This blog will help illustrate the performance impact of Processor and Memory selection on the PowerEdge FC630. First closely examine the table below, you will notice with higher TDP processors the heatsinks cover portions of memory channels 2 and 3 on each processors installed.

This reduction in available memory slots is normally not a problem given total memory configurations of less than 256GB, as long as only memory channel 1 (i.e. sockets with white release tabs) are populated.

But as you can see in the following figure when the 104mm heat sink is installed, four memory slots on each processor socket are physically blocked. These particular DIMM sockets span across memory channels 2 and 3 (i.e. the black and green release tab memory slots).

If any memory slots beside the white slots (A1-4 & B1-4) are then populated, an unbalanced memory configuration will occur resulting in memory clock rate reduction and a dramatic drop in overall bandwidth performance. See the following Blog for comparative measurements:

http://en.community.dell.com/techcenter/b/techcenter/archive/2016/09/23/13g-poweredge-server-performance-sensitivity-to-memory-configuration

The picture below better illustrates how the heat sink block the memory slots in the PE FC630


Since the 104mm heatsinks cover 4 of the black and green memory slots on both Processors, placing memory in the black and green memory slots can cause memory performance problems. The only way to correct this problem on higher-end processors with TDP’s of 135 watts and above is to use only the white release tab memory slots in the PE FC630, this may require the use of 64GB DIMMs on some memory configurations.

Unbalanced memory configuration will be caused in two ways, not enough DIMMS in a channel and too many channels being activated. The first occurs when there are not enough DIMMs populated in the channel. When the black release tab memory slots (i.e. memory channel 2) on both processors are used, the memory channel will be unbalanced because we are missing two DIMMS in channel 2 on both processors, this generally causes the memory performance to be cut by around 73%.

Three DIMMs per channel will happen, when memory channel 2 and 3 are used on both processors. This drops the memory bus speed from 2400Mhz on an E5-26xx v4 to 1866 MHz and from 2133MHz on an E5-26xx v3 to 1600 MHz in some cases. The memory bus speeds quoted here can be different depending on the ranking of the memory, see the table below to better understand how memory ranking choices can affect memory speed.


The last thing that can affect memory bus speed and performance is the maximum speed at which the memory controller in the processor can run, see table below for more details. The E5-2600’s memory controller has a wide range, at the bottom the low-end E5-2600 v3’s processors start at 1600 MHz and at the top the high-end E5-2600 v4’s processors go up to 2400MHz.

An Example would be the PE FC630 has 2400MHz memory installed in memory channel 1, but the system has E5-2609v4 processors, the 2400MHz memory with only run at 1866MHz on the current configuration and will fall to 1600MHz if you install memory in all 3 memory channels. Consult the table below to see the max memory bus speed of the processor that you might be considering or is currently installed in your system.

The table below shows how adding more memory can cause the memory bus speed to drop. For instance if you have sixteen 8GB 2400MHz R DIMMs in the FC630 for a total of 128GB and you need another 32GB of system memory. To do this you add eight more 8GB 2400MHz R DIMMs in the green release tab slots (i.e. memory channel 3) of the PE FC630, the memory speed will drop from 2400MHz to 1866MHz, this causes around a 23% decrease in memory performance. If your application is memory sensitive it would have been better to replace the DIMMs in memory channel 1 (i.e. sockets with white release tabs) with 16GB R DIMMs, instead of add the 8GB R DIMMs to Memory channel 3 (i.e. sockets with green release tabs)


 

In summary, the memory controller in the E5-26xx processor supports many DIMM configuration, but to maximize the memory performance on the PowerEdge FC630, you must consider all of these limitations. To better understand the memory configurations available on the PowerEdge FC630 please consult the User’s Guide at http://www.dell.com/support/home/us/en/19/product-support/product/poweredge-fc630/