what is cycle latency?

[XcatXboy]

i don't understand the cl16 cl19 cl21 and other

[trag1c]

CAS latency is how long from when a read command is issued to the data being available. It's how many clock cycles most occur before the first word of data is made available. So a CL16 kit will take 16 clock cycles before you can read data off of it. Basically the lower the CAS (CL) number the better the ram (assuming identical clock speeds).

[YoungBlade]

CL refers to the number of cycles that it takes to perform a specific type of memory operation. You usually see what are called "primary timings" listed, like 16-18-18-38, but there are dozens and dozens of different timings that are set beyond those. It can get insanely technical, and I will not claim to understand it fully, but those primary timings are generally the most important for performance, which is why people care about them.

 

Also important is that your RAM runs at a certain speed. For DDR RAM, the speed listed is double the clock speed of the RAM. DDR4-3200 actually runs at 1600MHz, although it transfers data twice per clock cycle, giving you 3200 mega-transfers per second, sometimes shown as MT/s. That's great, and it means that calling it "3200" is not false advertising or anything, however, when dealing with timings, the true MHz number is what actually matters.

 

Let's say you have DDR4-3200 CL16 memory. This means that, for that first primary timing, it takes 16 clock cycles to complete that operation. The reason that one is the one people most care about, is that it's arguably the most important timing. It is the "CAS latency," which is the time from when a command is sent to the memory to when it will begin to respond to the command. The CPU will ask the RAM for something, and it will take 16 clock cycles before the RAM can start to give it that information.

 

DDR4-3200 is actually running at 1600MHz, so there are 1600 million clock cycles happening every second. You can divide to find how long it will take to complete 16 cycles. In this case, the answer is 10 nanoseconds. So it takes 10 nanoseconds for the RAM to answer a request for data from the CPU because the CPU has that combination of speed and CL.

 

It is the combination of speed and timings that determines the latency of RAM. DDR4-3600 CL16 is not just faster than DDR4-3200 CL16 in terms of how much data it can transfer per second, it's also faster because it runs more clock cycles every second, in this case, 1800MHz, and so it only requires 8.89 nanoseconds to respond, because it takes a bit less time to complete 16 clock cycles. On the flip side, DDR4-2400 CL16 is much slower, as it's only running at 1200MHz, and so needs 13.33 nanoseconds. And if you had DDR4-3200 CL14 instead, it would still be running at 1600MHz, but it would only need 8.75 nanoseconds to complete 16 cycles. If instead you had DDR4-3600 CL18, its latency is actually the same as the first example, at 10 nanoseconds.

[Chris Pratt]

Already good explanations from others. I'll just add that the lower the CL value, the better, and that you should look for RAM with a CL no higher than 1/2000 the rated clockspeed for DDR4. For 3200MHz, for example, that's CL16 max. If you see a 3200MHz CL18 or higher kit, that's a sign that it's crap RAM, and you should avoid it.

[SkilledRebuilds]