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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WhoBeDaPlaya - Thursday, February 10, 2005 - link
Ain't no way you can get those repeaters out of there - that's already the optimum solution for driving the large load (interconnect). It probably equalizes the stage effort required (you can work out the math and find that for multi-stage logic, the optimal config is that each stage has the exact same effort level). Eg. instead of driving an interconnect with a "unit" inverter, it might be more feasible to drive it with a chain of them, each with different fan in/out. Repeater insertion is tricky and (as far as I know) can't readily be automated.Interconnects are getting to tbe point where traversal of a die diagonally can take multiple clock cycles. Some folks are suggesting that a pipelined approach could be extended to interconnects, esp. clock trees. But the most fun problem (for me at least :P) is the handling of inductance extraction - how in the h*ll do you model it accurately? High-speed digital design == Analog design. Long live analog / mixed-signal VLSI designers :P
fitten - Thursday, February 10, 2005 - link
[quote]Well-written multicore-aware code should have the number of cores as a _variable_, so you just set it to 1 on a uniprocessor platform.[/quote]Sometimes parallel algorithms aren't very good for serial execution. In these cases, you may actually have one algorithm for multiple processors and another algorithm for a single processor.
[quote]So, if Intel were to use less repeaters the heat output could be lowered significantly. [/quote]
Well... I'm sure the Intel engineers didn't just up-and-say one day, "Hey, I know something cool to do... let's put some more repeaters into the core." I'm sure there's a reason for them being in there. It would probably take a bit of redesign to get the repeaters out. (I'm pretty sure this is what you meant, but I just wanted to clarify that stuff like repeaters aren't just put into a CPU for no reason. Things like repeaters are put in because there wasn't a more viable solution to some signalling problem that's there.)
sphinx - Thursday, February 10, 2005 - link
So, the reason for the Prescott's shortcomings is the use of too many repeaters as shown in the image of the Itanium 2. If I remember correctly, the article said that the repeaters were using too much power as well. So, if Intel were to use less repeaters the heat output could be lowered significantly.AtaStrumf - Thursday, February 10, 2005 - link
Nice article and pretty easy to understand as well. I'm happy to hear that there may still be hope for controlling the power leakage, because without it I just can't see anybody getting beyond 65 nm, since even 65 nm will, without improvements, leak almost 3 times as much power as 90nm does now.Anxiously waiting for E0 A64 to see what AMD has managed to cook up.
mickyb - Wednesday, February 9, 2005 - link
There are plenty of multi-threaded apps out there. I am not sure pure single threaded apps exist any more outside of "Hello World" and some old Cobol/FORTRAN ports that are on floppy.Quake and UT have been multi-threaded for a while. Quake was multi-threaded when I had a dual Pentium pro. There were even benchmarks. The benefits seen with hyper-threading also show that many apps are multi-threaded. The performance gain was negligible due to the graphics drivers and OpenGL/DirectX not being thread optimized. I am sure that has been worked out by now.
Multi-threading is not all about making use of multiple CPUs. There are many conditions where a program would be stopped dead in its tracks waiting for a response from some outside program or hardware device. You can solve this with events, multi-process, multi-threading, call-backs, etc. Goal wise, they are related. In the Winders world, threading is the method of choice.
I really can't believe there are still arguments going on about programs not being multi-threaded. This is not that much of an issue any more. Even if your apps is not threaded, the OS is and it can run on one CPU while your app runs on the other. Or if you have 2 apps, then they can run on different CPUs.
With all that said, I agree with the thought that creating performance for all applications is better served using a faster single core CPU than dual CPUs. I think this way because when you have a unit of work to be done (even with multiple threads), it is more likely to be done quicker with a single CPU that is capable of the same computing power as 2 CPUs. I single unit of work will ultimately be smaller than a thread in all cases. The smallest is the instruction set.
Now...with that said, if the limiting factor is technology and they cannot obtain the equivalent performance of a dual core with a single core, then it makes since to go dual core to obtain it, especially with the power leakage. I like the thinking behind dual core on a laptop, but am skeptical about the part that says turning the CPU off and on rapidly to keep it cool and efficient. It will probably work if it isn't turned on and off too quickly, but heat spreads pretty quickly. You wouldn't even get past POST without a heat-sink and that silicon insulator keeps everything pretty cozy.
NegativeEntropy - Wednesday, February 9, 2005 - link
Johan, another excellent article, I'm looking forward to part 2.Evan Lieb - Wednesday, February 9, 2005 - link
It's pretty much impossible to get a "newbie" explanation of CPU architectures without a least a basic understanding of how CPUs work. Rand's suggestions were quite good, you should start there if you're overwhelmed by Johan's explanations IceWindius. It also wouldn't hurt to start with Anand's CPU articles from last year.Rand - Wednesday, February 9, 2005 - link
"I wish someone like Arstechinca would make something really built ground up like CPU's for morons so I could start understanding this stuff better."You may want to read parts 1-5 of "The Secrets of High Performance CPUs"
http://www.aceshardware.com/list.jsp?id=4
A bit outdayed now, as it was written in 99' if I recall correctly but it's still broadly relevant and a nice series of articles if your looking to get a better understanding of microprocessors without being drowned in the technical side of things.
ArsTechnica also has some good articles with a newbie friendly slant.
There are some excellent articles at RealWorldTech as well, but their definitely written for engineers rather then the average person.
Unfortunately most of the more noteable books like those by Hennessy & Patterson assume you've already some knowledge of computer architectures.
stephenbrooks - Wednesday, February 9, 2005 - link
#46, Well-written multicore-aware code should have the number of cores as a _variable_, so you just set it to 1 on a uniprocessor platform. I also think there already exists a multithreaded version of one of the big engines (Quake, UT?) that apparently does not lose any performance on a single core either.But I agree with the main thrust of your post, which is "Buy AMD".
Noli - Wednesday, February 9, 2005 - link
Not to belittle dual core development and I know there are a lot of people who run technical programs that will benefit from dual core on this site, but when I spend a small fortune on a pc, the primary driver is being able to play the most advanced games in the world. Unfortunately, I don't feel multi-threaded game code is going to get written for a longggggg time (what's the point of reducing potential customers?). How long till a very large percentage of users have dual cores? End of 2006 at the very earliest? So it's really a just a theoretical interest till then for me...