DDR4 RAM Latency

As a system builder, enthusiast, and gamer, one of the items we usually purchase in a new build is RAM (Random Access Memory). This RAM is used for fast temporary storage for your CPU (Central Processing Unit). Persistence storage such as HDDs and SDDs are very slow to process data directly plus they have a limited lifespan for reads and writes. Hence RAM is used between the CPU and the persistence storage, bringing faster application performance. We’ll take a look at finding the lowest RAM latency, which would be the faster response RAM and estimated cost per GB.

RAM is sold in terms of how fast the RAM is and CAS Latency. RAM speed is the built in clock cycles that refreshes the memory and is usually in terms of MHz or MT/s. CAS Latency is the delay time between memory controller asks for data and data is available on the RAMs output pins and is usually in nanoseconds.

Since usually RAM is sold with different speed and CAS Latency, the following formula will assist you on figuring out the relatively faster RAM as the 2 values do determine the performance of the RAM.

Latency (ns) = Clock Cycle Time (ns) x CAS Latency (CL)

Latency (ns) = (1/Module Speed(MT/s) x 2) x CAS Latency (CL)

To find the higher performance RAM, look at the least CAS Latency and higher speeds to minimize the latency as much as possible as shown below in the table. We’ve excluded some results due to not existing on the market or only found very few results to provide an good average price per GB.

Module SpeedCAS LatencyLatency (ns)Est Price $/GB (6/2019)
4400177.7320.937
4400188.1819.749
3600158.3316.281
4000178.515.745
4400198.6418.041
3200148.7511.149
3600168.8912.292
400018911.619
3000149.338.923
3200159.389.895
3600179.4410.635
4000199.515.706
2666139.7511.697
300015108.61
320016108.932
3600181010.754
36001910.566.484
30001610.677.868
24001310.836.906
26661511.257.023
30001711.335.577
24001411.676.454
266616126.582
21331312.196.242
24001512.57.432
21331413.139.76
24001613.335.82
21331514.067.466
24001714.177.164

The difference between the lowest and highest latency times are 7 nanoseconds apart. These differences might be noticeable, but very slightly.

As for our gamer audience, it is wiser to spend extra money on the GPU than on faster RAM. usually for gaming an individual computer only need s8 to 16 GB depending on the game and near 10 ns in latency before getting higher prices.

Officially, DDR4 memory limitation is set by JEDEC (https://www.jedec.org/) and the upper limit is at 3200 (MT/s) before needing to turn on XMP (Intel Extreme Memory Profile) in your motherboard settings to get the higher speeds on XMP Ready/Certified RAM.