How does Core2Duo CPU performance change based on Multiplier x Memory speed?

[GundamF91]

(This question stems from a thread I read in the System Memory forum) So if a C2D has 9x multiplier, then the most efficient way to overclock would be to use 400mhz memory to make it 9x400=3.6Ghz. So it sounds like that DDR2-800/PC2-6400 should be enough to push the envelope for all modern air-cooled LGA775 processors. Why would there be need for DDR2-1066/PC2-8500 memory then?

So now I'm wondering how it works with E2180 which has factory 10x multiplier? In case of a CPU with 10x multiplier, such as the E2180, what memory should be used? ? 10x400 would be 4Ghz which is impossible. It sounds like either memory needs to be slowed down, or reduce the multiplier.

For memory speed to go down, I think factory default is 200mhz, to make default as 10x200=2.0Ghz. In this case, would DDR2-800 be able to run at 200mhz speed? And doesnt' this mean DDR2-800 would be way overkill for E2180? the DDR2-667/PC2-5300 with its 333mhz speed should be more than enough to 10x333=3.3Ghz overclock.

For multiplier reduction, does Intel lock the multiplier from going down? I know AMD locks multiplier from up, but allows it to go down. Does Intel C2D allow changing multiplier down? Hypothetically, if the E2180's 10x multiplier can be dropped 8x, but then you can use the 400mhz DDR2, so that gives 8x400=3.2Ghz, same overclock, but more bandwidth. In that case, would you use the DDR2-1066/PC2-8500 memory to up the FSB to 533Mhz and lower multiplier even more?

After having 4 AMD processors all running on DDR, the Intel and DDR2 speeds are really confusing! I'd appreciate any help so I can get the right memory and multiplier combination.

 

[jjmIII]

You can set the memory faster than the cpu if you want. My q6600 is at 333bus, but I run my memory at ~1000. It isn't necessary thought. You are correct, a 333bus really only needs DDR2-667. I'd still try to get DDR2-800. My e2140 runs a 400bus, and therefore needs DDR2-800.

 

[djspl]

Intel multipliers can go lower. Ram can run lower. Having a 10x multi and high speed ram just gives you more options.

People that opt for ram faster than DDR667 are simply trying to go faster than 10 x 333 will allow.

If you have DDR800 and you are shooting for 10x360, then you know your ram is not going to cause problems because it's running well within spec.

 

[SerpentRoyal]

The highest FSB is often achieved with the default CPU multi. Core speed is KING. Use memory divider to raise RAM speed for a small boost in performance.

 

[etherealgrifter]

Check out the bottom of this guide:

http://www.tomshardware.com/fo...lock-quads-duals-guide

He's got some links to some quick studies about how higher FSB and memory bandwidth don't make much of a real world difference.
Summarized:



What is a better overclock?

Good question. I think that common thought is a higher bus rate and lower multiplier. Or is a low bus rate and higher multiplier better? For example, 9x333=3.0 GHz and 8x375=3.0 GHz.

I did a lot of testing on this one and concluded that there is no difference for real world applications. If you use a synthetic benchmark, like Sandra, you will see faster memory reads/writes, etc. with the higher FSB. This is great if all you do with your machine is run synthetic benchmarks. But the higher FSB comes at the cost of higher voltages for the board which equate to higher temps.

Which memory divider should I use?

Another good question! Common sense tells you that higher memory bandwidth should mean faster results, right? I set out to put this thought to the test looking at just two different memory dividers on my o/c'ed Q6600 system (333 MHz FSB).

1:1 a.k.a. PC5300 (667 MHz)
3:5 a.k.a. PC8888 (1,111 MHz)

Several apps were run and timed while completing various tasks. After seeing the data I generated on a quad core @ 3.0 GHz, I concluded that the higher memory bandwidth gave more or less no appreciable difference for real world applications. Shocked? I was.

