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While comparing Intel's two new i7 processors, I noticed that the only difference between them apart from the L3 cache size and CPU speed is that one has QuickPath Interconnect, and that seems to account for most of the massive price difference between the two ($600 vs $1000). I'm not interested in shopping advice, but I do wonder what the advantages of QPI are.

I've read the Wikipedia article and I understand that QPI essentially replaces the Front-Side Bus. The article says that the first QPI implementation achieved double the throughput of the contemporary FSB, but it's not clear that this increase in throughput results in an increase in performance in real-world situations.

I understand that a faster FSB enables faster processors and memory, so theoretically replacing the FSB with something faster (QPI) would also enable faster processors and memory. However, I haven't seen anything indicating there are significantly faster components that are only usable with QPI. In other words, a fast FSB seems to be "good enough" for all intents and purposes.

Is a FSB still considered a bottleneck anymore by anyone who isn't trying to sell me something? If yes, when does QPI offer a performance improvement over a FSB? QPI is apparently not used for memory access like a FSB is; what effect does that have?

Edit: I'm looking for an answer that can say "When gaming and communication with the video card is the bottleneck, you'll see X sort of improvement. When doing CPU-intensive operations like compiling, you'll probably see no effect unless it's also memory-intensive" or something like that. Theoretical maximums and performance under contrived situations isn't useful, nor is the exact percentage performance gain given specific hardware and a specific task like the first loading screen in Crysis :P. A happy medium, please! You can support generalizations with specific test results, though, of course.

Mega bonus points if you can also address the same issues for HyperTransport.

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Sadly, this is an question that has no deterministic answer. Without specific workloads, the real world differences might as well be zero. For example, suppose your "real world example" is staring at Facebook for six hours, which is actually understating usage data. But never the less, the point stands. –  surfasb Nov 17 '11 at 4:24
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@surfasb I did ask when it offered a performance improvement. It obviously offers an improvement when given a contrived task to test nothing but the throughput, which is worthless information. Likewise asking what the difference is given X processor and Y memory running Z benchmark is pretty worthless. The useful middle ground is what I'm aiming for; I'm sure someone can generalize with a "CPU-intensive like compiling" vs "graphics intensive gaming" vs whatever else style answer. If I ask for anything too specific I just eliminate any information that doesn't match those parameters. –  Matthew Read Nov 17 '11 at 4:35
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I edited the title to (hopefully) make it appear less about specific result expectations. –  Ƭᴇcʜιᴇ007 Nov 17 '11 at 5:44
    
@techie007 Thanks! –  Matthew Read Nov 17 '11 at 15:28
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2 Answers 2

up vote 3 down vote accepted

The QuickPath Architecture increases the bandwidth between physical processors, memory and I/O in (at least) three ways:

  • The memory controllers are moved from the chipset into the CPU, shortening the path from each processor to RAM, and preventing data from going a back and forth through a central, shared chipset to get at memory.
  • There are connections/paths added to the processors to pass data directly between them, again eliminating data trips though a central chipset to share data between CPUs.
  • There are now multiple paths to the chipset, which is what attaches you to other I/O.

These are connections are called QuickPath Interconnects (QPI), and are considerably shorter, wider and faster than traditional FSB connections.

QPA's mesh-like (point-to-point) architecture will also allow great future expandability without the bottleneck of overloading a single FSB.

"When will I notice the improvements?"

You should notice an increase in performance in anything that passes data between CPUs, to and from system Memory, and to and from system I/O. YMMV. :)

See here for best info out there about it.

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Thanks for the clear overview. My question may have been slightly premature if there's not much that truly takes advantage of QPI right now, but it certainly looks like it has a lot to offer for the future. –  Matthew Read Nov 17 '11 at 15:45
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QPI is a new technology for IO interconnect by Intel, introduced with the core i series of processors.

QPI acts like a IO bus with additional feature for processor interconnection. Now, since you want performance improvements in QPI over FSB, I can say that QPI can do wonder when compared to FSB. What happened in earlier architecture was that the memory was localized with a processor, what this means is that data residing with a memory chip; and the chip has proximity with a particular processor, was electrically nearer to a processor. Now with FSB, this is not the case. The data can move between processors faster and efficiently. You can have a dedicated link from each processor to every another. It suffices to make the cluster connected, but it really helps if the cluster is fully connected( every processor connected to every other).

For an Intel video, click here. For an introductory white paper by Intel here.

In summary, one can say that QPI enhances the performance in a multiprocessor PC tremendously. Even in a single processor PC, the IO is enhanced by QPI.

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Thanks for the link to the white paper, it's very useful. As I understand it, in a current single-processor system the performance improvement will largely come from decreased memory access times. There should be graphics improvements as well with PCI Express products that take advantage of the new architecture, and we can more or less expect every aspect of system performance to improve because the performance of the FSB was being pushed near maximum and was beginning to limit everything else. Very cool. –  Matthew Read Nov 17 '11 at 15:43
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