Hacker News Reader

Comment by cudder | Hacker News Reader

> DDR5-5600 when 2 modules are installed, DDR5-3600 when 4 modules are installed

Is this a new thing? I don't rememebr four sticks being slower than two on previous generations. Then again I haven't been paying too close attention these past few years.

View on HN

New with DDR5, and it's the reason no-one should consider upgrading memory by adding additional sticks on DDR5 platform -- instead, if you need more memory, you should replace your modules. The additional two slots are basically a newbie trap.

Next gen will do away with this because it moves to CAMM2 and there will only be a single connector on consumer motherboards.

I'm buying a server with 12 slots, should I go for 4x128 gb instead of 8x64 then?

Is the server using udimms or rdimms? With rdimms the additional memory controller load from more dimms isn't nearly as much as with udimms

No, you want to utilize the extra memory bandwidth of the EPYC server CPUs. Use all 12 channels!

The desktop CPUs only have two memory channels so you don't get a performance benefit by using four memory modules.

Thanks, I'll try and populate it fully!

You want to ideally have exactly one DIMM per channel. How many dimms this means depends on configuration, but modern EPYC platforms are 12-channel, so there is likely one slot per channel on your board.

You can run two DIMMs per channel without much downside. There's a dual-socketed server mobo out there that has 48 DIMM slots, making it possible to run 24TB of RAM in a 2U rack-mount

On current AMD processors, there is absolutely downside to running two DIMMs per channel. That's the discussion we're having here.

It's a DDR5 thing - four sticks have absolutely been slower on the Zen 4 too. I've tried 4x32 GiB on a 7950X and I would get exactly that speed after several minutes (!) of POST. Anything faster would not be stable, I ended up sending back half of it and settled with 64 GiB total, POST still takes 30s.

I have a similar setup and with 4x32 sticks the most I have been able get it 4800 with EXPO enabled. Is that your experience too?

It's not new (as 2 other commenters posted). See for example (DDR4 on X470): https://www.asrock.com/mb/AMD/X470%20Master%20SLI/index.asp#... and also (DDR4 on X570): https://www.asrock.com/mb/AMD/X570%20Steel%20Legend/index.as...

Scroll down to the tables listing UDIMM Memory Slot and you can see how different configurations of single rank vs. double rank play out with respective to what becomes the "max" frequency.

Reply on news.ycombinator.com