The Quest for More Processing Power, Part One: "Is the single core CPU doomed?"
by Johan De Gelas on February 8, 2005 4:00 PM EST- Posted in
- CPUs
CHAPTER 2: Why single core CPUs are no longer "cool"
The end of the single core CPU?
Right now, the leakage problem is by far the most urgent problem. As profit margins are low and cost is, in most cases, the decisive factor for consumers, expensive cooling systems are not practical.
Past experience has shown that complex superscalar CPUs need about twice as many transistors to achieve +/- 40% better performance. The conclusion of many industry analysts and researchers is that the single-core CPU has no future. I quote Shekhar Borkar, Intel Fellow, Director:
"Multiprocessing, on the other hand, has potential to provide near linear performance improvement. Two smaller processors, instead of a large monolithic processor, can potentially provide 70-80% more performance, compare this to only 40% from the large monolithic processor."
Note the word "monolithic", a word with a rather pejorative meaning, which insinuates that the current single core CPUs are based on old technology. So, basically the single core CPU has no future as it improves performance only by 40%, while doubling complexity and thus leakage. This reasoning explains why all of sudden Intel marketing does not talk anymore about 10 GHz CPUs, but about the "era of thread parallelism".
It should be noticed though that the 40% better performance of the "monolithic CPU" is achieved across a wide variety of applications, without the need of time-consuming software optimizations. The promised 70% to 80% of the multithreaded CPU can only be easily achieved in a small range of applications, while the other applications will see exponential investments in development time to achieve the same performance increase.
Of course, we agree that multiprocessors have benefits. It is easier to turn off a complete CPU than to manage the energy consumption of the different parts of one big CPU.
And you can run a single-threaded application on CPU1, and turn the CPU2 off. When CPU1 gets almost hot, you let CPU2 continue to do the work. As a result, you reduce the average temperature of one CPU core. As leakage decreases with lower die temperatures, this technique can reduce overall leakage power. The objective of using a dual core CPU is then primarily to reduce power consumption in situations where there is only one CPU intensive application. This is probably the reason why Intel sees a great future for dual core CPUs in the mobile market, although the mobile market is probably the last market where we will be able to benefit from dual core power. The last thing that you want is twice as much power dissipation because the two cores get active. In our humble opinion, dual core will be only dual when it is not working on battery power.
Trendy
The second argument used by the people who are hyping the multithreaded CPU is "the whole industry is moving towards multi-core CPUs". Considering that the server is the only market where non x86 CPUs play an important role, it is not very surprising. For companies such as SUN and IBM, it is only natural to ignore single-threaded performance somewhat and to invest as much time as they can in designs that can work with as many threads as possible. The software that runs on these SUN and IBM machines, Massive OLTP databases and HPC applications, are multi-threaded by nature.
SUN's Niagra CPU can run 32 threads at once, but it will not be the kind of CPU that you would like in your desktop. Single threaded performance is most likely at the level of one of the early PIIIs. Sun's own demo [6] shows a Niagra to be more than 4 times slower in a single-threaded application than an unknown single-threaded CPU, which is, hopefully for SUN, one of the current top CPUs.
Delving deeper
So, while there are definite advantages to CPUs that exploit Thread Level Parallelism, if we want to understand what is really going on, we need to delve a little deeper. First, we look if leakage can really kill all progress of "monolithic" single core CPUs; secondly, we will study the prime example of a "classic" single core CPU that had crushed into a wall of leakage: the Intel Prescott.
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Momental - Wednesday, February 9, 2005 - link
#41, I understood what he meant when he stated that AMD could only be so lucky to have something which was a technological failure, ie: Prescott, sell as well as it has. Even the article clearly summarizes that Prescott in and of itself isn't a piece of junk per se, only that is has no more room for evolution as Intel originally had hoped.#36 wasn't saying that it was a flop sales-wise, quite the contrary. The thing has sold like hotcakes!
