Does Latency Matter?

Track

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Ive been looking a lot at RAM lately, trying to find some that will fit in my PC.
Ive noticed that the price of lower latency RAM is much much higher than that of higher latency. For instense the Corsair DDR2-800 CL 4 modules cost around 200$, while the CL 3 modules cost over 350$.
So logically I assumed that the difference between CL 4 and CL 3 is huge.

However after looking at many reviews, it appears that the difference is actually less than 5%.
So from this i have to assume that latency DOES NOT matter, and i can have DDR-800 CL 10 and wouldnt be losing too much performance.
But then, whats with the much higher price for lower latency modules?

Thnx!
 

Track

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So the only reason to buy DDR2-800 CL4 instead of DDR2-800 CL5 is because the CL4 modules will allow u to overclock to DDR2-1066 CL5?
 

Mondoman

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Just as with CPUs, there is not a linear relationship between performance and price.
Partly this is due to the lower yield of chips near the performance extremes (e.g. high speed/low latency), partly this is because the manufacturers know that buyers who want/need items near the performance extremes are willing to pay a disproportionate price for them.
 

Track

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Just as with CPUs, there is not a linear relationship between performance and price.
Partly this is due to the lower yield of chips near the performance extremes (e.g. high speed/low latency), partly this is because the manufacturers know that buyers who want/need items near the performance extremes are willing to pay a disproportionate price for them.

But thats my point.
These "performance extreme" modules arent better than the ones normal ppl buy, wich is UNLIKE CPUs where there is a huge difference between 2Ghz and 3Ghz.
 

Mondoman

Splendid
...
These "performance extreme" modules arent better than the ones normal ppl buy
Yes, they are. They should run faster and/or at lower latencies.
..., wich is UNLIKE CPUs where there is a huge difference between 2Ghz and 3Ghz.
There's just as huge a difference in *memory* performance between DDR2-533 modules and DDR2-800 modules.
 

Track

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...
These "performance extreme" modules arent better than the ones normal ppl buy
Yes, they are. They should run faster and/or at lower latencies.

You're not listening to what im saying.. sorry to be rude.
YES, they are lower latency, but they do not have better performance because of this.
 

Mondoman

Splendid
With respect, I'm listening (reading) more closely than you are: you're not being clear because you're making unqualified blanket statements rather than precisely-qualified ones. I'm just trying to clarify things. :)

Of course RAM run at lower latencies has better *memory* performance than RAM run at higher latencies (at the same speed). The former also normally leads to increased *system* performance as well (except perhaps at FSB:memory throughput ratios slightly below 1:1).

However, (and I'm guessing this is your point) the *degree* of system performance improvement due to the lower latencies is often so small as to not be easily noticeable during normal use. It's useful to look at real-world data and decide for yourself where your money is best spent, also taking into account the possibility of re-using RAM on a future system with a likely-higher FSB.

Here's some data for Core2Duo systems (AMD x2 AM2 systems will have different results due to their built-in memory controller): http://www.madshrimps.be/?action=getarticle&number=1&artpage=1962&articID=472
 

Track

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However, (and I'm guessing this is your point) the *degree* of system performance improvement due to the lower latencies is often so small as to not be easily noticeable during normal use. It's useful to look at real-world data and decide for yourself where your money is best spent, also taking into account the possibility of re-using RAM on a future system with a likely-higher FSB.

Alright, so if we agree that the performance difference is so small that it is not noticeable, then why do CL3 modules cost so much more than CL4 modules? People arent stupid.. they wont buy something that is expensive just for that fact.

And since im going to have my FSB at 2133Mhz or higher, i doubt ill be re-using these modules for my next build, wich will likely have a much higher FSB.
 

cb62fcni

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Here's a little benchmark that should clear things up a bit.

http://gomeler.com/2007/02/22/memory-timing-and-frequency-comparison/
 

Ninjaz7

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Nice post...Madshrimps cleared up some questions I had,although there are a few more that are pretaining to ddr2.On my last build I went with corsair 5-5-5-12(actually 15)an it worked out pretty well oc'ing a gigabyte 945p-s3 with a d805.My best advice is go to reputable sites and see what they recommended for your application(dont know if your wanting to do some serious oc'ing or not) :D gl.
 

picho

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Interesting link. Don't fail to notice, however, that their bar graphs don't start at 0. There's a max differential of ~45% on the sandra tests, and about ~10% on the SuperPI tests.

