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RAM-CPU bottleneck???

post #1 of 21
Thread Starter 
I just realized this, and I feel like a n00b.
Using higher ratios(3:4, 2:3, etc) will just create a bottleneck towards the cpu, because the ram will be much faster than the fsb. Thats why the 1:1 ratio is the best, because there is no bottleneck.
For example, if you set a 2:3 ratio your ram will be 50% faster than your fsb, thus a ram-cpu bottleneck.

So would it be better to buy lower rated ram, for the low timings??
You change to the 1:1 ratio and Oc your cpu to the max. You will end up with a high fsb, high cpu clock, no bottleneck, and low ram timings.
post #2 of 21
Quote:
Originally Posted by DataX
I just realized this, and I feel like a n00b.
Using higher ratios(3:4, 2:3, etc) will just create a bottleneck towards the cpu, because the ram will be much faster than the fsb. Thats why the 1:1 ratio is the best, because there is no bottleneck.
For example, if you set a 2:3 ratio your ram will be 50% faster than your fsb, thus a ram-cpu bottleneck.

So would it be better to buy lower rated ram, for the low timings??
You change to the 1:1 ratio and Oc your cpu to the max. You will end up with a high fsb, high cpu clock, no bottleneck, and low ram timings.
Depends on how your bios is setup.

On mine it's DRAM:FSB ratio, so the senarios you put together would actually have the CPU running faster than the ram (this is the most common senario when using dividers). People run their CPU faster than the ram because their ram is not capable of any higher speeds. Running dividers limits memory bandwith though, because even though you're at a higher clock speed, you're still limited by your memory speed.

The best thing to do when getting ram is getting good quality ram that can achieve the highest bus speed possible (at least DDR500), while keeping at least decent timings (CAS 2.5 or less). That way you wont be limited by your ram when overclocking your CPU and you will be able to run a 1:1 ratio and let your CPU be the limiting factor so you know you're getting the most out of it.
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post #3 of 21
Thread Starter 
Quote:
Originally Posted by Evil XP2400
Depends on how your bios is setup.

On mine it's DRAM:FSB ratio, so the senarios you put together would actually have the CPU running faster than the ram (this is the most common senario when using dividers). People run their CPU faster than the ram because their ram is not capable of any higher speeds. Running dividers limits memory bandwith though, because even though you're at a higher clock speed, you're still limited by your memory speed.

The best thing to do when getting ram is getting good quality ram that can achieve the highest bus speed possible (at least DDR500), while keeping at least decent timings (CAS 2.5 or less). That way you wont be limited by your ram when overclocking your CPU and you will be able to run a 1:1 ratio and let your CPU be the limiting factor so you know you're getting the most out of it.
But either way, the only thing dividers do are create bottlenecks, towards cpu or ram. It's useless to have a higher speed on one or another thing; it's like mixing different rated ram... even if you have some DDR2-800, it will run at the speed of the lowest rated ram installed.

So you must buy your ram depending on the highest fsb your cpu can achieve for the 1:1 ratio.
post #4 of 21
Quote:
Originally Posted by DataX
But either way, the only thing dividers do are create bottlenecks, towards cpu or ram. It's useless to have a higher speed on one or another thing; it's like mixing different rated ram... even if you have some DDR2-800, it will run at the speed of the lowest rated ram installed.

So you must buy your ram depending on the highest fsb your cpu can achieve for the 1:1 ratio.
The only way of finding out what's the highest FSB your CPU is going to reach is to try, so it's best to buy ram that will out do the so you're not limited by ram. Also with ram that does very high FSB, you can usually drop the CPU multiplier a notch and that will allow you to get even higher FSB. Regardless of the slightly lower clockspeed, your computer will run faster because of the added memory bandwith.

My setup for example (all on a 1:1 ratio):

I have ram that's capable of DDR600 speeds.

I can either run my computer @ 240FSBx12 which is 2880mhz(2.5-3-3-8) or run it the way I'm running it now 260FSBx11 which is 2860mhz (2.5-3-3-8). Even though the first combination yields a faster clockspeed, the second one offers much better performance.

