I am planning to buy this memory. I don't understand what 9-9-9-24-2N means vs a timing like 9-9-9-20? Does it matter in performance?
A related question: what's the difference between buying the above 16GB vs buying two of these? The second option is cheaper.. Is this worth it?
To actually answer the question, I found what the numbers mean here: http://www.hardwaresecrets.com/article/Understanding-RAM-Timings/26/2
Here's the relevant portion:
The operations that these numbers indicate are the following: CL-tRCD-tRP-tRAS-CMD. To understand them, bear in mind that the memory is internally organized as a matrix, where the data are stored at the intersection of the lines and columns.
- CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and then returning it.
- tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
- tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the beginning of the access to another line of data.
- tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.
- CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not announced. It usually is T1 (1 clock cycle) or T2 (2 clock cycles).
So in your example of 9-9-9-24-2N, your CAS latency, RAS to CAS delay, and RAS precharge would all be 9 clock cycles, your active to precharge delay would be 24 clock cycles, and your command rate would be 2N (I think that means 2 clock cycles).
If you view the link above, the article goes in to more depth about what each of the numbers means. The article has a total of 6 pages, I linked to page 2. Pages 3-6 explain what the different timings are.
Wikipedia has a page on it too: Memory timings, but I found the first article I linked to explained it a little better.
Those numbers describe different latencies that are incurred during memory access. The larger the numbers, the slower the memory.
Here are the gory details: http://www.techpowerup.com/printarticle.php?id=131
Besides brand, the primary difference between your two choices is 1.5V versus 1.65V. This can be a real issue if your motherboard doesn't support one or the other.
Always check the motherboard manufacturer's memory support list. If your motherboard is new, you should be able to find memory that is guaranteed to work on your system. Older motherboards can be hard to find matches for given the short half-life of memory products.
At the very least, see if the preponderance of large DDR3 on the supported list is 1.5V or 1.65V.
In addition, since you are installing a very large amount of memory, verify that your Power Supply is of high quality and sufficient amperage on the system board side rails. Most power supplies become a bit unstable (large ripple and voltage variances) when taxed at their maximum on one rail. If you have two CPU's as well, your system may not be well supported by a gaming power supply that expects the big loads to be on the peripheral rails (dual video cards, RAID, etc.).
The numbers refer to clock cycles, so if you're comparing PC 1333 with PC 1600, larger numbers on the PC 1600 would actually indicate similar performance. so 7-7-7-20 on PC 1333 should perform similarly to 9-9-9-24 on PC 1600.
