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The Difference Between RAM Speed and CAS Latency

Memory (DRAM) performance is all about the relationship between speed and latency. While the two are closely related, they're not connected in the way you might think. Here's how speed and latency are related at a technical level – and how you can use this information to optimize your memory's performance.

If you're interested in how much memory your computer should have, read here.

The perception and the truth about latency

Perception

  • Many users believe CAS latency is an accurate indicator of real world latency performance
  • Many users also believe that because CAS latencies increase with speed gains, some of the speed gets nullified

Truth

  • Semiconductor engineers know that CAS latencies are an inaccurate indicator of performance
  • Latency is best measured in nanoseconds, which is a combination of speed and CAS latency
  • Both speed increases and latency decreases result in better system performance
     
    • Example: because the latency in nanoseconds for DDR4-2400 CL17 and DDR4-2666 CL19 is roughly the same, the higher speed DDR4-2666 RAM will provide better performance
    • Example: if the speed rating of a standard module and a gaming module is the same (i.e. DDR4-2666) but the CAS latencies are different (i.e. CL16 vs. CL19), then the lower CAS latency will provide better performance

The difference between the perception of latency and the truth of latency comes down to how latency is defined and measured.

Two race cars represent memory speed and CAS latency

The latency paradox

Latency is often misunderstood because on product flyers and spec comparisons, it's noted in CAS Latency (CL), which is only half of the latency equation. Because CL ratings indicate only the total number of clock cycles, they don't have anything to do with the duration of each clock cycle, and thus, they shouldn't be extrapolated as the sole indicator of latency performance.

By looking at a module's latency in terms of nanoseconds, you can best judge if one module is, in fact, more responsive than another. To calculate a module's latency, multiply clock cycle duration by the total number of clock cycles. These numbers will be noted in official engineering documentation on a module's data sheet. Here's what these calculations look like.

Technology

Module Speed (MT/s)

Clock Cycle Time (ns)

CAS Latency

Latency (ns)

SDR

100

8.00

3

24.00

SDR

133

7.50

3

22.50

DDR

333

6.00

2.5

15.00

DDR

400

5.00

3

15.00

DDR2

667

3.00

5

15.00

DDR2

800

2.50

6

15.00

DDR3

1333

1.50

9

13.50

DDR3

1600

1.25

11

13.75

DDR4

1866

1.07

13

13.93

DDR4

2133

0.94

15

14.06

DDR4

2400

0.83

17

14.17

DDR4

2666

0.75

19

14.25

DDR4    

2933

0.68

21

14.32

DDR4

3200

0.62

22

13.75

What is latency and the latency equation?

At a basic level, latency refers to the time delay between when a command is entered and when the data is available. Latency is the gap between these two events. When the memory controller tells the memory to access a particular location, the data must go through a number of clock cycles in the Column Address Strobe (CAS) to get to its desired location and complete the command. With this in mind, there are two variables that determine a module's latency:

•The total number of clock cycles the data must go through (measured in CAS Latency, or CL, on data sheets)
•The duration of each clock cycle (measured in nanoseconds)

Combining these two variables gives us the latency equation:

latency (ns) = clock cycle time (ns) x number of clock cycles

In the history of memory technology, as speeds have increased, (which means clock cycle times have decreased), the CAS latency values have also increased, however because of the faster clock cycle the true latency as measured in nanoseconds has remained roughly the same. By optimizing the balance between the maximum speed your processor is capable of and the lowest latency memory available within your budget, you are able to achieve a higher level of performance using newer, faster, and more efficient memory.

Which is more important: speed or latency?

Based on in-depth engineering analysis and extensive testing in the Crucial Performance Lab, the answer to this classic question is BOTH! Speed and latency both play a critical role in system performance, so when looking to upgrade we recommend:

  • Step 1: Identify the highest memory speed supported by both your processor and motherboard (including overclocking profiles).
  • Step 2: Select the lowest latency memory that fits within your budget at that speed, remembering that a superior (i.e. lower) latency means superior system performance.

 


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