RAM tested latency is a measure of how quickly your system memory responds to requests for data, typically expressed by a CAS Latency (CL) rating and quantifiable in nanoseconds.
Understanding RAM latency is crucial for PC performance, as it directly impacts how fast your processor can access information stored in memory. The lower the latency, the faster the data can be retrieved.
Understanding Memory Latency
According to the reference, memory latency can be thought of as "how quick will my system memory respond to a request". This response time is usually measured in units of nanoseconds (ns).
One of the primary ways RAM latency is specified is through the CAS Latency (CL) rating. This number, often seen in memory specifications like "CL16" or "CL18," represents the number of clock cycles it takes between when a command is issued to the memory controller and when the data is available.
The Role of CAS Latency (CL)
While the CL rating is a key indicator, it's important to note that it represents clock cycles, not a fixed amount of time. Therefore, a lower CL number doesn't automatically mean lower latency in nanoseconds, especially when comparing RAM with different speeds.
Calculating True Latency in Nanoseconds
To determine the actual latency in nanoseconds, you need to consider both the CAS Latency (CL) and the memory module's speed (data rate). The reference provides a formula for this calculation:
(Real Latency) = CAS Latency x 2000 / Data-rate
Here's a breakdown of the components:
- CAS Latency: The CL rating of the memory module.
- Data-rate: The effective speed of the RAM in MT/s (MegaTransfers per second), which is often advertised as the memory speed (e.g., 3200MT/s is commonly referred to as DDR4-3200).
- 2000: A constant used in the formula to convert clock cycles and data rate into nanoseconds.
Example Calculation
Let's compare two theoretical RAM kits:
- Kit A: DDR4-3200 CL16
- Data-rate = 3200
- CAS Latency = 16
- Real Latency = 16 x 2000 / 3200 = 32000 / 3200 = 10 ns
- Kit B: DDR4-3600 CL18
- Data-rate = 3600
- CAS Latency = 18
- Real Latency = 18 x 2000 / 3600 = 36000 / 3600 = 10 ns
In this example, despite having different CL ratings and speeds, both kits have the same real latency in nanoseconds. This illustrates why looking only at the CL number can be misleading.
Why Latency Matters
Lower real latency can contribute to better performance in tasks sensitive to memory access time, such as gaming, content creation, and running complex simulations. However, the impact of latency often needs to be considered alongside memory speed (bandwidth), as both play a role in overall system responsiveness.
