Re: DDR2 versus FBD

On 7 Jul 2006 17:52:11 -0700, "David Kanter" <dkanter@xxxxxxxxx>
wrote:

Ryan Godridge wrote:
On 6 Jul 2006 19:07:16 -0700, "David Kanter" <dkanter@xxxxxxxxx>
wrote:

At some point in time, some of the folks here doubted that DDR2 would
present a capacity problem. I would like to instruct those individuals
to read the following PDF, in particular, page 37.

http://www-5.ibm.com/ch/events/fachtagung/pdf/system_x_performance.pdf

Just so you don't miss the highlights:

DDR2-400/533 supports 8 DIMMs across two channels
DDR2-667 supports 4 DIMMs across two channels
DDR2-800 supports 2 DIMMs across two channels

Perhaps now it should become abundantly clear why FBD is in fact
necessary for high bandwidth. There's an illustration of this on page
42 as well.

DK

I'm no memory expert so take this with a shovel of salt, but as I
understand it.

Did you get a copy of the PDF? George mentioned he couldn't access
it...let me know and I'll email it.

I managed to get it on the third attempt - dunno what's going on.

Nice FB Dimm for dummies type video linked from here -

http://www.futureplus.com/products/fs2338/fbd_overview.html

Current FB Dimm is DDR2 memory with a different module interface. The
interface is point to point fast serial allowing more modules to be
attached for a comparable pin count.

So the issue isn't modules/pin, it's bandwidth/pin. Each pin costs
quite a bit of time/effort/money, so the lower the pin count, the
better. FBD is 70 pins/channel, DDR2 is 240 pins/channel.

In turn, fewer pins means more channels. More DIMMs/channel is a
result of going serial, as you noted below.

The interface does not have a
shared bus between modules.

Right, the address and command lanes are bypass routed to the next DIMM
in the channel. That's why the unloaded latency is higher, because you
have several hops to get to a DIMM.


"Unloaded latency" ?

It is the extra bus loading which
mandates the use of slower DDR2 with current bus architectures when
more modules are added.

That's right. In general for DDR, capacity decreases as bandwidth
increases.

The capacity issue is the number of pins on the controller required
for a memory channel.

Sort of. The capacity issue is two fold:

1. # of channels, which is what you identified above - FBD can have
more channels with a given number of pins --> higher total bandwidth

2. Number of DIMMs/channel. In FBD, you can have up to 8
DIMMs/channel for any speed grade, whereas in DDR2 there is a hard
limit that decreases as the bandwidth goes up.

Is that a question of the current controller implementations or
intrinsically in the i/o standard?

To be fair, the latency does get a bit worse with each DIMM you add,
but performance will actually increase for the 2nd DIMM and possibly
the 3rd, since the memory controller can play more interleaving
tricks...however, losing a bit of latency is not a huge deal compared
to letting your data set spill to disk (or having to buy the super high
capacity 4 and 8GB DIMMs, which cost an arm, leg and your first born
child).


It's swings and roundabouts again, but I think Intel are right with
this one. Parallel interfaces have got nowhere to go.

The extra bandwidth is due in part to being able to use higher speed
modules when larger numbers are used, and being able to use higher
numbers of modules.

Yes and no. Using more modules does not increase the theoretical peak
bandwidth, but will increase the average bandwidth. The real gain in
bandwidth is because Intel uses 4 channels of FBD where AMD uses 2
channels of DDR2. Now, in a 2S system, AMD will have a total of 4
channels of DDR2; so things will be somewhat more equal. However, the
capacity is definitely going to be a problem. For DDR2-800, a 2S
system would have 4 DIMMs, which means you cannot get nearly enough
memory.

I'm confused when you say more modules does not increase theoretical
bandwidth - is that a width / depth issue. I.e. if the interface was
wide enough adding extra modules would add bandwidth.

I think the bottom line is that it means instead of trading capacity
for bandwidth, you trade capacity for latency (and some extra heat).

This isn't DDR2 Vs FB Dimm - this is serial Vs parallel, and
everythings going serial 'cos in general it tends to be faster.

Yup, that's right. It really is a serial versus || kind of thing. The
last thing to mention, is that since the routing is easier for FBD, the
boards should be a bit cheaper and easier to make. I couldn't really
quantify that though...

DK

I'd like to think you're right there, but I bet we won't see the
difference :(

.

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