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by metadat 596 days ago
I was curious so I looked it up:

https://en.wikipedia.org/wiki/DDR5_SDRAM (info from the first section):

> DDR5 is capable of 8GT/s which translates to 64 GB/s (8 gigatransfers/second * 64-bit width / 8 bits/byte = 64 GB/s) of bandwidth per DIMM.

So for example if you have a server with 16 DDR5 DIMMs (sticks) it equates to 1,024 GB/s of total bandwidth.

DDR4 clocks in at 3.2GT/s and the fastest DDR3 at 2.1GT/s.

DDR5 is an impressive jump. HBM is totally bonkers at 128GB/s per DIMM (HBM is the memory used in the top end Nvidia datacenter cards).

Cheers.
2 comments
reliabilityguy 596 days ago
> So for example if you have a server with 16 DDR5 DIMMs (sticks) it equates to 1,024 GB/s of total bandwidth.

Not quite as it depends on number of channels and not on the number of DIMMs. An extreme example: put all 16 DIMMs on single channel, you will get performance of a single channel.

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metadat 596 days ago
Thanks for your reply. Are you up for updating the Wikipedia page?, because as of now the canonical reference is wrong.
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angoragoats 596 days ago
If you're referring to the line you quoted, then no, it's not wrong. Each DIMM is perfectly capable of 64GiB/s, just as the article says. Where it might be confusing is that this article seems to only be concerning itself with the DIMM itself and not with the memory controller on the other end. As the other reply said, the actual bandwidth available also depends on the number of memory channels provided by the CPU, where each channel provides one DIMM worth of bandwidth.

This means that in practice, consumer x86 CPUs have only 128GiB/s of DDR5 memory bandwidth available (regardless of the number of DIMM slots in the system), because the vast majority of them only offer two memory channels. Server CPUs can offer 4, 8, 12, or even more channels, but you can't just install 16 DIMMs and expect to get 1024GiB/s of bandwidth, unless you've verified that your CPU has 16 memory channels.

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metadat 596 days ago
Got it, thanks for clarifying.

Happy Halloween!

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sroussey 596 days ago
Yes, and wouldn’t it be bonkers if the M4 Max supported HBM on desktops?
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