The Quest for More Processing Power, Part One: "Is the single core CPU doomed?"

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.