PCI Express 2.0: Scalable Interconnect Technology, TNG
by Kris Boughton on January 5, 2008 2:00 AM EST- Posted in
- CPUs
Final Thoughts
As we have seen, PCI Express 2.0 is the next step in the quest for more bandwidth. This trend has been around since the beginning of time as each successive generation usually doubles the maximum bandwidth of the previous generation (or technology). This only makes sense as according to Moore's Law the number of transistors placed in an inexpensive integrated circuit approximately doubles every two years - improvements in interconnect technology necessary to support these advancement are the logical extension of this law. Looking for performance improvements in today's technology offerings with the next-generation interconnect technology (like PCI Express 2.0) is largely futile. The real benefit may come in the next round of video card releases - or perhaps Generation Next^2. Still, those that are quick to brush off PCI Express 2.0 as ineffective and unimportant should recall the past. The difference in performance experienced during the transition from AGP to PCI Express 1.x was not necessarily a revolution at the time, but there are benefits to using PCI-E instead of the aging PCI interface, and we may see new implementations of PCI-E technology that make the 2.0 revision more important.
Just about every interconnect technology is moving from high-pin count parallel interfaces to high-speed, low-pin count, serial, point-to-point interfaces. We have seen the incredible difference moving to a low-latency, high-bandwidth interfaces made in the transition from PATA to SATA as well as PCI/AGP to PCI Express (AGP was really just a kludged-together remake of PCI technology). Moreover, we will see it again in the near future when Intel leaves behind their antiquated FSB topology for QuickPath - something AMD did years ago with the release of their Opteron/Athlon 64 line featuring HyperTransport Technology. Removing the MCH altogether means moving the memory controller on die as well as relocating PCI Express resources to the CPU. The X38/X48 chipset will simultaneously be the first and last Intel MCH to make use of PCI Express 2.0 technology. (Unless the fabled P45 chipset, the 65nm die-shrink of P35 with additional improvements, see's the light of day.)
No doubt, those that run their systems with PCI Express 2.0 graphics cards installed will see an increase in MCH power consumption. Although we can't tell you exactly what share of the consumption figure is due to PCI Express circuit operation, what we can tell you is that it went up - those portions of the die that must run at the base link frequency of 5.0GHz have at least doubled from the days of PCI Express 1.x. What this means to the bottom line we don't know - perhaps this can help us explain why X38 seems to be so darn hot at times.
Maybe in the future we will see a mechanism that allows us to force PCI Express 1.x operation with PCI Express 2.0 capable graphics cards on some motherboards. (We do not recall testing many boards that had this option - allowing for PCI Express 1.1 or 1.0a.) This would be the only way for us to determine if the increased signaling rate makes a big a difference for today's products, but while we would hope it helps we remain skeptical at best. Purchasing a PCI Express 2.0 graphics cards at this time is future proofing more than anything else; we haven't noticed any performance differences when comparing PCI-E 1.x chipsets to PCI-E 2.0 chipsets with the newer GPUs, and we don't expect you to either. PCI-E 2.0 may prove more useful as a way to provide four x8 slots that each offer the same bandwidth as the older 1.x x16 slots. With Triple-SLI, Tri-Fire, and talk of GPU physics accelerators, including additional high-bandwidth slots on motherboards makes sense.
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kjboughton - Sunday, January 6, 2008 - link
Are you sure this isn't fiber or optical? Any supporting information you can provide would be great.Hulk - Saturday, January 5, 2008 - link
First of all great article. Great writing. You should be proud of that article.I see that currently the Southbridge can transmit data to the Northbridge at 2GB/sec max. In real world situations about how much bandwidth would the Southbridge require assuming a light, medium, and heavy loading situation?
kjboughton - Monday, January 7, 2008 - link
I can help you with some of the base information needed to calculate this yourself (since every system is different based on attached peripherals as well as their type) and we'll leave the rest to you as an exercise.For example, a 1Gbps Ethernet connection to the ICH would have a maximum theoretical sustained data transfer rate of 125MB/s (1Gbps x 1 byte/8 bits). A single SATA 3.0Gbps drive would be limited by the interface to three times this number, or about 375MB/s (although the disk to bus/cache transfer rate is much less, somewhere on the order of 120-140MB/s sustained) - but nevertheless, burst read speeds could easily saturate the bus in one direction (1Gbps). Then there's USB devices, possibly a sound card or other onboard solution...going through the numbers, adding up the maximum possible bandwidth for all your attached devices you should be able to get an idea for what would be "light, medium and heavy" loading for your system. Again, this is something that varies from system to system. Hope this helps.