My CPU cant handle 260FSB with a multiplier of 12, but it can when I drop it to 11. If my ram wasnt capable of 300FSB I would have never found out. That's why I said, it's better to get the fastest ram you can, even if you dont end up using it to it's full potential. It'll give you more headroom and different FSBxMulti combinations to play around with. In case your CPU cant do high FSB at all, you can always just tighten the timings and still get great performance out of your ram. If I decided to drop back to stock speed I could just set my FSB to 200 (stock), tighten my timings to 2-3-2-6 (the best my mem does) and call it a day.

Bottom line is, when it comes to overclocking, you'll need as much headroom as possible from your ram to be sure you're getting the best out of your CPU.
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post #5 of 21
Thread Starter 
Quote:
Originally Posted by Evil XP2400
The only way of finding out what's the highest FSB your CPU is going to reach is to try, so it's best to buy ram that will out do the so you're not limited by ram. Also with ram that does very high FSB, you can usually drop the CPU multiplier a notch and that will allow you to get even higher FSB. Regardless of the slightly lower clockspeed, your computer will run faster because of the added memory bandwith.

My setup for example (all on a 1:1 ratio):

I have ram that's capable of DDR600 speeds.

I can either run my computer @ 240FSBx12 which is 2880mhz(2.5-3-3-8) or run it the way I'm running it now 260FSBx11 which is 2860mhz (2.5-3-3-8). Even though the first combination yields a faster clockspeed, the second one offers much better performance.

My CPU cant handle 260FSB with a multiplier of 12, but it can when I drop it to 11. If my ram wasnt capable of 300FSB I would have never found out. That's why I said, it's better to get the fastest ram you can, even if you dont end up using it to it's full potential. It'll give you more headroom and different FSBxMulti combinations to play around with. In case your CPU cant do high FSB at all, you can always just tighten the timings and still get great performance out of your ram. If I decided to drop back to stock speed I could just set my FSB to 200 (stock), tighten my timings to 2-3-2-6 (the best my mem does) and call it a day.

Bottom line is, when it comes to overclocking, you'll need as much headroom as possible from your ram to be sure you're getting the best out of your CPU.
That's exactly what I was thinking about. For now my system is 255*15=3825Mhz, at 1:1 with timings of 4-3-3-8. Cheap samsung memory... But I'm not buying new ram now, I'm planning to upgrade to P4EE 3.72; that means higher fsb, lower multi. So who nows whats it's max fsb.
But I don't think you should buy the fastest ram you can, because it will just have higher timings. Better use some cheap ram to find your max fsb, then buy the adequate ram with the tightest timings.
And you don't need "fast" ram to have enough overclocking headroom, since my ram has stock 4-4-4-11 timings, it overclock from 266 to 350Mhz. Enough to find max fsb.
post #6 of 21
RAM running faster than CPU will give you marginally better results than 1:1. If such a divider is at the expense of raw GHz, then 1:1 is the best. If you have some uber fast RAM that even 4:5 or 3:4 can't hinder, then go for the higher divider.

Why waste money on an EE, when you can get a 3.2 or 3.4 6**-series, drop the multiplier, and get the same exact processor?
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post #7 of 21
um....2:3 divider got me this:

DDR2-675 -> DDR2-800 on 5, 5, 5, 15 timings

+ screeny
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post #8 of 21
^How about a comparison between that, 1:1, and any other >1:1 dividers you may have available? And having two instances of CPU-Z open in the picture, one of the CPU tab and one of the memory tab, would also be helpful. That picture only tells us bandwidth...no notation anywhere of the FSB or divider.
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post #9 of 21
it wasn't stable, but it was a 270fsb, with a 2:3 divider


and if u don't believe me...try and beat it with anything less
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post #10 of 21
I wasn't questioning your results, I asked for a comparison on your system (with all settings equal, except for the divider) for the general knowledge of all....

I have already provided a similar comparison, for the sake of reference, on more than a couple occassions, but this thread is new, you're in front of your primary system.

:edit: nm, here's the comparison I did a while back.
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