General OpenPOWER Hardware > General CPU Discussion

max memory speeds

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pocock:

AbstractConcept made this edit in the wiki

I read somewhere that RAM operates at different speeds:

1 channel filled: 2666MHz

2 channels filled: 2400MHz

4 channels filled: 2133MHz

but the edit by AbstractConcept implies that RAM is always clocked at 2666MHz

Can anybody clarify?

Borley:
A recent personal catastrophe has had me scouring the wiki, and I've discovered that the firmware version may be a limiting factor. I cannot confirm this at the moment, as my system is currently in the dry dock.

EDIT: Found the same to be true for the Talos firmware.
--- Quote ---Enable 2666MHz DDR4 memory DIMMs
--- End quote ---

ClassicHasClass:
My understanding (perhaps Raptor can clarify) is that both are right, in a manner of speaking. You get the full 2666MHz with a single DIMM per channel, 2400 for 2, 2133 for 4. However, the controller for each processor has 8 channels and no Raptor system has more than 8 DIMM slots per CPU, so it's physically impossible to get multiple DIMMs on one channel. On a large system with more than 8 DIMM slots per CPU socket, however, it's possible.

chatcannon:
I have a related question: I have noticed that sometimes 3200 MHz RAM is cheaper than the corresponding 2666 MHz DIMM. What happens if you use a 3200 MHz DIMM with a Talos / POWER9 board? Will every 3200 MHz chip support 2666 MHz operation or will it end up running at some slower speed like 2400 MHz or even 1600 MHz?

ClassicHasClass:
As a general rule 3200MHz should run fine at 2666MHz, but I don't see any 3200MHz RAM on https://wiki.raptorcs.com/wiki/POWER9_Hardware_Compatibility_List/Memory . That doesn't mean it doesn't work, but you'd be a pioneer.

Also, I want to correct a miscount in my prior post: each pair of DIMM slots in the T2/T2 Lite is a channel, so the T2/T2 Lite have 4 per CPU. However, the pair of DIMMs in that pair of slots counts as one, so it's still 1:1. The Blackbird, however, is indeed one channel per slot.

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