Why cant P3's use the extra DDR speed?

[MadAd]

I am just about to upgrade my mobo from my trusty old BX6-2

Wot a nightmare

I want RAID, I would like to use SDRAM as well as DDR ram, and I want to be able to overclock (pref via a softmenu in bios)

I cant stay BX, id like 1/2 AGP divider and need ata100

However i see that the most mobos are still on SDRAM even in the 815 and Apollo areas. And i keep hearing that P3's wont be benefitted by DDR SDRAM

Why?


Ive read the rambus article (parts 1 and 2) and understand the peak bandwidth Vs Latency issues ... and sure wether SDRAM or DDR SDRAM both types will struggle to be pushed bandwidth wise with todays usage.

BUT wont DDR ram have a much flatter latency slope thru the 0.5 and above areas, the point SDRAM goes tits up??

So therefore with a P3 system under load with DDR, the delivery at mid to peak bandwidth should be greatly improved latency wise over the struggling SDRAM


Is that how it goes?? Am i talking out of my backside? Can someone explain simply if DDR is a waste of money on an Intel Platform and why???

I intend to keep whatever i get now for approx 1 year till the P4/A4 is mature enough to trust. (aiming for a Ghz P3 and a new mobo)


Oh, and my useage? Im a gamer first HL-TFC, UT, Superbike - Fast action good looking sims mainly (hence why if DDR will help at all, i want it). Also I work in 180Mb a time scans and jpeg files, and regularly manipulate 500Mb Wav files to/from mp3, use Vox-to-Text and generally curse how slow everything is every time i have a 10 min wait to finish an operation. The GXP helps a lot


thanks everyone

 

[MadAd]

ok, ive found a bit more, apparently the avg latency of DDR is slightly higher than SDRAM avg for delivery purposes

also that P3 bandwidth depends on the northbridge fsb as the bottleneck so eliminating the potential gains from faster-than-fsb memory.

But i still cant see why anand says in this weeks memory and mobo round up that DDR being as cheap as SDRAM should be a kicker for someone in the market for a DDR board, i have to upgrade my mobo pretty soon but whats the point if i get no improvement??

Since i dont want to end up with a BA in RAM theoretics to find if a DDR mobo will be snappier and/or faster playing OGL/D3D memory busters and in general use (ive only got a V5500, theres still some traffic from V/C to CPU/RAM) hopefully one of you guys can enlighten me please?

How small is small, and whats the techheads chioice on a DDR P3 board atm pleaseeeeee????? And why wont P3s benefit? or do they?



(if you see a headline of 'man breaks down upgrading PC' thats me - just bury me with my BX plz?)

 

[MadAd]

anyone?

 

[HansXP]

The reason is simple: P3s have a front-side bus running at only 133 MHz, which goes well with PC133 RAM. But when you effectively double the speed of the RAM to 266 (PC2100 DDR RAM) the front side bus on a P3 stays at 133 MHz. Therefore, there is no where for the extra RAM bandwidth to go.

 

[MadAd]

ty for your reply, but im wondering more at the performance at or around the peak for a SDRAM based system vs a DDR P3 system ... can you or anyone add light to an assumtion i drew from the performance charts? i said it up there, ill requote it

"So therefore with a P3 system under load with DDR, the delivery at mid to peak bandwidth should be greatly improved latency wise over the struggling SDRAM"

if you look at Anands rambus article the graphs appear to suggest at peak operation (BX SDR) that SD has a crippling latency issue, a little before the peak 133 bandwidth whereas the DDR should go sailing thru and beyond (with a big gap of un-use it appears from what your saying as the extra potential to 266 gets wasted) and allow the fsb to be the bottleneck, and not the SDRAM latency


excuse me while my head explodes - can someone tell me if thats right, or wrong, and why plz?

 

[ElFenix]

the latency might be caused by a saturated bus and it simply has to wait for the other data to clear out before it can get new data in. so the FSB is still the reason.

