i7-6700, Reduced memory performance with 4 (DDR3) modules compared to 2

[DDimi4]

Hello, I have a performance problem:

I have 4 same memory modules of:

8GB DDR3L 1600 CL11 1.35v Dual-Rank

with the i7-6700 processor.

In my B150 motherboard (Asrock B150M Pro4S/D3) if I use 2 Modules in Dual-Channel mode I get the expected performance, but not with all 4.

With 2 Modules (16GB) in dual channel "winsat mem" built-in windows 10 memory benchmark utility reports:

~22000MB/s

If I use all 4 modules (32GB) in dual channel mode "winsat mem" reports:

~15000MB/s

And just for comparison with 1 module in Single Channel mode:

~12000MB/s~

The performance difference is visible in various applications, not just that benchmark.

Is this a CPU limitation or a motherboard problem?

The processor documentation says that the skylake cpus has a total of only 4 memory rank controllers. Is this the reason for the performance degradation? All 4 modules have 8 ranks total.

Thanks.

NOTE: There are no stability/reliability problems in both cases, this is just a performance issue.

[idata]

Hello DSInt ,

Make sure you are using same memories with same speed, when using different speed DIMMs; the slowest memory timing is used. Also, I suggest you to contact your motherboard manufacturer to find out if your motherboard supports this configuration. I can see at the AsRock website that only supports Dual Channel DDR3/DDR3L Memory Technology, but I recommend confirming this information with them. Regards, Ivan.

[DDimi4]

Hello again,

As I already mentioned all 4 DIMMs are exactly the same. When only 2 of the 4 are installed in different channels (in any combination of the 4 parts) I get the expected performance.

I don't understand your comment about my motherboard supporting Dual Channel DDR3/DDR3L Memory Technology, also I have informed Asrock about the problem but they haven't replied yet.

Can you confirm if the 6th Gen core family processors only has 4 rank controllers? The documentation (datasheet) is not clear:

First it says in the System Memory Interface section:

"number of ranks per channel are SKU dependent", but I can't find anywhere the differences in SKUs.

And later in the DRAM Clock Generation section:

"Every supported rank has a differential clock pair. There are a total of four clock pairs driven directly by the processor to DRAM."

By comparison the 5th gen core datasheets clearly says:

"Up to 64 simultaneous open pages, 32 per channel (assuming 8 ranks of 8 bank devices)"

and

"The IMC supports a maximum of two DDR3/DDR3L DIMMs per channel; thus, allowing up to four device ranks per channel." which means 8 ranks total.

If that is the case, it can explain the performance degradation with 8 ranks total (as in my case), because of the slow rank switching.

Thanks.

[idata]

What I mean about Dual-channel, Triple-channel mode etc, you can see it here:

http://www.intel.com/content/www/us/en/support/boards-and-kits/000005657.html http://www.intel.com/content/www/us/en/support/boards-and-kits/000005657.html When you have four RAMs installed, you will be doing Quad-channel mode. Please check the DataSheet page 22 it explains the system memory organization modes:http://www.intel.com/content/www/us/en/processors/core/desktop-6th-gen-core-family-datasheet-vol-1.html http://www.intel.com/content/www/us/en/processors/core/desktop-6th-gen-core-family-datasheet-vol-1.html Regards, Ivan.