Why's XDRAM so slow?

[Mordecaii]

Ummm I'm kinda confused Shifty... are you saying that this isn't the XDR that will be in the PS3? Or are you saying that the bandwidth listed in the article wasn't correct? (which is because they are only counting 1 XDR module when there will be 4 in the PS3)

 

[mckmas8808]

Yeah I'm confused too. So Shifty you're saying that the one listed above is actually slower than what the PS3 will have right?
________
Plymouth cranbrook

 

[DuckThor Evil]

Yeah I'm confused too. So Shifty you're saying that the one listed above is actually slower than what the PS3 will have right?

No, the speed of XDR in PS3 is nothing special. Read the whole thread.

 

[Mordecaii]

True, it's not really the BW but the latencies that make XDR nice.

 

[Nite_Hawk]

I think Vaan is saying XDR has ridiculously LOW latencies, but people overlook this and only look at bandwidth, which is a point as has been raised. XDR is needed to keep the SPE's happy. I still don't understand why 3 GHz memory of a faster design is only about as fast as 700 MHz DDR, mostly 'coz the thread got derailed. But on this note, what are the comparative latencies between the XDR and DDR that PS3 uses?

I remember seeing a chart somewhere, but I can't find it at this point. Basically it showed the various latencies for ddr, gddr3, and xdr. I'm not sure who it came from though so take all of this with a grain of salt. The XDR memory had something like half the latency of GDDR3 according to that chart. Rambus also advertises it for use in situations that require low latency memory. From that standpoint, it would make sense why they'd use it for Cell, but use the cheaper GDDR3 memory for the graphics chip (which really is more concerned with higher throughput than lower latency).

Nite_Hawk

 

[Carl B]

Well, I think it can be generally agreed that the current XDR implementation within PS3 is not the ideal, but the XDR still has it's benefits. Now, whether from a costs perspective it will have proven to be worth it in the end, we'll just have to wait and see - but on that same topic actually I'm just going to give a preemptive 'yes.' My reasoning is that though Rambus is licensing their current flagship technology to Sony and Sony's memory partners, their must surely be some perceived advantage on their end to having Sony use XDR as opposed to GDDR3 all the way through; somethign which I feel could have translated into favorable licensing terms. I could see PS3 being quite the selling point for Rambus as they seek to expand their marketshare into graphics at large and possibly an attempt to retry the desktop assault. It's speculation on my part, to be sure, but I have a feeling that XDR for Sony won't be the bank-breaker everyone suspects. Does Cell need XDR more than Rambus needs Sony? I wonder...

(obviously since the memory controller on Cell IS Rambus specific, the current version of Cell absolutely requires it. My point though was that that didn't have to be the case originally, and Rambus must have done at least some selling to get it in there. I could see favorable terms for Sony/Sony partners being part of that deal)