Epyc Rome IFOP limited to 1467MHz?

diogin

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Mar 28, 2018
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AMD released a Workload Tuning Guide: https://developer.amd.com/wp-content/resources/56745_0.80.pdf

2.2.3 Memory Clock Speed
Benefit: By default, the BIOS for EPYC 7002 Series processors will run at the maximum allowable clock frequency by the platform. This configuration results in the maximum memory bandwidth for the processor, but in some cases, it may not be the lowest latency. The Infinity Fabric will have a maximum speed of 1467 MHz (lower in some platforms), resulting in a single clock penalty to transfer data from the memory channels onto the Infinity Fabric to progress through the SoC. To achieve the lowest latency, you can set the memory frequency to be equal to the Infinity Fabric speed. Lowering the memory clock speed also results in power savings in the memory controller, thus allowing the rest of the SoC to consume more power potentially resulting in a performance boost elsewhere, depending on the workload.
So, as the doc says, the Infinity Fabric will have a maximum speed of 1467 MHz (lower in some platforms).
AMD has advertised Epyc Rome to supporting 3200MHz DDR4. Why does it limit IFOP to 1467MHz instead of 1600MHz? Anyone knows the details of IFOP on Epyc Rome?
 
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TXAG26

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AMD released a Workload Tuning Guide: https://developer.amd.com/wp-content/resources/56745_0.80.pdf



So, as the doc says, the Infinity Fabric will have a maximum speed of 1467 MHz (lower in some platforms).
AMD has advertised Epyc Rome to supporting 3200MHz DDR4. Why does it limit IFOP to 1467MHz instead of 1600MHz? Anyone knows the details of IFOP on Epyc Rome?
Do Supermicro H11 boards allow adjustment of the memory speed from 3200MHz to 2933MHz to take advantage of this optimization?
 

diogin

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Do Supermicro H11 boards allow adjustment of the memory speed from 3200MHz to 2933MHz to take advantage of this optimization?
Yes they do. 2933MHz memory modules fit natively. 3200MHz memory modules fit by reducing frequency to 2933MHz, you can adjust it in BIOS.
 

sno.cn

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I was going to ask the same question, I saw that 1467 MHz number in the tuning guide too.

I have one running 2666 but I'm doing another build right now and I might go with 2933 instead of 3200 to save a few bucks.

Yes they do. 2933MHz memory modules fit natively. 3200MHz memory modules fit by reducing frequency to 2933MHz, you can adjust it in BIOS.
H11 BIOS has 3200 MHz as a manual memory speed option.
 

IamSpartacus

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Mar 14, 2016
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My RAM runs at 3200 when set to auto on my H11 but now I'm wondering if irs better to manually down clock it to 2933.
 

TXAG26

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I guess it’s a question of bandwidth vs latency. With all 8 channels populated, I would think most applications would have a hard time maxing that out, whereas latency would probably have a larger positive impact on performance. But I would guess that varies wildly between workloads.