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FSB vs RAM clock

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November 12, 2009 11:37:12 PM

This is probably a stupid question, but if the FSB is the classic bottleneck - does memory speed, beyond that of the FSB - really lead to performance improvements?

Essentially, if I'm looking at a core i5 with a 1333MHz FSB, does getting 1600MHz ddr3 ram represent any practical improvement over 1333MHz ddr3 ram?

If this question reveals a fundamental misconception regarding memory and the FSB, could someone please enlighten me?

Thanks! :) 

More about : fsb ram clock

a b B Homebuilt system
November 13, 2009 3:40:40 AM

1600Mhz is overclocked ram. If you want to overclock your CPU, you'll need ram that will also overclock well. You can overclock your ram without overclocking your CPU, but if you overclock your CPU, most of the time the RAM always needs to be tweaked.

Another thing is that there is no more FSB for the 1156 and 1366 CPUs. It's all built into the chip. What you're dealing with now is the base clock.
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a b } Memory
November 13, 2009 5:52:46 PM

xmp is similar to spd, the difference is that spd has one set of settings and the xmp has 3 different sets of setting for 3 more speeds
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November 13, 2009 10:42:55 PM

505090 said:
xmp is similar to spd, the difference is that spd has one set of settings and the xmp has 3 different sets of setting for 3 more speeds


Thanks. Are timings as important as speed? I found a much better deal on 9-9-9-24 ripjaw ddr3 1600 memory after looking at 8-8-8-24 ripjaw ddr3 1600 memory: they both seem identical in all other respects...
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November 13, 2009 10:55:06 PM

believe it or not, the loser the timings are on ddr3 the better off you are for i5/i7. Cas latency is really not an issue on nehalems.
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November 13, 2009 11:56:56 PM

kg4icg said:
believe it or not, the loser the timings are on ddr3 the better off you are for i5/i7. Cas latency is really not an issue on nehalems.


Huh. Not sure I understand that, but I'm not sure I completely understand the timings in the first place - good to hear in any case! This ram + mobo combo deal is around $40 less than I was going to buy otherwise apart and with the other ram.

Thanks for all the help everyone!
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a b } Memory
November 14, 2009 1:08:22 AM

kg4icg said:
believe it or not, the loser the timings are on ddr3 the better off you are for i5/i7. Cas latency is really not an issue on nehalems.



that makes absolutely no sense, would you care to explain your logic
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November 14, 2009 1:39:09 AM

505090 said:
that makes absolutely no sense, would you care to explain your logic


I don't understand how clock speed and latency for ram can be different? Intuitively, high latencies might prevent a clock speed above a certain threshhold, but if we're in a situation where some ram is running at some XYZ clock rate, why would it matter - on top of that - what the latencies are unless we want to overclock?

I've tried to find information on these timings but I'm finding mostly laymans info that doesn't really illuminate it's effect on performance... for me. Can anyone help me, or maybe point to a good explanation?
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a b } Memory
November 14, 2009 5:46:19 PM

Hmmm OK simple while answering the question

Different speeds and latency.
Is mainly the result of quality control by the chip manufacturer, same as intel chips they all come out of the same die (within the same core type) and depending on how each comes out (and demand) determines what speeds intel specs it to. Ever make an omelet, if you don't do it just right the edges burn and the middle doesn't finish cooking. Well same thing with making chips it's one big disc they cut chips out of and it doesn't come out even. FYI they are working on a molecular level.

Now speed and cas.
Speed is measured in MHz which is a measurement of how far apart the peaks are in the signal (just assume it's a simple sine wave for now). Now speed matters because our signals travel on that sine wave; so the faster it moves the faster our signal get there.
Cas also know as latency is the delay that is not accounted for in the previous paragraph. it is a measurement of how many of those sine waves it takes to actually do the task. Now in the real word there are 4 primary cas numbers CL, tRCD, tRP, and tRAS (there are actually about 8 but we don't worry about those). For clarity cas is the delay for the action not the length of the action.

How it works
So we have a speed of 600 and memory timings of 2-2-2
So that's 600 waves we can send things on. But it takes us 2 waves time for each of the first, second, and third steps. Making a total latency of 6, which means it takes the time of 6 waves to accomplish the task, so we only get to send info on 100 waves.
This is a gross oversimplification but it should get the basic concepts across.

Real world
We are dealing in nano seconds here. When you move to a higher speed ram your latency increases. This parallel increase takes up most of your gain but not all.

Hope this helps, if you are still curious I have highlighted several keywords for you to research.
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a b } Memory
November 14, 2009 5:49:31 PM

kg4icg said:
believe it or not, the loser the timings are on ddr3 the better off you are for i5/i7. Cas latency is really not an issue on nehalems.


I am still curious as to how this could be
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November 14, 2009 6:24:13 PM

505090 said:
Hope this helps, if you are still curious I have highlighted several keywords for you to research.


It does. I didn't understand how slower clock-rate memory could have a lower CAS. Now it makes sense: a combination of the mhz and the latencies would better describe the real performance of the ram.

Thanks!

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a b } Memory
November 14, 2009 6:39:34 PM

glad to help
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January 13, 2010 5:27:13 PM

Hey, man, this is one of the best analogy/answer/explanations I've ever seen on the timming and cas latency! You rock. I have a friend who's told me lowering the cas would improve the whole machine performance but, still, I knew it would reduce the overall oc possibilities on the same machine and that just didn't make too much of sense at the time but now things are a little clearer.
I was looking into buying cas 7 memory while I'm using OCZ cas 8 1600MHz and now I see that that wouldn't make a lotta improvement for my system.

It always pay to research but it is a bless when we can find someone who's known the in's and out's of the subject to explain with some simple analogy as you've done here.

Thanks for your input and keep up the good work! Great to know there are people out there not scared of sharing their experiences beyond any knowledge!!!
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a b } Memory
January 13, 2010 7:26:22 PM

always nice to hear I could help
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