Articles: Memory

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Clock Rate vs. Timings

Every time we try to choose the best memory for our computers, we have to face the choice between higher clock rates and lower timings. However, this time around we are going to avoid detailed tests of DDR3 SDRAM modules with different timings because memory timings have come to affect the overall performance much less in the newer platforms than clock rate does.

There are two reasons for that. First of all, at higher clock rates the minimum timings are anyway quite high, so the value you can add is relatively small. When you add a couple of cycles to a timing that was originally set at 3 or 4 (like with DDR2 SDRAM), you are going to observe a bigger effect than if that timing was 7 or 8 cycles by default (as is the case with DDR3 SDRAM): the latency grows by 50-70% in the first case and by a mere 25-30% in the second case. Thus, different sets of memory timings are closer to each other with today’s memory.

The second reason is the overall improvement in how the CPU communicates with memory. There are more data caching levels and the amount of cache memory has steadily been increasing. This masks the actual latency of system memory, shifting the focus to its bandwidth instead.

As a matter of fact, the makers of overclocker-friendly memory kits have already realized that there's no need for low timings with high-frequency DDR3 SDRAM. Low-latency kits have disappeared and it’s rather hard to find DDR3 SDRAM modules with latencies below 9 cycles. The number of products with high latencies and high clock rates is, on the contrary, on the rise.

Well, we don’t want to be pure theorists, so we carried out a practical test to compare identical PC configurations with a Core i5-3570K and with DDR3-1600 and DDR3-1867 SDRAM that had different timings.

The diagrams illustrate what we’ve written above. Increasing the memory clock rate by 266 MHz turns out to be more effective than lowering each timing by 3 or 4 cycles. DDR3-1867 with 9-9-9-27 timings turns out to have a better effective latency than DDR3-1600 with aggressive timings of 7-7-7-21. As for effective bandwidth, DDR3-1600 can’t match the higher-clocked alternative under any circumstances.

Thus, memory timings have indeed become a rather insignificant parameter for today’s computers. When choosing DDR3 SDRAM for an Ivy Bridge processor, you should consider its clock rate in the first place whereas a low CAS Latency and other timings are in fact unimportant. The same goes for tweaking and overclocking: you should first focus on increasing the clock rate of your DDR3 SDRAM and only then minimize its latencies if you want to.

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