 

[MadAd]

good point El, seems noones got a definative answer tho ... and the benchmarks on Toms bear out the fact that not one ddr A266 gives a useful or consistent gain

it just irks me having to spend on SDRAM again, and to have to think of buying another 'normal' board when the markets certainly getting interesting right now

what breakthru is it going to be to suddenly deliver DDR bandwidth to a fixed FSB im wondering - it cant be long off??

 

[GundamF91]

u have to realize the time when P3 came out was when Intel was in bed with Rambus til the end of time....so they didn't care for other possibilities. They were devoted to a P4 with Rambus and beyond.

Now that P4's their flagship model, I doubt they'd redesign anything around the P3 architecture.

Oh, maybe Rambus will rise again to give us some ultrafast ram solutions.

 

[MadAd]

what about Via? they have an interest with the A266 to somehow invent something to give some performance there??

especially with the new range of P3 intel are releasing in the new die, i dont think its quite dead yet as far as intel are concerned, just under review again

 

[AndyHui]

The P6 GTL+ system bus is the limiting factor and bottleneck.

Data must go from the RAM -> chipset -> processor. The step from chipset to processor only provides a nominal bandwidth at 133MHz. DDR SDRAM -> chipset speed is 266MHz nominally, so the bottleneck is pretty obvious.

You can't redesign the chipset -> processor link, unless you increase the FSB of the Pentium III processor, ie you won't get advantage out of DDR unless the P3 had a 266MHz FSB.

 

[MadAd]

AAAAAAAAAAHHHHHHHHHHH

/me does much head slapping


why id come to the alter-conclusion that fancy tricks with DDR would alter the cpu itselfs internal fsb bandwidth i do not know, lol @ me .... ty Andy for putting that last piece in place, after studying an 815 flow diagram and all the reading ive done since i started this thread its all clicked

so thats it ... SDRAM copes with the 133 bus nicely, its the right tool for the job, no matter how outdated


Soooo ... the top performance on P3 is in getting a stable 133fsb part, in whatever flavour multiplier you fancy and make it SCREAM to 160Mhz plus???


The CPU is limited to the quality of silicon (speed), the speed is limited to your wallet (money) Sooooo the chip to go for is the one that gets 160 FSB and stays within the likelyhood of getting a good clock on that wafers limit.

For the P3 (current process) wafers now thats what, 1.1Ghz? So, we know that 11x166 may well fail POST ( ... 133 is 25% of the whole, 100-166 is 40% of the new whole, so 40% of 11x is 4.4x minus the 11x =

a 6.6x multiplier 133 part run up to 166!!!! (6.6x166=1.09Ghz)

In street terms thats as close as Intel do a 864Mhz in the EB range of P3

HUZZAHHH!!!




So, I need SDRAM thatll crack the 160s (crucial CAS2 133's do the job i hear?)
A Mobo thats stable up to 160FSB (815 probably, dunno what tho)
AHH

What about my PCI and AGP busses at 166? Thats 20% out of spec to 133 ... on a 1/4 the PCI makes it need a 1/5 PCI and on a 1-2 AGP thats 83Mhz since i dont expect bus snap-back at 166

question to answer is does the FSB snap back @ 166, and IF NOT, which (if any) of the 815/133/266 boards offer a 1/5PCI and 2/5AGP, and IF NONE, then get the stablest one at 166FSB

The safe guess would be fix on a healthy 150 i guess and get the next multiplier up, to be realistic if the AGP/PCI is tight




ok, ill stop waffling now - time to go read some web, ty those that helped me on my way, i hope this dialog has helped some others to figure the P3 Vs DDR area right now

 

[AndyHui]

Currently, no motherboards offer a 1/5 PCI divider and 1/3 (or so) AGP divider.

Yes, you will be better off with a Pentium III operating at a much higher FSB.

 

[MadAd]

yeah, thx for replying, ive just figured that one out ... im stuck to screaming the poor little cards again like i've been doing for over a year at 124-9fsb on this BX6-2, ho well ... same doo-doo, different mobo, lmao

 

[Athlon4all]

Exactly, PIII is lagging behind Athlon and P4 because of the only 133MHz FSB, and thus doesn't have enough free FSB bandwidth to take advantage of the additional Memory Bandwidth.