Author: Robert Hyatt
Date: 11:40:52 05/23/00
Go up one level in this thread
On May 23, 2000 at 13:12:05, Tom Kerrigan wrote: >On May 23, 2000 at 09:21:31, Robert Hyatt wrote: > >>On May 23, 2000 at 04:08:16, Tom Kerrigan wrote: >> >>>On May 23, 2000 at 03:08:52, Bruce Moreland wrote: >>> >>>>On May 23, 2000 at 02:39:30, Tom Kerrigan wrote: >>>> >>>>>For some reason, people seem to believe that unless you're using 100% of your >>>>>processor all the time, you could make do with a slower processor. >>>>> >>>>>Okay, maybe your processor spends the VAST majority of its time waiting on you. >>>>>But that's not what matters. What matters is how much time you spend waiting on >>>>>the processor. >>>>> >>>>>I have an 800MHz Pentium III at work. It is very noticably faster than my 400MHz >>>>>Celeron at home. Even if I'm just browsing the web, I definitely prefer using my >>>>>work computer because I have to wait on it half as long. >>>> >>>>I'm all for faster computers, I just don't see what the benefit is of having two >>>>processors in a computer that's used by the typical home or business user. I >>>>may have to add "yet" to the previous sentence, but maybe not. >>>> >>>>People need more processors if: >>>> >>>>1) They do two CPU-intensive things at once. The typical user doesn't do this >>>>very much. >>>> >>>> or >>>> >>>>2) The application they are using can divide work. The typical application does >>>>not divide work. There are lots of reasons for this. We have a chicken and egg >>>>problem due to most computers being single processor. Another reason is that >>>>many tasks are inherently sequential. Another reason is that many tasks take >>>>very little time to execute, so multiprocessor overhead doesn't make a lot of >>>>sense. Another reason is that many tasks that take a lot of time take a lot of >>>>time due to bottlenecks other than the processor. And finally, if you have a >>>>task that is significant enough that you could notice time saved due to an >>>>additional processor, and it is able to be broken down into parts that can be >>>>done concurrently, multiprocessor code is often difficult to design, build, >>>>debug, document, and maintain. Adding multiprocessor features to most programs >>>>is a poor design decision. >>>> >>>>I am not a Luddite. If you have your normal single-threaded compute-bound app, >>>>and you double processor speed, the improvement is immediate, and I approve of >>>>this kind of improvement, most definitely. But take one of the many commercial >>>>chess apps and put it on a quad processor machine and see what happens -- >>>>nothing. This is also true of essentially *any* current app you can buy that >>>>does anything. >>>> >>>>Chess is actually a great case for multiprocessor computers, so for us, they are >>>>cool. >>>> >>>>But it will be a while until the typical user sees much advantage to adding >>>>another processor, I think. >>>> >>>>I see that multiple processor machines are becoming more common, but I can't >>>>understand why, and I can't understand why this trend would continue. >>>> >>>>bruce >>> >>>Sorry, I didn't read your post carefully and didn't realize you were talking >>>specifically about multiple processors. >>> >>>"Multiprocessing" will hopefully become widespread soon thanks to >>>multi-threading processors. This is an idea that [mainly] Compaq/Alpha has been >>>promoting. The idea is that if you have a processor with 2 execution units, you >>>can do one of two things: >>> >>>1) Use both units to execute a single thread; unfortunately, one of the units >>>will probably be idle a lot of the time. This is what most chips do now. >>> >>>2) Use one unit per thread, in the case that two threads are running. That way, >>>neither unit spins. >>> >>>The chip real-estate overhead for multithreading is evidently pretty small. >>>Hopefully it will be incorperated in mainstream chips soon. >>> >>>-Tom >> >> >>That is the main case we were discussing. I pointed out that within the next >>3-5 years, most microprocessors will probably follow this path, so that each >>single processor chip will have 2 (or even 4) cpus internally. Multi-threading >>CPUS are a bit different, but not much. Some designs don't really have multiple >>cpu cores, but they are able to contain multiple processor states, so that >>context-switching is essentially free. > >On-die multiprocessing and processor-level multithreading are two very different >things. > >-Tom Only conceptually...
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