LTG - Saturday, January 5, 2008 - link
Excellent article, good tech level.Would you believe "simple ecards" benefit from PCI-E 2.0 right now?
At most sites when you send an ecard it just e-mails a link to a flash animation to someone.
However when you send an ecard at the site below, it's rendering and compositing custom photos and messages into a 3d scene on the fly for each card sent.
Because this is a web site all of this runs on the server side for many users at once.
PCI-E 3.0 will be welcome :).
http://www.hdgreetings.com/preview.aspx?name=count...">http://www.hdgreetings.com/preview.aspx?name=count...
or
www.hdgreetings.com (sorry, link buttons not working)
JarredWalton - Saturday, January 5, 2008 - link
I don't know that e-cards would really benefit much - especially right now. The FSB and memory bandwidth aren't much more than what an x16 PCI-E 2.0 slot can provide in one direction (8GB/s). I would imagine memory capacity and the storage subsystem - not to mention network bandwidth - are larger factors than the PCI-E bus.Are you affiliated with that site at all? If so, I'd be very interested to see a performance comparison with a single 8800 GTX vs. an 8800 GT on a PCI-E 2.0 capable motherboard. The 8800 GTX even has a memory and performance advantage, but if as you say the bottleneck is the PCI-E bus, it should still see a performance increase from the 8800 GT.
LTG - Sunday, January 6, 2008 - link
Hi Jared, yes I'm a developer on the site - (pls don't think of my post as spam, i've been a reader at AT forever and it just seemed relevant :)You could be right, we are just now starting to test pci2.0 so the benchmark you mention will definitely shed some light.
The network is not a bottleneck because cards are rendered and compressed on a given server node.
The disk IO is a 6 drive RAID0 array (no data is at risk because the nodes just render jobs) with the Segate 7200.11 drives max out at 100MB/sec transfer rate each, which is less than 600MB/sec total, however I have "heard" that the effective PCI-E video card bandwidth is much less than the theoretical limit.
I wish there were a utility to easily measure PCI-E bandwidth but currently I only know of indirect experiments as you mention.
Thanks again for the nice article.
PizzaPops - Saturday, January 5, 2008 - link
I can't help but be amazed by the speed at which hardware is improving. I remember when we were stuck with just PCI and AGP for what seemed like forever. Now the speeds are getting ridiculous. Can't wait to see what the future has in store.Very informative article and not too difficult for the average person to understand either. Now I know why my X38 gets so hot.
Spoelie - Saturday, January 5, 2008 - link
Why does the 790FX stays so cool then?Besides, there hasn't been a review of that one yet on AT.
Gary Key - Saturday, January 5, 2008 - link
The 790FX does not have the memory controller on-board among other items, so the additional power required for PCI-E 2.0 is minimal at best as are resulting thermal increases. AMD also took a very elegant approach on the 790FX in regards to PCI-E 2.0 (they had time to ensure proper integration, Intel's is fine, just they had a lot to cram into the chipset this time around ;) ) that we will cover shortly.NVIDIA's current approach on the 780i is to use a bridge chipset that is creating a few problems for us right now when overclocking both the bus and video card. We will have a complete 790FX roundup the week of the 14th along with a "how to" guide on getting the most out of Phenom on these boards.
Comdrpopnfresh - Monday, January 7, 2008 - link
So by bridge chip, some intermediate chip slows things? Or creates asymmetric latencies leading to unbalanced clocks (like initial SATA drives implementing connections and features like NCQ natively on PATA with a cross-over to SATA)?If so, I read an article dealing with a similiar problem with raid-spanning of SSDs somewhere... have to dig up the link...
The problem began with one dive on the Intel ICH..9 (I understand there is a workaround now). So the tester switched to an add-in discrete raid-handling card. When they began adding more and more drives (believe they went 1-2-3-4), these mucho-expensive raid cards were zapping throughput one after another (with successively higher prices of course) because the companies shaved on the onboard processing power because before these SSDs, the throughput on a RAID spanning standard HDDs just wasn't nearly as great. When they got to four drives, and something like a > $900 add-in card they stopped- one hell of an expensive review!