Need help understanding memory speed

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I need help understanding memory speed.
I don't want to overclock it, just want it to run at
rated speed.
Dell Dimension 4600 P4 2.4GHZ 533FSB Northwood
2GB PC3200 RAM, matched pair of 1GB CL2.5
Vista SP1 OS.
SIW, PCwizard and CPUZ tell me that my memory clock is 166MHz.
but that the memory is rated for 200MHz.
Why isn't it running at 200MHz clock?  There are zero options
in the bios to change anything.  Yes, latest bios.

Is there anything I can do about this?
I wouldn't sweat it, but I'm trying to run a HDTV tuner.
The cpu is maxed out and it's on the ragged edge of not working.
10% improvement would help.

I have the option to change the processor to one that's
2.8GHz. 800FSB.  It's 10W hotter.  I'd rather not go
to the expense if I'm not
convinced that it will also let the memory go it's max rate.

My friend has an asus motherboard.  2.4GHz. 533MHz FSB Prescott but 200MHz
memory clock XP OS.  The ram benchmarks from PCWizard run slightly slower
than mine...but the memory speed reported by memtest86+
reflects the 200/166 increase in clock speed.
I don't understand what I've learned because I don't understand
the internal workings of either program.

I think I understand how the system clock, FSB rate and CPU
multiplier relate.  I've not been able to find anything definitive
on how the motherboard decides what memory clock to use or what
to do to change it or would it be a step backwards in performance
if I override the bios decision??

Thanks, mike

Re: Need help understanding memory speed

spamme0 wrote:
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The chipset on the motherboard is 865PE.

These are available memory dividers for 875/865. What
this means, is FSB800 processors allow three different
memory divider options, so if a PC3200 stick is present,
it can be run at DDR400. If the PC3200 stick was installed
with a FSB533 processor, the fastest divider option is DDR333
and below the capabilities of the stick (it still works, but
not at full speed). Overclocking the processor a bit, can
bring up the memory speed as well (on motherboards where the
clock can be set).

  FSB800  | FSB533 | FSB400
          |        |
  DDR400  |        |
  DDR333* | DDR333 |
  DDR266  | DDR266 | DDR266

* One of the entries there is actually running at DDR320,
as noted on page 6 here. This is the 865PE memory guide from Intel.

The difference isn't enough to get worked up over. On the following page,
taking the ratio of the bandwidth numbers (1.08) and dividing by 3,
you get about 3% more application performance [this equation is my own
heuristic]. You wouldn't even notice that. On the other hand, a faster
processor, if you can find one that is supported by the motherboard,
can potentially give a bigger jump. (So, say 2.8/2.4 = 1.17)
The only time a jump like that might be noticed, is if you had
a game that was stuttering a bit, and a little extra processor
like that may be enough to fix it.

On something like a retail motherboard, they have tables of
upgrade options, so you can easily check what CPU will work. With
Dell, that isn't likely to be readily available. For one thing,
some Dell processor upgrades, use a different cooler with one
extra heatpipe in it. There may not be room to fit a high
performance aftermarket cooler. (Maybe you can judge from your
current CPU operating temp, as to what impact another 10W would
have with your current cooler.)


Re: Need help understanding memory speed

Paul wrote:
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Quoted text here. Click to load it
Thanks for the pointers.
I'm having trouble understanding where you got the 1.08 and divide by 3.

If I go from 2.4/533/ddr266
to 2.8/800/DDR400
my math tells me I have 16% more cpu cycles
and 1.5x the memory bandwidth and fsb bandwidth.
I have no idea were the video decoding bottleneck is,
so can't tell if that's a significant improvement.

But the answer appears to be that (with this motherboard)
I can't get the memory
speed without the 800 FSB and that the 2.8/800 processor
should make the ram go DDR400.
I didn't buy this Dell system because it was high performance.
I bought it because it was $4 at a garage sale.
Now, I'm paying for that mistake.

My DSL runs at about 740Kbps
and that's just under what's required for online TV.
Processor can't fix that.

This Dell has a  passive heat sink and a hood over it that
couples the airflow to the case fan.  I'd be going from 60ish watts
to 70ish watts.  I need to drag out the thermocouples and do
some tests.


Re: Need help understanding memory speed

spamme0 wrote:
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Memory bandwidth only has a small effect on application performance.
You need a "measured" bandwidth, to estimate the performance improvement.
If the measured memory bandwidth increases by 8%, the rule of thumb is
you see 1/3rd of that in the application. Or about a 3% improvement.
The rule of thumb is only accurate for small memory bandwidth
changes. I haven't looked at the effects with extreme changes.

The CPU clock rate change in your example, gives a larger improvement,
and would be your main incentive for changing. Your total improvement
is 16% plus 3%.

Some motherboard designs have a SuperI/O chip which has a hardware
monitor interface. That interface measures voltages, temperatures,
and fan rotation speed (for fans with RPM output). You can use
an application like Speedfan from to try to get a
readout. A diode in the CPU can be tied to the SuperI/O and give
CPU temperature readout, with some amount of error. Speedfan probes
the low speed busses on the motherboard, looking for the
signature of the supported SuperI/O devices.


Re: Need help understanding memory speed

Paul wrote:
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I  have a couple of programs that report temperature and fan speed,
but there's no data from this motherboard shown.
I'll try the ones you linked.

The plan was to take the 2.0GHZ/100FSB processor out of my secondary
system and replace it with the 2.4/533FSB processor from the primary
sytstm and update the primary to 2.8/800FSB.  The motherboards support this,
but if the memory bandwidth isn't going to be the bottleneck, I can
save half the price of the processor just upgrading the secondary system.

Re: Need help understanding memory speed

spamme0 wrote:
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Try working out the bandwidth balance, to get an idea as to why it works
as well as it is now. The trick is the dual channel part.

Say we had an FSB800 processor. The interface is 64 bits wide on
the processor, or 8 bytes in total. Multiplying the two together
gives 6400MB/sec of theoretical bandwidth.

A PC3200 stick gives 3200MB/sec, and two channels gives 6400MB/sec.
The PC3200 stick cannot sustain that for very long, which is why the
measured values are different than those numbers. That might be
referred to informally, as the "efficiency", which won't be 100%
but some lesser number.

If I have a FSB533 processor, that is 4264MB/sec. Two channels of
PC2100 just balance that. Two channels of PC2700 have more bandwidth than
the FSB, but the memory efficiency means this won't be wasted. The PC2700
will give slightly better numbers than the PC2100 would.

If the system only had a single memory channel, or if you plugged
your existing sticks into the wrong slots, then that could change things.
Then the bandwidth wouldn't be quite as balanced, and there would
be more room for improvement.


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