Home > Rants & Opinions > QuickPath owns

QuickPath owns

When Gophur was building my desktop workstation about a year ago, there were various considerations. While it was fun having multiple PCs, it gets old when it comes to moving stuff around between them. I could also better justify a heavier-hitting machine by having one PC to run not only my desktop but also my other machines virtually.

Somehow, though, I let Goph talk me down to an i3 or i5 – I don’t even remember. What I do know is that it is not an i7.

The i-core CPUs feature Intel’s new QuickPath technology. Short version? Replace the single-lane, cobblestone paved connection between processor and memory, disk, etc with a 2 or 4 lane highway.

If you remember AGP, QuickPath is kinda AGP for everyone and AGP on steroids.

Before QuickPath your processor had a single pipeline (“bus“) for talking to memory, disks, etc.

QuickPath provides a way for the CPU to have multiple conversations going on with different hardware devices (sound chips, graphics card, disks, memory, etc) etc. It also allows the chip to have an onboard memory controller so that the CPU can talk to multiple sticks or banks of memory at the same time.

Which is handy for the CPU if it plans on, say, multi-tasking.

Today’s computers run really, really fast. Much faster than the memory chips available. If you have cable, broadband, DSL or better – try and imagine surfing today’s web with an early dialup modem…

Hence all the talk about cache — Intel and AMD have been taking up precious CPU circuits providing little chunks of buffer-memory onboard of the processor unit. “L1 Cache” (Level 1 – the first of several levels) is CPU that serves as really, really fast memory (as fast as the CPU itself). But it’s costly. I’m guessing here, but I’d speculate that it increases heat usage and decreases energy efficiency of CPUs too.

The actual memory in your computer runs much slower. Faster than disk, but slower than the CPU. So if an application needs to fetch data that isn’t in the cache, and given that cache is very small that will be most of the time, the CPU has to wait for the memory to receive the request, fetch the information, and send it back.

Multi-tasking operating systems will tend to do something else while your application is waiting, but this can backfire. You see it when your computer is starting up and it slows to a crawl. This is because all of the applications are accessing new data that most definitely won’t be in the tiny little cache; worse, they are pulling the data off disk into memory. That’s a whole extra bottleneck.

Because QuickPath allows the CPU to talk directly to memory, and to be much smarter about it’s interactions, and because it can talk to multiple memory sticks and/or banks simultaneously, it is much less wasteful of precious CPU cycles. If a program is about to read big chunks of memory, the CPU can be smart and start reading the next big chunk of information info buffers, then start reading the first chunk of that information into the Level 1 cache. By the time it’s done with the first chunk, the next chunk will be ready in the buffers and the CPU can scream along.

It also means that things can start up faster. On an older CPU, the CPU has to fetch some data from disk, then write it into memory. It can’t do both at once. With QuickPath, it can set up a fireman’s chain, because it can read and write at the same time.

There’s a limitation though: Think of cars on a freeway. Each one needs its own little pocket of space, and you can’t have two cars in the same lane in the same pocket at the same time.

So if your CPU has multiple cores, you’re going to have contention over who gets to use the QPI (QuickPath Interconnect).

The i3 and i5 CPUs come with 2 QPIs – that means that two cores at a time can be doing I/O (Input/Output).

The i7, however, comes with 4 QPIs. Which means it can really zing.

In terms of number-crunching power, it’s not that much more powerful than a 3 or 5. But in terms of multi-tasking power… <sings> Rocket Powah!

So, yeah. This box is so not an i7.

I have an i7 at home. I routinely have SmartSVN, Visual Studio, VMWare running Ubuntu 10.10, Chrome, Thunderbird and one or two games loaded (WWIIOL, EQ2 and/or World of Tanks). The machine is almost always responsive.

Like they say – you don’t know what you’ve got till you lose it.

I’m going to alt-tab now. I may be gone some time.

Categories: Rants & Opinions
  1. coppertopper
    September 20, 2011 at 1:37 pm

    Interesting! This sounds like a must-have feature, yet I’ve never heard of it. Does AMD have any sort of equivalent in the works?

  2. September 21, 2011 at 12:58 am

    QuickPath is rather similar to AMD’s HyperTransport, which they have been using since around 2004.

    Also, QuickPath itself doesn’t allow the CPUs to talk to any more memory banks at a time than before – the memory controller, although it’s on-die – still sits between CPU cores and memory. Where you do have more memory bandwidth with QuickPath are multi-socket servers, where each CPU package has a memory controller, and CPU cores can talk to memory controllers that are off-chip. However, a modern desktop system has only one CPU package, and e.g. Core i7 2600 and Core i5 2500 both have pretty much exactly the same memory bandwidth.

  3. September 22, 2011 at 10:02 am

    I think you’ve misunderstood quickpath, Taz; Gophur was operating under the same premise when he chose the CPU for my box. Even tho it is a generation newer than my home i7, even tho it is nearly 1Ghz faster than my home i7 … My home i7 blows it away when it comes down to load-bearing across multiple processes. Yes, the number of cores helps there, but that’s only because thinking primarily in terms of “bandwidth” is a mistake when it comes to understanding throughput with QuickPath.

    – Oliver

  4. September 23, 2011 at 3:39 am

    Which specific CPUs are we talking about here?

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