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Why is it recommended to have a 1:1 ratio of Front-side Bus and DRAM? Does it really affect the performance of the CPU?

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2 Answers 2

up vote 5 down vote accepted

Many people advice 1:1 ratio, but there are also testimonies for the opposite.
For example, see So much for 1:1 fsb:dram ratio being the best.

I quote from FSB:DRAM 1:1 ratio question:

The higher clocked the RAM is, the more performance one will get from the RAM. the 1:1 thing is a bunch of bull. Other devices besides the CPU request and store in RAM. Just look at any modern day video card and you will see the RAM clocked 2 or 3X higher than the GPU. Considering the bus of the P4 is quad pumped (4 signals per clock) compared to DDR, which is only at 100, 133, 166, or 200 (double pumped, ie 100 = 200DDR). There is QUITE a bit of lag between the CPU and RAM. So PLEASE stop saying the 1:1 ratio BS around here. The faster the RAM, the faster the access time and the more data can be sent in and out. I am NOT trying to insult anyone here, but I am trying to stop the BS rumor from spreading any further!

Just run Sisoft Sandra at different RAM speeds and see how much more data one can pump through it. Or run any benchmark (3Dmark for example) with the different RAM settings, the higher the RAM clock, the higher the score.

So please STOP spreading this rumor.

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This advice doesn't make any sense for processors with integrated memory controllers, this includes the latest Intel processor and AMD processors since the first Athlon 64 and Opteron processors.

With the memory controller on the North Bridge or Memory Controller Hub, higher frequency memory should still improve performance because:

  • The processor has multiple levels of very high speed memory (cache) which stores recent memory accesses
  • The processor prefetches memory to the cache so that the instance where the processor is waiting on memory in minimized
  • The front-side bus isn't necessarily saturated if the DIMM frequency is higher for example (using Wikipedia as a reference) an Intel Core 2 with a 400 MHz FSB had a bandwidth of 11.92GB/s while 400 MHz DDR2 RAM has a single channel data rate of 6.4 GB/s (12.8 GB/s for dual channel -- but that doesn't help my point :-) ).

The only way to really be sure is to measure the performance before and after the change with a trusted test, though.

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There is no absolute test. To cut a long story short: My advice is to get the best RAM and CPU one can get to get the best performance. And don't mind no ratios. –  harrymc Oct 14 '09 at 12:53

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