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CAS latency vs Speed

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tolham

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which is faster: DDR3 2000 (PC3 16000) with CAS 9, or DDR3 1600 (PC3 12800) with CAS 6?

i don't fully understand RAM. the two models i'm comparing are CORSAIR XMS3 and G.SKILL PI.


also: does it make a difference to have 4x 2GB sticks vs 2x 4GB?
 

mrmaia

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To put it on simple maths:

The frequency is expressed in Hertz, which means "cycles per second". So, the DDR3 2000 will perform 2000 cycles a second while the DDR3 1600 will do, well, 1600.

The CAS latency is given in cycles. So, a CAS9 RAM will take 9 cycles to respond and the CAS6, 6 cycles.

Now putting it together: the DDR3 2000 CAS9 will take 9/2000 seconds, which is equal to 0,0045 seconds, to respond while the DDR3 1600 CAS 6 will take 6/1600, which is equal to 0,0038 seconds, to respond. Thus, the 1600 one is faster.

Less RAM sticks generally means less load to the memory controller. So, 2x4GB is better than 4x2GB.
 

mrmaia

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Now that you got it, a correction must be made:

Frequencies are expressed in MEGA Hertz, which means a million cycles per second. So while the 1600 is still faster and stuff, the response times are a millionth of what I wrote :)
 

beenthere

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...but unfortunately with DDR3 RAM having higher frequencies and bandwidth, the frequency and RAM latencies have very little impact on system performance because they are not a bottleneck as can occur with slower DDR/DDR2 RAM.

Typically increasing RAM speed from 1333 MHz. to 1600 MHz. results in ~ 1% system performance. Tom's and Anandtech have both done benches on RAM frequency and latencies for gaming and seen minimal gains so there's little point in spending more money for almost no system performance gain unless you can do a major RAM OC like going from a default of 1066 MHz. to 2133 MHz.
 

Fern2899

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I'm not a pc hardware pro, but since I kinda have a question about this subject, is there really a noticeable difference between say 1600 ddr3 w/ Cas 7 latency vs same ram but being Cas 8 or even a Cas 8 vs Cas 9? Is it even worth the cost or is it mainly for bragging rights... Reason I'm asking is cause I'm planning on purchasing ddr3 1600mhz ram and as I'm reading I see different latency types... And I mean almost $50 price difference from Cas 9 and cas7... Now is it really that much of a noticeable difference that's it's worth paying almost $50 more?
 

gijoe262

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What a great explanation. Just set out to figure this out today and this pretty much wraps it all up. Thanks, and good for you!
 
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