Archived from groups: alt.comp.hardware.homebuilt (
More info?)
kony wrote:
> On Mon, 28 Mar 2005 13:55:58 GMT, Phisherman
> <nobody@noone.com> wrote:
>
>
>>Can anyone recommend any articles about memory? I want to buy two
>>sticks of 512MB PC3200 for a EPoX EP-5EPAJ board, and the prices (and
>>timings) are different. For example,
>>
>>2.5-3-3-6
>>2.5
>>3-4-4
>>2.5-4-4
>>2-6-3-3
>>
>>I know the first number is CAS, and the lower numbers the better.
>>Some vendors only give the first number. What is the significance
>>of these numbers, how will they affect performance, which ones run
>>cooler, and which one is a "best buy?" TIA
>
>
>
> "Best buy" would of course depend on the prices, which you
> failed to mention and might vary depending on
> who/when/where/how-much you buy.
>
> Generally a name-brand budget grade 2.5,3,3,6 is the best
> buy. Contrary to the previous poster's estimates about
> performance differences most put the difference at closer to
> 10% performanace between slow and fast memory except in very
> limited, specific apps that are memory bottlenecked. In
> most common uses the differences are even less, closer to
> 3%, but then for those uses a modern system is overkill too.
>
> You mentioned the board but not the CPU. A Celeron with
> lower FSB and performance doesn't necessarily warrant
> higher-end memory as much, and further, if you bought PC3200
> and then ran it at PC2700 speed, the originally specs
> timings may go lower @ PC2700 speed. That is, PC3200
> 2.5,3,3,6 memory might run at 2,3,3,5 (or some other timing,
> this is just an example) when the motherboard is set to this
> lower MHz memory bus speed.
>
> Overall one often considers what's appropriate compared tot
> he rest of the system... If you paid premium for a high-end
> CPU and video card, drives, etc, then premium memory makes
> more sense. If the rest of the system was budget/value
> price optimized then it makes less sense to blow out that
> value by spending a disproportionate amount on memory.
Actually, a 'budget' system with onboard graphics that shares system
memory would see a bigger increase in performance in the graphics area
than one with a high end AGP card. The difference can be very impressive
indeed with onboard graphics systems in FPS and smoothness.
At very high resolutions & bit rates the biggest bottleneck is that the
AGP/PCI-E graphics card has saturated it's fill rate and you wont see a
lot of difference in a high end AGP/PCI-E card with that. If you get 1-2
fps worth of improvement you are doing very well, it's maxed and that is
pretty much all there is to that. At lower resolutions and bit rates
you'll see more of a difference as there is some room for system calls
to the memory controller to have an impact. With onboard graphics the
difference is much more dramatic as it's calls are entirely dependant
upon the system bus & memory controller and this is a bottleneck for it,
whereas your high end AGP card wont be making anywhere near as many
calls to system memory(as it has it's own very high speed memory)and
wont see the types of gains for that reason. Though we are talking
nanoseconds of access time, we are talking about a lot of accesses in a
short amount of time.
CAS System Bus Speed Cycle Time Real World Latency
2.5 200 MHz 5.0 ns 12.5 ns
4.0 333 MHz 3.0 ns 12.0 ns
4.0 400 Mhz 2.5 ns 8.0 ns
2.0 400 Mhz 2.5 ns 5.0 ns
4.0 500 Mhz 2.0 ns 8.0 ns
(There are flaws in this example, mainly that the other timings would be
equal in this example and it only shows advantage for CAS but I'm
figuring out how to measure RAS impact is far harder than I want to get
into. X-2-2-5-1T is used here, each of the other settings has it's own
impact as well.)
Even with 40% less bus speed the CAS 2.5 memory is nearly keeping up
with 4.0 CAS memory with that huge bus speed advantage. I should point
out though, that this is only for access. You can pump more data through
a faster bus. So it doesn't mean the throughput is as good as faster
bus (it isn't quite there because you do get more burst with faster
bus), but you can greatly accelerate the loading and unloading process
by reducing the latency.
Due to DMA the processor isn't the only thing which can access the
memory controller directly -- almost all of your devices can - network
cards, hard drives, sound cards, graphics cards & chips. Impacts on
different systems vary, but the entire system benefits from reduced
latency. Anything where memory is a bottleneck is going to benefit.
There is another number usually attached to memory timings (but you
almost always have to visit the manufacturers site to get) and that is
command time, which is generally represented as 1T and 2T. 1T means the
entire process basically takes 1 cycle to sync up, and 2T means it takes
2 cycles to sync up (load, call the row, time the row has to remain in
memory, time to flush it, time to sync). So if a cycle takes 2.5 ns to
complete a 1T stick can do it in 1 cycle(2.5 ns) and a 2T stick will
take 5ns or 2 cycles to do it. This can have as much impact as CAS on
memory performance. Most brand name memory is 1T.
And as Kony stated, if your systems calls for PC2700 and you put in
PC3200, it may be able to run at more aggressive timings at the lower
speed. Even if you can not get the system to be stable with a CAS of 2
(or whatever is lower than the listed CAS), I would still try lowering
the RAS as you can get some measurable benefits there. Just make sure
you run some stability test & benchmark it. Sometimes you get lucky and
get a good set of sticks that run error free without the added expense.
More often it's trial & error testing setting the RAS & CAS with lower
rated sticks for optimal performance and stability.
--Timbertea
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