Author Topic: mixing memory sizes on the same CPU?  (Read 463 times)

pocock

  • Sr. Member
  • ****
  • Posts: 276
  • Karma: +30/-0
    • View Profile
mixing memory sizes on the same CPU?
« on: August 30, 2022, 03:00:09 am »

I understand that each CPU has four memory channels, therefore, optimum performance is achieved when all four memory banks have an identical size and they are accessed in parallel.

Nonetheless, is it safe or practical to mix memory sizes in the memory banks on the same CPU?

For example, could the user have 2 x 16GB and 2 x 32GB connected to a single CPU?

Would the CPU ignore the extra capacity in the larger modules and use them all as 16GB?  Or would it somehow access the RAM using two channel speeds?
Debian Developer
https://danielpocock.com

ejfluhr

  • Newbie
  • *
  • Posts: 32
  • Karma: +3/-0
    • View Profile
Re: mixing memory sizes on the same CPU?
« Reply #1 on: September 01, 2022, 02:38:42 am »
Useful picture here:  https://www.itjungle.com/2018/03/05/deal-power9-memory-entry-servers/



The P9-Nimbus processor has 8 channels.  Sforza pins out 2 per right/left side; the other 2 on each side are unusable.

First factor:  does the system mainboard connect 2 DIMMs to the same memory channel or 1 DIMM per memory channel for each of the 4?   If 1 per channel, that is best performance and most flexibility, opening the possibility that mismatched DIMM sizes would each be used at full capacity.   If 2 DIMMs are connected on one channel, they must be fully matched (size, speed, maybe even manufacturer).

As seen here in this orientation (rotated 90-degrees from picture above):


The DIMM interfaces on the processor die are split half-top and half-bottom; the individual memory controllers are grouped on their respective edge and must run the same (maybe slowest plugged DIMM?) clock rate.  I -think- top group vs. bottom group are independent enough to run different sizes and even speeds.

It is also very dependent on the firmware + software.  Unclear to me if that will properly set up & use what the hardware sees.
« Last Edit: September 01, 2022, 02:45:47 am by ejfluhr »

MauryG5

  • Hero Member
  • *****
  • Posts: 680
  • Karma: +21/-1
    • View Profile
Re: mixing memory sizes on the same CPU?
« Reply #2 on: September 01, 2022, 09:15:04 am »
I don't know if this can give you an answer to your question but I personally have a blackbird with 24 GB of which one bank is 16 and the other is 8 and Raptor herself told me that I could do it without any problem. I can't tell you, however, that the performance changes, I use it regularly with all 24 GB and it has never given me problems, I just don't remember exactly which slot I put the 16 on and on which the 8 unfortunately ...

chatcannon

  • Newbie
  • *
  • Posts: 10
  • Karma: +3/-0
    • View Profile
Re: mixing memory sizes on the same CPU?
« Reply #3 on: October 27, 2022, 09:47:39 am »
I personally have 2x 32 GB and 2x 64 GB connected to a single CPU (inserted in the recommended slots as per the Talos II motherboard user guide) and have had no problems using all the memory capacity. I haven't tried any memory benchmarks to detect if there is any slowdown.