It's too bad they didn't do any media encoding or gaming benchmarks.
 

Mondoman

Splendid
...
Alright, so if we agree that the performance difference is so small that it is not noticeable, then why do CL3 modules cost so much more than CL4 modules? ....
CL3 is pushing the margins of DDR2 technology at speeds like DDR2-800, so the yield of chips able to run at that latency has got to be substantially less than the yield of CL4-capable chips.
Also, while you and I wouldn't do so, some people ARE willing to pay very high price increments for only a small performance increment. For example, a QX6700 costs $970, 80% more than the Q6600 for only a roughly 11% clock speed boost.
 

cb62fcni

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Yea, it's a quick and dirty benchmark, kind of only confirms what most of us already know.

Remember, when you increase frequency there's a massive bandwidth gain that you can't get from tighter timings. High freq, loose sticks will always be faster than low freq tight.
 

Ninjaz7

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I've heard both sides of that theory but I usually buy the tighter latency modules,most of my machines run faster than others in real world applications :D .
 

Track

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Alright, so if we agree that the performance difference is so small that it is not noticeable, then why do CL3 modules cost so much more than CL4 modules? ....
CL3 is pushing the margins of DDR2 technology at speeds like DDR2-800, so the yield of chips able to run at that latency has got to be substantially less than the yield of CL4-capable chips.
Also, while you and I wouldn't do so, some people ARE willing to pay very high price increments for only a small performance increment. For example, a QX6700 costs $970, 80% more than the Q6600 for only a roughly 11% clock speed boost.

So ppl actually pay twice as much money for a 5% performance increase?
Thats insane.

Your CPU analogy is wrong however. Because i can easily gain an 11% overclock on the Q6600, but i will never be able to get to 1066 CL4 from 667 CL4.
 

Track

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I've heard both sides of that theory but I usually buy the tighter latency modules,most of my machines run faster than others in real world applications :D .

That graph clearly demonstrates that tighter timings are meaingless and offer no performance increase at all.

How can u possibly disspute this?
 

cb62fcni

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Tighter timings DO offer a performance increase, it's just not generally as big as higher frequency. I generally try to purchase low-latency myself, simply because it tends to overclock further as it is more tightly binned. If I have to loosen it up to get the higher freq, well, so be it.
 

cb62fcni

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Did I say that? No. That's a little excessive.

However, there's nothing crazy about getting 4-4-4-12 DDR2-800 2.0V for $135 and bumping it to 1066 5-5-5-18 @ 2.2V, right?
 

Track

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Did I say that? No. That's a little excessive.

However, there's nothing crazy about getting 4-4-4-12 DDR2-800 2.0V for $135 and bumping it to 1066 5-5-5-18 @ 2.2V, right?

But then why do ppl buy 1066 4-4-4-12?
 

memorymaster

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Just settle this debate with numbers....

Say you have a 800 CL5 and a 800 CL4....

Now the true clock frequency is 400 MHz, so one clock cycle is 1/(400 MHz) = 2.5ns.

So 800 CL5:

2.5ns x 5 clock cycles = 12.5 ns

800 CL4:

2.5ns x 4 clock cycles = 10 ns.

Even though it is a little more complex than this, when comparing the two Cas Latencies, you can see that CL4 is theoretically faster by a small margin.... So if you want to go faster CL4 IS FASTER THAN CL5.
 

cb62fcni

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That's never been a debate. The numbers speak for themselves. Dropping one latency level usually nets a 1-5% increase, depending on the app. However, and a big however, frequency matters MUCH more than latency in the majority of applications, especially on an Intel system.
 

Track

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Just settle this debate with numbers....

Say you have a 800 CL5 and a 800 CL4....

Now the true clock frequency is 400 MHz, so one clock cycle is 1/(400 MHz) = 2.5ns.

So 800 CL5:

2.5ns x 5 clock cycles = 12.5 ns

800 CL4:

2.5ns x 4 clock cycles = 10 ns.

Even though it is a little more complex than this, when comparing the two Cas Latencies, you can see that CL4 is theoretically faster by a small margin.... So if you want to go faster CL4 IS FASTER THAN CL5.

Thank u for that.
But i have no idea what that means..

Yes, 5 clock cycles on 400Mhz are 12.5ns, but what does that mean?
 

cal7

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And after all those answers , i would suggest you buy the "slower" ram and get yourself a better CPU or GPU to actually see a difference. :)
 

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