i7 8700 processor with six cores clocked 3.20 - 4.60 Ghz and 32Gb ram ddr4 2666 mhz. In your opinion, could the calculation time be halved? Thanks for the time taken to read the message. Astan |
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December 24, 2018, 11:29 |
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#2 |
New Member
Join Date: Aug 2017
Posts: 28
Rep Power: 8 |
I don't think so, benchmarks evaluates them as 60% faster, and the downside is that they both are dual channel. I would rather look for i7 5820k quad channel CPU build.
Reference: https://cpu.userbenchmark.com/Compar...700/3940vs3887 |
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December 24, 2018, 18:36 |
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#3 |
Member
Andrew
Join Date: Mar 2018
Posts: 82
Rep Power: 8 |
Hi Pibil1 Merry Christmas, thanks you for you answer.
how can i deduce from the link you sendt me that if I join those two pc speed is increased by 60%? |
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December 25, 2018, 06:12 |
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#4 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,400
Rep Power: 47 |
I don't think pibil1 necessarily meant connecting these two machines.
An I7-5820k can be twice as fast compared to an I7-7700 all by itself. Did you investigate if the cases you run fit into memory? 40 million cells with 40GB of RAM could be quite close. |
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December 25, 2018, 17:31 |
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#5 |
Senior Member
Join Date: Oct 2011
Posts: 242
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If you can have a second workstation for a few bucks and it time is cheap then it may be a good option. This is what I do with unused workstations. Otherwise, as stated by the other forumers it may be cheaper to go for a more powerful setup.
I agree with the remark of flotus 40milion cells for 40gb ram, you may be on the edge |
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December 26, 2018, 09:58 |
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#6 |
Member
Andrew
Join Date: Mar 2018
Posts: 82
Rep Power: 8 |
hi naffrancois and flotus1
Thank you very much for your answers and happy holidays. Yes, on a single node I'm at the limit of ram. But I thought that on 2 nodes, reaching 72 GB of ram that would no longer be a problem. However, i'm not sure about the improvement of the calculation speed. Based on a study of "speed up" and parallel efficiency, when in your opinion would be better to increase the number of cores? |
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January 2, 2019, 06:07 |
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#7 |
Member
Andrew
Join Date: Mar 2018
Posts: 82
Rep Power: 8 |
hello guys, happy new year!
These days I have done some simulations on the cavity and motorbike test cases. I performed simulation with 1 core and 2 cores with i7 7700. I calculated the speed up: clocktime (1 core) / clocktime (2 cores) and I get 1.6 and not 2. What can this problem depend on? Astan |
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January 2, 2019, 06:20 |
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#8 |
Senior Member
Join Date: May 2012
Posts: 548
Rep Power: 15 |
Most likely because you are not bandwidth limited on 1 core in combination with the fact that 1 core turbo is higher than 2 core turbo (i.e. the CPU is running at a higher frequency on 1 core compared to 2 cores).
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January 2, 2019, 09:09 |
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#9 |
Member
Andrew
Join Date: Mar 2018
Posts: 82
Rep Power: 8 |
Hi Simbelmynė thank you for your answer, i have tried to turn off the speed boost and done the simulation again. But the speed up is still less than 2.
What could i try to solve the problem? Astan |
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January 2, 2019, 12:14 |
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#10 |
Senior Member
Join Date: May 2012
Posts: 548
Rep Power: 15 |
If you have made sure that the frequency is not dynamically altered then I would guess that your memory is limiting the results. You could try to under-clock all cores to test this if you are accustomed to this kind of tinkering.
Also, a speed-up of 1.6 is not all that bad either since most i7 CPUs seem to be in the 1.7-1.8 span when going from 1 to 2 cores (with memory @ 3200 MHz). |
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January 2, 2019, 12:30 |
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#11 |
Senior Member
Join Date: May 2012
Posts: 548
Rep Power: 15 |
*double post - please delete at convencience*
Last edited by Simbelmynė; January 2, 2019 at 12:36. Reason: double post |
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