The Dark Knight: Intel's Core i7
by Anand Lal Shimpi & Gary Key on November 3, 2008 12:00 AM EST- Posted in
- CPUs
Thread It Like Its Hot
Hyper Threading was a great technology, simply first introduced on the wrong processor. The execution units of any modern day microprocessor are power hungry and consume a lot of die space, the last thing you want is to have them be idle with nothing to do. So you implement various tricks to keep them fed and working as often as possible. You increase cache sizes to make sure they never have to wait on main memory, you integrate a memory controller to ensure that trips to main memory are as speedy as possible, you prefetch data that you think you'll need in the future, you predict branches, etc...
Enabling simultaneous multi-threaded (SMT) execution is one of the most power efficient uses of a microprocessor's transistor budget, as it requires a very minimal increase in die size but can easily double the utilization of a CPU's execution units. SMT, or as Intel calls it, Hyper Threading does this by simply dispatching two threads of instructions to an individual processor core at the same time without increasing the available execution resources. Parallelism is paramount to extracting peak performance out of any out of order core, double the number of instructions being looked at to extract parallelism from and you increase your likelihood of getting work done without waiting on other instructions to retire or data to come back from memory.
In the Pentium 4 days enabling Hyper Threading required less than a 5% increase in die size but resulted in anywhere from a 0 - 35% increase in performance. On the desktop we rarely saw a big boost in performance except in multitasking scenarios, but these days multithreaded software is far more common than it was six years ago when Hyper Threading first made its debut.
This table shows what needed to be added, partitioned, shared or unchanged to enable Hyper Threading on Intel's Core microarchitecture
When the Pentium 4 made its debut however all we really had to worry about was die size, power consumption had yet to become a big issue (which the P4 promptly changed). These days power efficiency, die size and performance all go hand in hand and thus the benefits of Hyper Threading must also be looked at from the power perspective.
I took a small sampling of benchmarks ranging from things like POV-Ray which scales very well with more threads to iTunes, an application that couldn't care less if you had more than two cores. What we're looking at here are the performance and power impact due to Hyper Threading:
| Intel Core i7-965 (Nehalem 3.2GHz) | POV-Ray 3.7 Beta 29 | Cinebench R10 1CPU | Race Driver GRID | |||
| HT Disabled | 3239 PPS | 207W | 4671 CBMarks | 161.8W | 103 fps | 300.7W |
| HT Enabled | 4202 PPS | 233.7W | 4452 CBMarks | 159.5W | 102.9 fps | 302W |
Looking at POV-Ray we see a 30% increase in performance for a 12% increase in total system power consumption, that more than exceeds Intel's 2:1 rule for performance improvement vs. increase in power consumption. The single threaded Cinebench test shows a slight decrease in both performance and power consumption (negligible) and the same can be said for Race Driver GRID.
When Hyper Threading improves performance, it does so at a reasonable increase in power consumption. When performance isn't impacted, neither is power consumption. This time around Hyper Threading has no drawbacks, while before the only way to get it was with a processor that was too hot and barely competitive, today Intel offers it on an architecture that we actually like. Hyper Threading is actually the first indication of Nehalem's true strength, not performance, but rather power efficiency...
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Gary Key - Monday, November 3, 2008 - link
"The 920 to 3.6/3.8 is a nice overclock but I wonder what you mean by proper cooling and how close you came to crossing the 80C "boundary"?"It was actually quite easy to do with the retail cooler, in fact in our multi-task test playing back a BD title while encoding a BD title, the core temps hit 98C. Cinebench multi-core test and OCCT both had the core temps hit 100C at various points. Our tests were in a closed case loaded out with a couple of HD4870 cards, two optical drives, three hard drives, and two case fans.
Proper cooling (something we will cover shortly) consisted of the Thermalright Xtreme120, Vigor Monsoon II, and Cooler Master V8 along with the Freezone Elite. We were able to keep temps under 70C with a full load on air and around 45C with the Freezone unit.
Th3Eagle - Tuesday, November 4, 2008 - link
Wow, thats interesting. Can't wait to see the new article. Always nice to see an article about coolers.Thanks for the reply.
Anand Lal Shimpi - Monday, November 3, 2008 - link
Gary did the i7-920 tests so I'll let him chime in there, we're also working on an overclocking guide that should help address some of these concerns.-A
whatthehey - Monday, November 3, 2008 - link
Tom's? You might as well reference HardOCP....Okay, THG sometimes gets things right, but I've seen far too many "expose" articles where they talk about the end of the world to take them seriously. Ever since the i820 chipset fiasco, they seem to think everything is a big deal that needs a whistle blower.
Anandtech got 3.8GHz with an i7-920, and I would assume due diligence in performance testing (i.e. it's not just POSTing, but actually running benchmarks and showing a performance improvement). I'm still running an overclocked Q6600, though, and the 3.6GHz I've hit is really far more than I need most of the time. I should probalby run at 3.0GHz and shave 50-100W from my power use instead. But it's winter now, and with snow outside it's nice to have a little space heater by my feet!
The0ne - Monday, November 3, 2008 - link
TomHardware and Anandtech were the one websites I visited 13 years ago during my college years. Tom's has since been pushed far down the list of "to visit sites" mainly due to their poor articles and their ad littered, poorly designed website. If you have any type of no-script enable there's quite a bit to enable to have the website working. The video commentary is a joke as they're not professionals to get the job done professionally...visually anyhow.Anandtech has stayed true to it's root and although I find some articles a bit confusing I don't mind them at all. Example of this are camera reviews :)
GaryJohnson - Monday, November 3, 2008 - link
Geez, calling a core 2 a space heater. How soon we forget prescott...JarredWalton - Monday, November 3, 2008 - link
I think overclocked Core 2 Quad is still very capable of rating as a space heater. The chips can easily use upwards of 150W when overclocked, which if memory serves is far more than any of the Prescott chips did. After all, we didn't see 1000W PSUs back in the Prescott era, and in fact I had a 350W PSU running a Pentium D 920 at 3.4 GHz without any trouble. :-)Griswold - Tuesday, November 4, 2008 - link
Funny comparison. If it was just for the space heater arguments sake (well, 150W is by far not enough to qualify as a real space heater to be honest), I could follow you but saying the 150W of a 4 core, more-IPC-than-any-P4-can-ever-dream-of, processor should or could be compared to the wattage of the infamous thermonuclear furnace AKA prescott, is a bit of a long stretch, dont you think? :pRyan Smith - Monday, November 3, 2008 - link
Intel can call it supercalifragilisticexpialidocious until they're blue in the face, but take it from a local, it's Neh-Hay-Lem. Just see how it's pronounced in this news segment:http://www.katu.com/outdoors/3902731.html?video=YH...">http://www.katu.com/outdoors/3902731.html?video=YH...
mjrpes3 - Monday, November 3, 2008 - link
Any chance we'll see some database/apache benchmarks based on Nehalem soon?