Sun has high expectations for Niagara

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Code with threads which need to synchonize 'often'

will get minimal benefit from chip conmcurrency. What is the benefit to have multiple threads 'active' only to wait to serialize themselves?

The objective of the simultaneous concurrent threading is to have more than one thread producing useful work simultaneously. In the classical multithreaded execution, a thread executes in the processor until it has to stall for possibly 100s of clock cycles (due to a cache miss, due to mispredicted branch, or lack of available functional units, etc.). The stalls are the big enemy of pipelines (especially the 'deep' ones, errrhhhhmmm Pentium 4...) which mean that h/w stays idle without producing useful work for several clock cycles. When the stall is for > 100s of cycles (page fault) the OS switches to another thread. Since stalls and page faults are VERY common events, especially in servers, a lot of h/w remains idle.


The simultaneous multithreading simply allows the context of multiple threads to be 'known' to the processor simultaneously, so that even if one thread stalls, other(s) cam still make positive progress.

It is a good idea but there are some pitfalls: with k active thrads the memory throughput should be AT LEAST k times that of the processor which can have one thread. Another is that the threads better be independent, or you end up serializing execution 'too often'.

Let's see how well Sun can pull this off.....


--m

Posted by: michael_t   Posted on: 10/26/05 You are currently: a Guest | Members login | Terms of Use

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