Re: DDR2 versus FBD
- From: "David Kanter" <dkanter@xxxxxxxxx>
- Date: 7 Jul 2006 19:25:10 -0700
[snip]
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" ?
So unloaded latency would be the time it takes to access data in memory
when the system is otherwise idle. So you remove any queuing delay.
When someone says that an MPU's memory latency is X nanoseconds,
usually they mean unloaded. Loaded latencies can be substantially
higher, depending on the load. Just as an example, with DDR2 you have
bus turn around (on top of queuing), which is where a bubble is
inserted between R and W activities. FBD supports simultaneous R and W
transactions.
[snip]
Sort of. The capacity issue is two fold:Is that a question of the current controller implementations or
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.
intrinsically in the i/o standard?
I'm not sure which you are referring to, so I'll answer for both. 8
DIMMs/channel is a hard limit for FBD. DDR2 the limit is in the I/O
standard. Fundamentally the issue is the number of DRAM devices that
each channel can support, which has dropped off rather substantially.
Going from SDR-->DDR halved that, going to DDR2-400 halved that again,
and DDR2-667 and DDR2-800 will only make it worse.
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.
Yup I agree. The one thing that AMD is right about is that the heat
can be an issue. The initial implementation of AMB's are not as cool
as everyone would have liked.
I'm confused when you say more modules does not increase theoreticalThe 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.
bandwidth - is that a width / depth issue. I.e. if the interface was
wide enough adding extra modules would add bandwidth.
Yes. Going wider (i.e. more channels) adds bandwidth. Going deeper
(more DIMMs/channel) increases effective bandwidth.
Time for ASCII diagrams
------|--|--|
CPU-|
------|--|--|
Will have better effective bandwidth than:
------|
CPU-|
------|
Both will have better peak bandwidth (and better real bandwidth) than:
------|--|--|
CPU-|
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 :(
Probably not. A lot of times 'cheaper' really seems to mean "we won't
jack up the price and make you bleed", rather than "it will be less
expensive".
DK
.
- References:
- DDR2 versus FBD
- From: David Kanter
- Re: DDR2 versus FBD
- From: Ryan Godridge
- Re: DDR2 versus FBD
- From: David Kanter
- Re: DDR2 versus FBD
- From: Ryan Godridge
- DDR2 versus FBD
- Prev by Date: Re: DDR2 versus FBD
- Next by Date: Re: DDR2 versus FBD
- Previous by thread: Re: DDR2 versus FBD
- Next by thread: Re: DDR2 versus FBD
- Index(es):
Relevant Pages
|
