Why are ram timings in DDR4 ram so much slower than DDR3?

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I've been looking over RAM timings today, and one thing I noticed is that, even when considering very high-end DDR4 RAM, the timings are absolutely massive compared to DDR3 - I can buy very cheap DDR3 RAM with 9-9-9-24 timing, but DDR4 RAM (at nearly ten times the cost) might have a timing of 19-24-24-46. I have two questions about this:

  1. Why are the timings so much higher in DDR4?
  2. Does the speed of DDR4 ram overcome the massive timing increases?

Kulahan

Posted 2016-10-07T21:34:05.370

Reputation: 193

they aren't actually slower, the number is just larger. Timings are in clock ticks, which are the period of time that it takes for a single cycle in the frequency. a 1GHz tick is 1 1 billionth of a second. a 2GHz tick is half that (0.5 billionths). so as the frequency goes up, the tick time goes down, so the timing figures take more ticks to do a task, but a tick is a smaller period of time. – Frank Thomas – 2016-10-07T21:39:32.740

See the answer here about how timings can be calculated in Seconds of time, to allow an apples to apples comparisons: http://superuser.com/questions/593772/calculate-performance-of-ram-using-timing-and-speed

– Frank Thomas – 2016-10-07T21:42:25.733

@FrankThomas so basically, the DDR4 RAM has such a high clock speed, the timings still come out to a smaller number of nanoseconds? That makes sense. I'll start applying that formula. thanks. – Kulahan – 2016-10-07T21:52:56.890

yeah, in this case the DDR4 has a frequency of 2.625 times that of the DDR3, so I'm pretty sure the numbers will work our favorably for it, though I haven't crunched the numbers. – Frank Thomas – 2016-10-07T21:54:29.857

you can almost assume that because the timing values are not quite at 2.5x of the DDR3, that the operations will be faster, if only marginally. as long as the frequency and datatransfer rate are higher, you are getting the value from the upgrade, as long as the timings are not proportionally worse. in fact, it may be worth it even if the timings are just a hair slower (proportionately) anyway. – Frank Thomas – 2016-10-07T22:00:32.797

@FrankThomas, it actually is slower... my 5.5 year old system has DDR3 at 1600 MHz with CL=6. I'm looking at getting a new system with DDR4 at 3200 MHz but the fastest latency I can find is 14. Double the clock rate and double the latency would be the same in real time, so even 14 is a little slower latency than my old DDR3. – psusi – 2016-10-07T22:41:32.697

That is very true, but remember, Latency isn't the end-all-be-all metric. having a faster data transfer rate and command rate (as a result of the increased frequency) does provide meaningful benifets, if we are looking at things this closely. https://en.wikipedia.org/wiki/CAS_latency#Effect_on_memory_access_speed

– Frank Thomas – 2016-10-07T22:55:37.110

@psusi So then what's the point of it? Is it just the newest standard with higher potential? Like, maybe a few years from now, it'll kick the crap out of the DDR3, but for now it's just not worth investing in? – Kulahan – 2016-10-07T22:55:37.140

Because the commands can be pipelined ( you can issue the next read command while the first one is still processing ). That means if you pipeline well ( as happens for large sequential memory access ), then the higher clock rate means you can transfer more data in less time. For random access, you are stuck with one command at a time and waiting for the long latency. – psusi – 2016-10-08T01:12:37.503

Answers

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1600MHz  9-9-9-24       1.25e-9 seconds per clock tick
CL = (1.25e-9 * 9)   =  1.125e-8 seconds
RtC = (1.25e-9 * 9)  =  1.125e-8 seconds
RtP = (1.25e-9 * 9)  =  1.125e-8 seconds
tRAS =(1.25e-9 * 24) =  3.0e-8 seconds

4266MHz 19-24-24-46     4.688e-10 seconds per clock tick
CL =  (4.688e-10 * 19)  = 8.907e-9 seconds
RtC = (4.688e-10 * 24)  = 1.125e-8 seconds
RtP = (4.688e-10 * 24)  = 1.125e-8 seconds
tRAS = (4.688e-10 * 46) = 2.156e-8 seconds

So on timings, the DDR4 is the clear winner, being at worst equal to the DDR3 latencies, and at best is in an different order of magnitude.

Frank Thomas

Posted 2016-10-07T21:34:05.370

Reputation: 29 039

Little bit of an apples to oranges comparison there: you are comparing the highest clocked DDR4 with "normal" clock, high latency DDR3. Compare it with CL6 DDR3 and the latency is worse for DDR4, even at that max clock rate. – psusi – 2016-10-08T01:08:09.703

those are the models that the OP indicated in their links. I agree it is comparing extremes, and you are quite right that you need to exceed more than 2x frequency, or you may be suffering a hit on latency when all else is equal, but for the specific models the op indicated, the ddr4 has the advantage. I do wonder if they have a CPU/board that will support RAM at that speed, but that is not part of the post. – Frank Thomas – 2016-10-08T02:05:25.183

Asked: 2016-10-07T21:34:05.370

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Active: 2016-10-07T23:16:22.177