HPC upgrade - # of cores or GHz?

Chris13 · January 17, 2017, 04:26

Hi everyone,

I was asking myself if the number of cores or a higher frequency would be a better choice for upgrating the HPC cluster at my workplace. In detail, I'm currently running 4 nodes, each with 2x Xeon E5-2630 v2 and 64 Gb memory.
I want to add 2 more nodes and need to decide wether to use 2x 14-Core E5-2680V4 @ 2.4GHz with 64 or 128 Gb memory - or 2x 8-Core E5-2667V4 @ 3.2 GHz with 64 or 128 Gb memory. The software I'm using is Ansys CFX - Fluent. Does anyone can give me an advice which would be a better choice, independent from any price related issues?

Thanks
Chris

flotus1 · January 17, 2017, 08:39

Without considering the hardware prices these are the best dual-socket processors for CFD when you have significant license costs.
E5-2687W v4 - 12 cores - 30MB Cache - 3.2 GHz all core turbo
E5-2667 v4 - 8 cores - 25MB Cache - 3.5 GHz all core turbo
E5-2643 v4 - 6 cores - 20MB Cache - 3.6 GHz all core turbo
E5-2637 v4 - 4 cores - 15MB Cache - 3.6 GHz all core turbo

If you have more than 16 parallel licenses, the E5-2687W v4 is your best option.
To answer you question in a more general sense: Frequency is usually more important than the number of cores since higher number of cores increase the parallelization overhead. So going for a higher number of significantly slower cores is usually not a good idea for CFD. Additionally, maximum memory bandwidth is the same for all these processors so you are more likely to cause a bottleneck here with high core counts.

Chris13 · January 17, 2017, 08:55

Hi Alex,

thank you for your answer. So I probably go with the 8 core CPU's. I'm going to contact Ansys support though and see what they say just to get another point of view. Thank you!

flotus1 · January 17, 2017, 09:51

Feel free to sum up their answer here. I am pretty sure what it will be, but as you suggest it is always good to have a second opinion.

Chris13 · January 31, 2017, 05:42

Hi again and sorry for the late reply.
So the Ansys customer service didn't give a specific answer to my problem about the number of cores vs. frequency question... what they did say was, that CFX scales well with a number of Xeon E5 26xx processors which is not very helpful. But they also said, that CFX was generally invented for parallelization so I should rather go with more cores if the jobs get big and if I have sufficient memory ( about 4 Gb / core ) and licences.
From a number of configurations, the (2x nodes) 2x 14-Core Broadwell E5-2690v4 Xeon with 128 Gb memory option was the one Ansys advised and that's probably what I'm going to choose.

Please everybody, do not see this as a recommendation of what to buy! It's what I am choosing for my specific task and with the capabilities I have.

Best regards
Chris

January 17, 2017, 04:26	HPC upgrade - # of cores or GHz?	#1
Chris13 New Member Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 9	Hi everyone, I was asking myself if the number of cores or a higher frequency would be a better choice for upgrating the HPC cluster at my workplace. In detail, I'm currently running 4 nodes, each with 2x Xeon E5-2630 v2 and 64 Gb memory. I want to add 2 more nodes and need to decide wether to use 2x 14-Core E5-2680V4 @ 2.4GHz with 64 or 128 Gb memory - or 2x 8-Core E5-2667V4 @ 3.2 GHz with 64 or 128 Gb memory. The software I'm using is Ansys CFX - Fluent. Does anyone can give me an advice which would be a better choice, independent from any price related issues? Thanks Chris

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January 17, 2017, 08:39		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,398 Rep Power: 46	Without considering the hardware prices these are the best dual-socket processors for CFD when you have significant license costs. E5-2687W v4 - 12 cores - 30MB Cache - 3.2 GHz all core turbo E5-2667 v4 - 8 cores - 25MB Cache - 3.5 GHz all core turbo E5-2643 v4 - 6 cores - 20MB Cache - 3.6 GHz all core turbo E5-2637 v4 - 4 cores - 15MB Cache - 3.6 GHz all core turbo If you have more than 16 parallel licenses, the E5-2687W v4 is your best option. To answer you question in a more general sense: Frequency is usually more important than the number of cores since higher number of cores increase the parallelization overhead. So going for a higher number of significantly slower cores is usually not a good idea for CFD. Additionally, maximum memory bandwidth is the same for all these processors so you are more likely to cause a bottleneck here with high core counts.

January 17, 2017, 08:55		#3
Chris13 New Member Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 9	Hi Alex, thank you for your answer. So I probably go with the 8 core CPU's. I'm going to contact Ansys support though and see what they say just to get another point of view. Thank you!

January 17, 2017, 09:51		#4
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,398 Rep Power: 46	Feel free to sum up their answer here. I am pretty sure what it will be, but as you suggest it is always good to have a second opinion.

January 31, 2017, 05:42		#5
Chris13 New Member Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 9	Hi again and sorry for the late reply. So the Ansys customer service didn't give a specific answer to my problem about the number of cores vs. frequency question... what they did say was, that CFX scales well with a number of Xeon E5 26xx processors which is not very helpful. But they also said, that CFX was generally invented for parallelization so I should rather go with more cores if the jobs get big and if I have sufficient memory ( about 4 Gb / core ) and licences. From a number of configurations, the (2x nodes) 2x 14-Core Broadwell E5-2690v4 Xeon with 128 Gb memory option was the one Ansys advised and that's probably what I'm going to choose. Please everybody, do not see this as a recommendation of what to buy! It's what I am choosing for my specific task and with the capabilities I have. Best regards Chris