Author: Robert Hyatt
Date: 10:37:19 10/05/01
Go up one level in this thread
On October 05, 2001 at 13:27:04, Miguel A. Ballicora wrote: >On October 05, 2001 at 00:53:07, Robert Hyatt wrote: > >>>>compare that to a machine with twice the resources that will run that >>>>algorithm well. That isn't what this algorithm analysis is about. It is _very_ >>>>safe to assume that the dual-cpu machine and the single-cpu-running-two-threads >>>>cases are _identical_ in their overhead costs from the machine perspective. >>>>Which means it can be ignored... >>> >>>I do not see why it is safe. Particularly, when you don't know how the >>>hardware is going to be in 10 years. 10 years ago branch misprediction >>>was not an issue, today is. Can you guarantee that ignoring that overhead >>>is safe in the future generation of computers? yes or no?. >>> >>>Miguel >> >>I can guarantee you that future machines will have _worse_ memory bottlenecks. >>And worse bus conflicts. That will always be more expensive than context >>switching. New cpus will have zero context switching overhead as they will >>begin to do threading inside the processor... > >Then, regarding this point it all comes to how well you predict the architecture >of the processors of the future. > >Miguel Not really. I claim, and this is based on actual tests, that the parallel processing overhead will get worse, while the context-switching overhead will become smaller. Based solely on what chip designers are doing now for future generation chips. But even if these two overheads remain static, it doesn't change the argument about the super-linear discussion at all. Everything must remain static except for the addition of a second processor, for this comparison to be reasonable. That is a common flaw in many reported parallel speedups. IE the Waycool guys at CalTech added another processor _and_ another block of memory. And they didn't separate the gain from the additional processor from the gain from the additional hash table size... Here we fix _everything_ except the number of cpus...
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