[Eug]

As far as I can glean:

1) Xserve G4 CPUs cannot use DDR so it's pointless for them.
2) Direct memory access for DDR will be of benefit to system performance, but it will be minor.
3) No current G4s out there can use a true DDR-capable FSB. What about other chips, like the nebulous 7470?

[ 05-15-2002: Message edited by: Eug ]

[x user]

Originally posted by Eug:
<STRONG>As far as I can glean:

1) Xserve G4 CPUs cannot use DDR so it's pointless for them.
2) Direct memory access for DDR will be of benefit to system performance, but it will be minor.
3) No current G4s out there can use a true DDR-capable FSB. What about other chips, like the nebulous 7470?

[ 05-15-2002: Message edited by: Eug ]</STRONG>

It has a point if it sells the server...

[macaddled]

Mac Ach

As usual, it's BadAndy you wanna read.

[Eug]

Originally posted by macaddled:
<STRONG>Mac Ach

As usual, it's BadAndy you wanna read.</STRONG>

Good thread, but BadAndy speaks over my head.

[krove]

Well, he cleared up the one issue I've seen repeated several time in the last few days: How can G4s and DDR-SDRAM go together?

Well, believe it, they do.

[edit] I must clear up that the DDR solution isn't the best (or fastest) because of the dual processors, but it does offer more performance than PC-133 nonetheless.

[ 05-16-2002: Message edited by: krove ]

[MasonMcD]

This is from The Motley Fool AAPL board, by Greg Titus, OmniWeb developer:

Recommendations: 16

LongHook says:
Okay, hope that explains it all. Probably too long winded, but I just wanted to be thorough =)

Summary: DDR = fast, FSB = slow, CPU = very fast

An image is worth a thousand words. Check out this one for a graphical depiction of what Hook is talking about:
http://litterbox.zawodny.com/~jzawod...0001688-md.jpg

That's a shot of an XServe architecture diagram at the announcement today. Notice the 2.1 GB/s from the DDR SDRAM to the system controller, but only 1 GB/s can get from the system controller to the dual G4s. Thus, the DDR isn't buying you any additional memory bandwidth to the CPUs. (I.e. everything LongHook just said.)

However!

A server usually isn't about fast CPUs. Look at that architecture diagram again and imagine the 1 GB/s FSB is saturated -- the CPUs are pulling 1 GB/s from the RAM. On a G4 workstation, that would also saturate the memory bus. But on the XServe you've got an extra 1.1 GB/s leftover for direct DMA transfers from the disks into RAM (up to 533 MB/s) and depending on how smart the system controller is, possibly for direct DMA transfers out the ethernet ports (up to 2 GB/s).

So the DDR is there not to speed up the CPUs (you'll get essentially the same CPU performance on an XServe as on a desktop dual G4), the DDR ram is there to get you maximum performance from the disks and other I/O while still providing bandwidth to the CPUs.

Since the purpose of a server is usually to get stuff off the disk and shove it out onto the network as efficiently as possible, it seems pretty reasonable for Apple to have done this.

In short, no, the DDR on this system isn't just a stunt, it has real performance benefits, but also, no, it won't actually speed up the CPUs any.

--Greg

[ 05-16-2002: Message edited by: MasonMcD ]

[Big Mac]

Okay, all of this sounds interesting, but I want to know the upshot. Basically, I'm wondering the following:

1) Is the XServe a proper implementation of DDR, or is it not?
2) If not, what is holding it back?
3) How far behind is it?
4) Is the G4 the bottleneck?
5) What are the implications for the next revision Power Macs?

[echoes]

Originally posted by Big Mac:
Okay, all of this sounds interesting, but I want to know the upshot. Basically, I'm wondering the following:

1) Is the XServe a proper implementation of DDR, or is it not?

Depends on what you mean by proper. The implementation has two advantages -- it gives Apple's marketing department a buzzword to work with and it gives more bandwidth to the HDD's, giga-eth and AGP card.

It doesn't have the more traditional advantage of faster memory though which is greater bandwidth between the CPU and memory to improve processor performance which is pretty lame.

2) If not, what is holding it back?

The front side bus, the bus between the CPU's and the memory/system controller is only operating at 133MHz (~1 GB/sec) just like the current Quicksilvers.

3) How far behind is it?

Intel currently has motherboards with an effective 533Mhz FSB (~4.26GB/s) and there are Athlon XP boards with an effective 333MHz FSB -- so even if they had implemented a proper double pumped FSB they would have still been behind.

4) Is the G4 the bottleneck?

I'm not sure but I've seen speculation that the current MPC7450 doesn't support faster bus speeds?

5) What are the implications for the next revision Power Macs?

My hope is that it has little implication and the pro towers will have a FSB with enough bandwidth to support any DDR implementation they will probably get at MWNY... more realisticly though I'd think this doesn't bode well for whatever revision occurs.

[ 05-17-2002: Message edited by: echoes ]

[JLL]

Originally posted by echoes:
<STRONG>Intel currently has motherboards with an effective 533Mhz FSB (~4.26GB/s) and there are Athlon XP boards with an effective 333MHz FSB -- so even if they had implemented a proper double pumped FSB they would have still been behind.
</STRONG>

The G4 has one (big) advantage, and that's the L3 cache which the processor can access with 4GB/s. A faster FSB would still help a lot though.

[Eug]

I'm not sure but I've seen speculation that the current MPC7450 doesn't support faster bus speeds?

I'm no expert but it would seem that these chips are simply not designed to make use of a double-pumped FSB. Faster bus speeds per se are not the real issue. (Maybe it can run a faster bus speed, but it'd still be single-pumped. ie. Overclocked to 150 it's still going to operate fine theoretically, but at 150, single-pumped.) See the first post for a link with sketchy info on the 7470 which supposedly can use a double-pumped bus.

[ 05-17-2002: Message edited by: Eug ]

[echoes]

Originally posted by JLL:
<STRONG>
The G4 has one (big) advantage, and that's the L3 cache which the processor can access with 4GB/s. A faster FSB would still help a lot though.</STRONG>

I wouldn't consider that much of an advantage. It's like having 2MB of DDR memory with the proper bandwidth to it across the whole system (instead of the possible 1.5GB of RAM if they had a double pumped (well to equal that much bandwidth almost quad pumped) system bus). The 2.53GHz P4 accesses it's 512KB of L2 with 81 GB/s of bandwidth.

[JLL]

Originally posted by echoes:
<STRONG>

I wouldn't consider that much of an advantage. It's like having 2MB of DDR memory with the proper bandwidth to it across the whole system (instead of the possible 1.5GB of RAM if they had a double pumped (well to equal that much bandwidth almost quad pumped) system bus). The 2.53GHz P4 accesses it's 512KB of L2 with 81 GB/s of bandwidth.</STRONG>

What's the speed of the G4's L2 cache? I don't know how much RAM the processor needs, but the 2 MB L3 cache should help a lot in certain areas.

[echoes]

Originally posted by JLL:
<STRONG>

What's the speed of the G4's L2 cache? I don't know how much RAM the processor needs, but the 2 MB L3 cache should help a lot in certain areas.</STRONG>

I would guess it's fullspeed L2 cache so whatever speed the processor is at -- 256K in size I think.

[ 05-18-2002: Message edited by: echoes ]

[[APi]TheMan]

Originally posted by Eug:
<STRONG>

Good thread, but BadAndy speaks over my head. </STRONG>

What was he talking about...? Man that guy's got a lot of intelligent things to add to a discussion.