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name Punditsdkoslkdosdkoskdo

Can more memory than I need slow down my server?

We're about to install a memory intensive application and will purchase a server for it. The server will have at least 4 CPUs, with multiple cores. While the application is memory intensive, the consultants we're speaking with said that 32GB should be sufficient. They also said that if I add more memory than that, the server will slow down because of the "extra wiring" to connect all of the memory. I'm not planning on putting VMs on the hardware so I don't need extra memory to partition for virtual machines. However I am concerned that 32GB may not be enough.

Does the consultant's assertion that more memory than I need will slow the machine down correct?

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    • WHY are you concerned that 32GB "may not be enough"? What logical analysis of your environment and use case has lead to that conclusion?
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    • Perhaps you have a system that wants triple channel memory, but has a number of sockets that aren't evenly divisible by 3 or something? So if you fill up all the sockets you will not be able to take advantage of the triple channel feature? The 'extra wiring' bit sounds like crap though.
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    • FWIW it's a BI system for a data warehouse that does all of its work in memory. Supposedly it's highly compressed, but it still worries me.

I've not heard the terminology "extra wiring" but adding more ranks to a memory channel may slow it down. There should be documentation on the server that covers memory configurations, including what bus speed is supported with varying amounts of rank on each bus. It's pretty common for the speed to drop after about 3 to 4 ranks.

See also:

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It sounds to me like the consultant is blowing smoke. Either that, or they're not qualified to be IT consultants. The only time that adding more memory has ever slowed anything down in my experience is going above 3.5 GB (i.e. from 2 GB to 4 GB or more) on an x86 (32-bit) system, which requires a different memory addressing scheme. If you're starting with 32 GB RAM, going to 64 GB won't slow anything down.

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Depending on the exact specs of a server, it is possible that the memory bus is clocked down when additional modules are plugged in (the 'extra wiring' bit is bs though). Specifically, the maximum memory clock speed depends on the number of ranks in each channel.

The memory controller has a specific driving power, e.g. clocking up to four ranks with full speed, running up to six ranks with one clock step slower, and eight ranks with two steps slower.

Rules change when buffered DIMMs (RDIMMs or LRDIMMs) are used - they take bus load off the controller, so more modules can be used or the same number can be clocked higher (for servers supporting multiple flavors, usually not mixed). Check the server's manual for specific limitations.

So, when buying RAM you should be looking out for the least number of DIMMs and ranks, and good buffering.

However, memory clock speed consequences differ greatly with application or workload. When memory runs low and swapping starts you'll gladly trade in a slight speed degradation due to downrated memory speed for a large speed degradation due to swapping - which may be 100x slower than the slowest memory.

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Warm tip !!!

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