I, like many others here, literally got dizzy as I struggled to keep up with all of the technical terminology and mathmetical formulas. My brain is, as of this moment, threatening to strike if I don't get it a better health and retirement plan along with a shorter work week. ;)
Ivo - Wednesday, February 9, 2005 - link
1. About the multiprocessing: Of coarse, there are many (important!) applications, which are more than satisfied with the existing mono-CPU performance. Some other will benefit from dual CPUs. Matrix 2CPU+2GPU combinations could be essential e.g. for stereo-visualization. Probably, desktop machines with enhanced voice/image analytical capabilities could require even more sophisticated CPU Matrices. I suppose, the mono- and multi-CPU solutions will coexist in the near future.2. About the leakage problem: New materials like SOI are part of the solution. Another part are the new techniques. Let us take a lesson from the nature: our blood-transportation system consists of tiny capillaries and much thicker arteries. Maybe it could make sense to combine 65 nm transistors e.g. in the cash memory and 90 nm transistors in the ALU?
Noli - Wednesday, February 9, 2005 - link
"Netburst architecture is very innovative and even genial"genius-like?
If by genial you mean 'having a pleasant or friendly disposition', it sounds weird. It can mean 'conducive to growth' in this context but that's not so intuitive because a) it wasn't and b) at best it was only theoretically genial.
Presumably it's not genial as in 'of or relating to the chin' :)
Agree monolithic was confusing but it was the intel dude who said it - I thought it meant 'large single unit' rather than 'old (as in technology)' as in: increasing processing power by increasing the size and complexity of a single core is now not as efficient as strapping two cores together - a duallithic unit :)
Sorry to be a pedantic twat.
Xentropy - Wednesday, February 9, 2005 - link
Some of the verbage in that final chapter makes me wonder how much better Prescott might have done if Intel had just left out everything 64-bit and developed an entirely different processor for 64-bit. Especially since we won't have a mainstream OS that'll even utilize those instructions for another few months, and it's already been about a year since release, they could have easily gotten away with putting 64-bit off for the next project. It's pretty obvious by now even the 32-bit Prescotts have those 64-bit transistors sitting around. Even if not active, they aren't exactly contributing to the power efficiency of the processor.I think one big reason Intel thinks dual core will be the savior of even the Prescott line is supposedly dual cores running at 3Ghz only require equivalent power draw to a single core at 3.6Ghz and should be just as fast in some situations (multitasking, at least). Dual core at 85% clockspeed will be slower for gaming, though, so dual core Prescott still won't close the gap with AMD for gaming enthusiasts (98% of this site's readership), and may even represent an even further drop in performance per watt. Here's hoping for Pentium-M on the desktop. :>
piroroadkill - Wednesday, February 9, 2005 - link
#36 -- You really didn't read the article and get the point of it. It wasn't a failure from a sales point of view, and this article was not written from a sales point of view, but a technical point of view, and how the Prescott helped in furthering CPU technology.Thus, a failure.
ViRGE - Wednesday, February 9, 2005 - link
Although I think I sank more than I swam, that was a very good and informative article Johan. I just have one request for a future article since I'm guessing the next one is on multi-core tech: will someone at AT run the full AT benchmark suite against a SMP Xeon machine so that we can get a good idea ahead of time what dual-core performance will be like against single core? My understanding is that the Smithfields aren't going to be doing much else new besides putting 2 cores on one die(i.e. no cache sharing or other new tech), so SMP benchmarks should be fairly close to dual-core benchmarks.Griswold - Wednesday, February 9, 2005 - link
Point and case as to why the marketing department is the most important (and powerful) part of any highly successful company. It's not the R&D labs who tell you what works and what comes next, it's the PR team.quidpro - Wednesday, February 9, 2005 - link
Someone needs to make a new Tron movie so I can understand this better.tore - Wednesday, February 9, 2005 - link
Great article, on page 3 you talk about BJT transistor with a base, collector and emitter, since all modern cpu's use mosfets should you talk about a mosfet with a gate, source and drain?Questar - Wednesday, February 9, 2005 - link
"The Pentium 4 "Prescott" is, despite its innovative architecture, a failure."AMD wishes they had a "failure" that sold like Prescott.