Pentium 4 and memory bandwidth

[Turbonium]

As I understand it, due to the "deep pipeline" architecture of the Pentium 4, the memory being used should match, or exceed, the bandwidth of the FSB for optimal performance, because it keeps the CPU "fed" with instructions, instead of the CPU "wasting" cycles doing nothing.

Assuming I described it correctly, what makes the P4 so special? Doesn't any CPU benefit from having memory match or exceed its FSB speed? Or is it that the architecture of the P4 simply makes it take more of a hit than the average CPU to have memory slower than the FSB speed?

And is synchronous memory speed better than faster, asynchronous memory speed?

Examples...

Synchronous memory setups:
400MHz FSB (100MHz quad-pumped) : 400MHz RAM (PC3200)
533MHz FSB (133MHz quad-pumped) : 533MHz RAM (PC2100 dual-channel)
800MHz FSB (200MHz quad-pumped) : 800MHz RAM (PC3200 dual-channel)

Asynchronous, but faster memory setups:
400MHz FSB (100MHz quad-pumped) : >400MHz RAM
533MHz FSB (133MHz quad-pumped) : >533MHz RAM
800MHz FSB (200MHz quad-pumped) : >800MHz RAM

Something tells me I've got something wrong. I could use some clarification.

 

[Stumps]

you have the ram speeds wrong for starters.

400mhz FSB - 200mhz DDR (PC1600)
533mhz FSB - 266mhz DDR (PC2100)
800mhz FSB - 400mhz DDR (PC3200)

Using Dual channel doesn't increase the speed which the memory runs at, only the bandwidth due to the increase from a 64-bit memory bus to a 128-bit memory bus.

1:1 is considered optimal for most CPU's however certain setups can benfit from faster ram speeds.

The P4 benfitted more from having a 1:1 or faster memory ratio simply because the long pipeling was hard to keep fed with data, while the memory controllers used for the P4 (especailly the i875 and SIS655) were quite fast, but paled in comparison to AMD's intergrated memory controllers found on the A64 cpu's.

The AMD Athlon XP always benfitted from having it's memory bus running at 1:1 running the ram any faster in most cases did very little for peformance and in the case of the old KT400A chipset even slowed things down a bit.

 

[Turbonium]

Originally posted by: Stumps
you have the ram speeds wrong for starters.

400mhz FSB - 200mhz DDR (PC1600)
533mhz FSB - 266mhz DDR (PC2100)
800mhz FSB - 400mhz DDR (PC3200)

Are these RAM speeds you listed assuming dual-channel mode?

 

[Stumps]

Originally posted by: Turbonium

Originally posted by: Stumps
you have the ram speeds wrong for starters.

400mhz FSB - 200mhz DDR (PC1600)
533mhz FSB - 266mhz DDR (PC2100)
800mhz FSB - 400mhz DDR (PC3200)

Are these RAM speeds you listed assuming dual-channel mode?

yes...single channel setups are pretty much a joke with the P4, although the good ole SIS648 was pretty fast especially when used with either DDR333 (PC2700) or DDR400 (PC3200) ram.

From my experience's with P4's using a 533 FSB with DDR333 (PC2700) always seemed a much better match than using the DDR266 (PC2100) ram in dual channel, but the peformance gains from using faster ram with the 800 FSB/PC3200 P4's seemed to be much less.

I didn't really care much for the 400 FSB models cause they were crap compared to the Athlon T-bird and XP.

 

[Turbonium]

Now that you mention it, I'm trying to put together an old P4 system based on the i845 chipset, which is what led me to posting this thread in the first place.

So if I have a P4 with a 533MHz FSB, and I pair it with DDR333, is the FSB only "50% fed"?

 

[Stumps]

Originally posted by: Turbonium
Now that you mention it, I'm trying to put together an old P4 system based on the i845 chipset, which is what led me to posting this thread in the first place.

So if I have a P4 with a 533MHz FSB, and I pair it with DDR333, is the FSB only "50% fed"?

pretty much, you would need to use 533mhz ram to have a 1:1 bandwidth match on the old i845.

What version are you using (i845, i845D, i845PE etc)?

 

[Turbonium]

Originally posted by: Stumps
What version are you using (i845, i845D, i845PE etc)?

i845GE (motherboard is the D845GEBV2/D845PESV)

Do you reckon a tangible increase in performance if I go from a 400FSB P4 chip to an equally clocked 533FSB P4 chip? Because I have a 400FSB chip as it stands, but was thinking of getting a 533FSB version instead. The motherboard manual says PC2700 defaults to PC2100 with a 400FSB chip, so I'm assuming yes.

 

[Stumps]

What speed 400 FSB P4 are you using?

the 533FSB did make a good deal of difference but the main advantage of it is that the 533 FSB P4's ran higher clock speeds than the older 400FSB models due to the change from the 180nm process to a 130nm process.

 

[Turbonium]

I have a 1.8GHz 400FSB P4. Was thinking of picking up something along the lines of a 2.4GHz 533FSB P4.

 

[Stumps]

if you are going to seriously get a different P4 go for the 3.06ghz model it's pretty good for a P4.

I have one running at 3.45ghz on a GA-SINXP1394 (SIS655) mobo with a 7800GS, it still can run games pretty good.

 

[Turbonium]

Originally posted by: Stumps
if you are going to seriously get a different P4 go for the 3.06ghz model it's pretty good for a P4.

Well the system is going to be on 24/7 (server), and I don't want a chip that runs too hot or too loud. I want to be able to use a passive cooling solution, or a very low RPM HSF solution. I can only really achieve that with a lower speed chip.

I figured 2.4GHz is a good balance between speed and heat output (I used to have a 2.4GHz 800FSB chip so I know how hot it gets, especially compared to the 3.2GHz chip I have today, which is likely comparable in heat output to the 3.06GHz chip you suggested).

 

[Stumps]

Ok, sounds pretty good. IIRC the stock HSF units that came with the old non HT P4's are pretty quiet.

 

[Turbonium]

Do you think I could use the 1.8A P4's HSF on a 2.4B?

 

[Stumps]

IIRC they where the same HSF unit, I've own both those CPU's in the past and I remember them having identical HSF units.

 

[Turbonium]

Well that makes me search for a 2.4B a whole lot easier, since I won't have to worry about finding one with a HSF.

 

[rogue1979]

If your motherboard doesn't have any overclocking options, you can bsel mod to a 533MHz fsb and get 2.4GHz. Unless it is one of the very first 1.8GHz P4's, just about every Northwood (socket 478) 1.8 will hit 2.4 at stock voltage (not a Williamette socket 423!). The heat output will still be the same as a stock 2.4GHz.

Any Northwood at 2.4GHz puts out a significant amount of heat, a passive solution doesn't sound like a good idea.