The difference between RAM speed and CAS Latency explained header graphic.

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
•As speeds increase, latencies decrease or remain roughly the same, meaning faster speeds deliver 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

335

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

18

13.50

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 it is executed. 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, clock cycle times have decreased, resulting in lower latencies as technology has matured, even though there are more clock cycles to complete. Because speeds are increasing and latencies are remaining roughly the same, you're able to achieve a higher level of performance using newer, faster, and more energy efficient memory.

At this point in the discussion, we need to note that when we say “latencies are remaining roughly the same,” we mean that, for example, from DDR3-1333 to DDR4-2666, latencies started at 13.5ns and returned to 13.5ns. While there are several instances in this range where latencies increased, the gains have been by fractions of a nanosecond. In this same span, speeds have increased by over 1,300 MT/s, effectively offsetting any trace latency gains.

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 speed. In general, as speeds have increased, latencies have remained approximately the same, meaning faster speeds enable you to achieve a higher level of performance. Learn more about compatibility between memory and other components.

Optimize your system by installing as much memory as possible, using the latest memory technology, and choosing modules with as much speed as is cost-effective and relevant for the applications you're using.

 


©2018 Micron Technology, Inc. All rights reserved. Information, products, and/or specifications are subject to change without notice. Neither Crucial nor Micron Technology, Inc. is responsible for omissions or errors in typography or photography. Micron, the Micron logo, Crucial, and the Crucial logo are trademarks or registered trademarks of Micron Technology, Inc. All other trademarks and service marks are the property of their respective owners.


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