Memory Latency Impact on Performance

We just looked at the impact of memory bandwidth on performance, but what about latency?  Let's first by adjusting the CAS latency from our default of 2 clocks up to 3 clocks.  Almost all DDR400 these days is CAS 2 memory, but older memory may have a higher CAS latency or you may have to increase your CAS latency when overclocking to gain more memory bandwidth, so what kind of a performance hit is there when going from CAS 2 to CAS 3?

At worst, CAS 2 memory seems to be about 5% faster than CAS 3 memory when looking at at_c17_12, our most CPU intensive test.  While 5% alone isn't anything major, combine that with a number of other performance tweaks and they can definitely begin to add up.

Now let's look at keeping Tcl (CAS latency) fixed at 2 clocks, but vary Trcd timings from 3 up to 6 clocks:

For the most part we saw no real changes when adjusting Trcd, the one exception being at_coast_05 which actually showed a pretty big difference between a Trcd value of 2 and higher latency values.

Next we'll look at adjusting Trp:

Here we see very little impact on performance.

Putting them all together we can see what the overall impact on using fast DDR400, higher latency DDR400 and extremely high latency DDR400 will be:

Our standard 2-2-2-10 memory does actually offer reasonable performance benefits in Half Life 2 compared to DDR400 with higher timings such as 3-3-3-10 or the unrealistically high 3-6-6-10. 

First and foremost Half Life 2 does appear to be rather dependent on memory bandwidth, but it is also quite appreciative of low latency memory as well.  If you're wondering whether being able to run memory at low timings and high clock speeds is important, when it comes to Half Life 2 performance it is.