[mythtv-users] Anyone care to double check my shopping basket?

Johnny Russ jruss at mit.edu
Thu Jun 12 14:01:41 UTC 2008


>
> Anyone else know about this multiple of 400MHz thing? I'll dig around
> the net now.
>

I am not sure how much the actual performance would be different but I
believe I know the reasoning behind the 400 MHz clock. If you are using DDR2
800 (ie PC6400) RAM then ideally you would want the front side bus to be
running at 400 MHz. Then the combined speed of a matched pair of DDR2 memory
will be 2 x 400 = 800 MHz. If you have a processor that is not clocked to
run at some multiple of 400 MHz then the front side bus won't be running at
400 MHz. For example I have a Core 2 Duo that comes factory clocked at 2.13
GHz. That comes from a 266 MHz FSB x 8 multiplier = 2.13 GHz. So to get each
of my RAM modules to run at 400 MHz, the clock for the RAM is running at 3:2
ratio of the FSB, (266 x 3/2 = 399 MHz). Generally when your memory isn't
running 1:1 with the FSB then your performance won't be quite as good. There
weill be higher latency etc. This is generally an issue that overclockers
deal with, trying to strike a balance between getting your FSB and processor
as fast as possible and keeping your RAM 1:1 with the FSB. An ideal
overclock for my Core 2 Duo would bump the FSB up to 400 MHz. Then the
processor would be running at 8 x 400 = 3.2 GHz and the RAM would be running
1:1 at 2 x 400 = 800 MHz.

Generally with a given processor and RAM you will get better performance if
your FSB and RAM are running 1:1. Keep in mind that you can overclock to get
this for many of these processors. However, choosing one processor over
another just because it runs on a 400 MHz FSB isn't necessarily going to
give you better performance. It would be hard to say whether an extra 100
MHz or 200 MHz in processor speed will affect performance less than the
clock ratio on your RAM being 1:1, without actually testing the systems out.
It certainly wouldn't be one of my prime considerations when selecting a
processor.
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