 

[Acanthus]

Originally posted by: etherealgrifter
Check out the bottom of this guide:

http://www.tomshardware.com/fo...lock-quads-duals-guide

He's got some links to some quick studies about how higher FSB and memory bandwidth don't make much of a real world difference.
Summarized:



What is a better overclock?

Good question. I think that common thought is a higher bus rate and lower multiplier. Or is a low bus rate and higher multiplier better? For example, 9x333=3.0 GHz and 8x375=3.0 GHz.

I did a lot of testing on this one and concluded that there is no difference for real world applications. If you use a synthetic benchmark, like Sandra, you will see faster memory reads/writes, etc. with the higher FSB. This is great if all you do with your machine is run synthetic benchmarks. But the higher FSB comes at the cost of higher voltages for the board which equate to higher temps.

Which memory divider should I use?

Another good question! Common sense tells you that higher memory bandwidth should mean faster results, right? I set out to put this thought to the test looking at just two different memory dividers on my o/c'ed Q6600 system (333 MHz FSB).

1:1 a.k.a. PC5300 (667 MHz)
3:5 a.k.a. PC8888 (1,111 MHz)

Several apps were run and timed while completing various tasks. After seeing the data I generated on a quad core @ 3.0 GHz, I concluded that the higher memory bandwidth gave more or less no appreciable difference for real world applications. Shocked? I was.

PC memory is probably the lowest bang for buck technology out there.

As long as you have enough of it, there is virtually no difference between $40 a stick memory and $150 a stick memory in real world applications.

 

[myocardia]

Originally posted by: etherealgrifter
Which memory divider should I use?

Another good question! Common sense tells you that higher memory bandwidth should mean faster results, right? I set out to put this thought to the test looking at just two different memory dividers on my o/c'ed Q6600 system (333 MHz FSB).

1:1 a.k.a. PC5300 (667 MHz)
3:5 a.k.a. PC8888 (1,111 MHz)

Several apps were run and timed while completing various tasks. After seeing the data I generated on a quad core @ 3.0 GHz, I concluded that the higher memory bandwidth gave more or less no appreciable difference for real world applications. Shocked? I was.

While most people don't need to run their RAM any faster than the FSB, some people do, as admitted by graysky, the guy who wrote that post:

I will admit that there might be special cases where running at high memory dividers may produce more substantial gains: apps such as folding@home or seti@home, etc. may benefit from the higher memory bandwidth since they tend to make exclusive use of the system memory bandwidth and rely much less on the hardrive. I have no data to back-up this though. Also lacking in my experiments are any game data. I'd be interested in knowing if the higher bandwidth can be leveraged by game engines such as UT3, Crysis, etc. but I also didn't look at these here.

Also, one thing he didn't consider is the fact that he was using only one hard drive, then he was running HD-intensive apps, like PhotoShop and different video encoding/recoding apps. Obviously, he'd have improved his performance in all of those apps considerably more by having two hard drives. But for people who already have 2 or more HD's, the faster RAM would make a difference.

Now, this doesn't mean that everyone should got out and buy extremely expensive PC10400 RAM, but it does mean that people who can benefit from it probably should be spending a couple of $$ more per GB for PC6400, as opposed to PC4200.

 

[Denithor]

And BTW, 4GHz isn't impossible, you just need considerably better cooling than provided by the stock cooler. Just look at xtremesystems.org for lots of examples of C2D chips running at or above (sometimes waaay above) 5GHz.

To answer your question, for general use memory only has to run fast enough that at a 1:1 ratio you can push your cpu to its maxium safe fsb (which is mostly determined by your motherboard and cooling). For chips with a 9x or 10x multiplier on stock cooling, for example, you probably aren't going to go much over 333fsb so you can get away with cheap/generic DDR2-667 memory.

And note that just because memory is rated for DDR2-667 or DDR2-800 doesn't mean it won't clock above that speed: with a little extra voltage and loose timings most 667 memory will easily hit 800-900 speeds (or even higher). My e6400 is running 2.8GHz at 350fsb on the super cheap ($8.75/1GB after MIR) HP DDR2-667 memory.