cpu/fsb ratio

[hot dog]

Why does it increase system performance when the cpu/fsb ratio is low or 1:1?

The most common answer I've heard is that the cpu does not have to "wait" on other system components so the system throughput is higher...but why?   I can accept that the cpu won't have to wait on other components with a low ratio, but I can't help wondering why.  What is the reason? 

[hot dog]

I figured out why I'm so confused...

For some reason I thought that the "ratio" everyone refers to is between the FSB and the CPU, where the ratio is really between the RAM and CPU.... and ideally it should be 1:1     I don't know how I figured it the way I did before...

So then what if the RAM is 500 MHz and the CPU is 1000 MHz ( 1:2) ...for example

The cpu has to wait for data from the ram, but would that slow down the system throughput? 

[Aardobard]

I figured out why I'm so confused...

For some reason I thought that the "ratio" everyone refers to is between the FSB and the CPU, where the ratio is really between the RAM and CPU.... and ideally it should be 1:1     I don't know how I figured it the way I did before...

So then what if the RAM is 500 MHz and the CPU is 1000 MHz ( 1:2) ...for example

The cpu has to wait for data from the ram, but would that slow down the system throughput? 

Almost.  The most common RAM today is DDR2.  In your example, that means that the 500MHz DDR2 RAM is really operating at 250 @double data rate (or x2) to equal 500.
The Intel CPU talks to the motherboard at x4 (I don't remember why, maybe someone can help us both out ), so the fsb in the bios will be 250 (250x4=1000).

In this example, the CPU and RAM are at 1:1. 

[hot dog]

The Intel CPU talks to the motherboard at x4 (I don't remember why, maybe someone can help us both out  , so the fsb in the bios will be 250 (250x4=1000)

I think what you are referring to is quad pumping  the cpu   

http://en.wikipedia.org/wiki/Pumping_%28computer_systems%29


I should  have been a little more specific too...   I believe cpu/ram ratio refers to the memory bus speed and the Internal cpu fsb speed...  If the external fsb is 100 MHz, then 100 x 4 =  400 MHz would be the Internal cpu bus speed.  Then if your DDR is 200 MHz, it would equate to 400 MHz, making the ratio 1:1


So, in your example the ratio would actually be 1:2, unless I interpreted it incorrectly.
Your RAM is running at 500 MHz (250 x 2) and your Internal CPU BUS is running at 1000(250 x 4) .................thus, 1:2   

[hot dog]

I copy and pasted this from another forum...   I think this is likely the issue with speed when the ratio is something other than 1:1..


"I think the idea stems from bottlenecks.. Think about having 100Mhz bus of Memory that was sending things to and from the Processor that was also running with a bus of @100Mhz.. You'll have a very smooth flow of information.. Now if either of those numbers are different things can back up.. think of a traffic jam.. It's not that the information won't be processed, but it's the slower of the two numbers that will end up being the speed that the system can process that information.. either the CPU is running faster and it's just sitting there twiddling it's metaphorical thumbs waiting for the memory to send it something to do, or the memory is running faster than the CPU and the memory has a line at the grocery store with 20 people in it waiting to pay.."