Questions on PCIe

[JB73]

skuzzy,

i have purposely NOT thought about hardware upgrades because these new PC architectures are comming out.

if i want to in; the next 3 months; buy a system that will run for 3+ years, only getting a video card upgrade in the future is that possible?

basically if i were to by an atholn 64 system, and an 8x AGP card, will they still be making AGP cards for my system in 2 years? or will i have to dump my MB / processer / sound card, all that for a new architecure?

[StarOfAfrica2]

Considering that they still make and sell video cards with as little as 8MB of RAM on the shelf at the box stores, I'm betting they will have AGP around for awhile.

[Kev367th]

Just noticed there are now PCI-E cards by Gigabyte with TWO GPU's on them.
Apparently the latest with 2x6600's on it is slightly faster than the XT and is only $400 complete with a motherboard!

Wonder how two dual GPU cards (total 4 GPU's) running SLI would perform?

[bloom25]

Skuzzy, you've got me a bit confused.  Absolute theoretical bus bandwidth (if expressed in bits or bytes per second) does not change with the width of the data you are transferring.  If you are looking at this in terms of how many bus clock cycles it takes to transmit 64 versus 32 bits, then sure that doubles, but the bandwidth hasn't changed.  The maximum bus bandwidth for a PCIe 16x slot is 4 GBytes/sec up and down.  It's 2.133 GBytes/sec for AGP 8x.  If anything the effective bandwidth of the PCIe bus would be higher with 64 bit datasets.  (Less address commands needed for a given amount of data being transferred.)

The big reason to move to serial busses is really that of cost and design complexity.  

On a serial bus you can make use of differential signalling to transfer data at a very high clockspeed.  Differential signalling uses 2 wires per data line.  It is the voltage difference between those two lines that determine whether you are sending a 0 or a 1.  The reason for doing this is that it is effectively immune to what is known as common mode interference.  (I'd be happy to explain what this is, but it would take more time than I have at the moment.)  With a parallel bus you have to worry about crosstalk between lines, and implementing differential signalling to deal with this on a 32 bit parallel bus would require 64 lines.  That would take up a ton of space on a board and make for very expensive cabling and connectors.  (Some SCSI cables use low voltage differential signalling.  Anyone who has dealt with them know how expensive they are...)

[Skuzzy]

Bloom, I know what you are saying, but if the native data width of any given operation goes from 32 bits to 64 bits, then the effective performance of a serial bus is cut in half as the operation cannot commence until the op is transferred.  The number of ops does not necessarily change either.  They just get bigger in many cases.

And yes, a parallel bus is wrought with issues, especially if it using passive termination.  Active termination helps, and differential is definately the best and, as you pointed out, the most expensive.

The 4GB/s you spec for the PCI-e bus is dependent on the clock rate of the bus.  Typical devices I have seen are running at 2.5Ghz per channel, which yeilds the figure I came up with.  Some of the cheaper motherboards look to be using 2Ghz parts as well.
Seems to be a grab bag at this time.

All that aside, where the heck you been?

[bloom25]

Skuzzy, where haven't I been is the question.  I design data communications systems, primarily for public safety.  Its been keeping me on the road (in the air) a LOT recently, so I haven't had much time to browse the forums.  ( I was in Dallas last month.  I thought about stopping by...)  I wish I could show you guys some of the "toys" I get to play with these days...

Another advantage of a serial bus that will become more and more important is that of RF emissions and interference.  With a serial bus it is a easier to use a signalling protocol that minimizes the number of transitions on the bus, reducing RF emissions.  A serial bus using differential signaling is also much less prone to interference from other devices.  This is something that will become more significant as data rates go up and you need to minimize error rates.

It wouldn't surprise me in the not to distant future if all components in a computer are linked together using a common high speed serial bus with a hub in the center acting as a router for traffic to each endpoint.  That would be a LOT less expensive and much more flexible than what is done currently.  (You are already starting to see this with Hypertransport, etc.)

[Skuzzy]

Well, it's good to see you around.  I know how the traveling bit can keep you busy.
--
I am not saying the serial bus is not a good way to go.  I am just pointing out there are limitations.  The single limitation of the native datum width will not show itself until the change from an 'integer' being 32 bits to 64 bits.  It will not incur a 50% reduction in performance, as opcodes may not change that much from where they are now, but I could see a 30% hit pretty easily.
Of course, the same hit would occur on computers using a parallel bus fixed at 32 bits wide.

The other downside of serial is the various clockings hidden from the user.  Just because you have a PCI-E does not mean you are getting 3Ghz on the channel.  It can be 2Ghz and up.  In an ideal world, they would all perform the same, but just like the $2 video chips OEM's use today, if they can find a cheaper way to implement it, they will.

Overall it is definately cheaper and easier to implement higher clock rates, but it is not without its own limitations.

[Black Sheep]

I'm gonna go spend about 1000-1200 on a new system. Should I jump to PCI-E or stick with AGP? I already have a decent Radeon 128mb that I can swap...Some of the new HP's dont even have an AGP slot anymore.