Thursday, May 10, 2012

Live hardware threads ^ kernel threads - Barrel processor

Exec summary -- in one cpu cycle a Barrel-processor machine (like T2000) can have
- 8 executing threads
- 32 Live hardware threads
- hundreds of kernel threads
- thousands of userland threads
-----------
I worked for a while on the Sun T2000 server supporting 4 cpu threads per core. http://en.wikipedia.org/wiki/Barrel_processor explains

- Such a cpu-core does not allow execution of multiple instructions in one cycle. So "Simultaneous" is misleading.
- Each thread running at roughly 1/4 the original CPU speed

A single-tasking processor spends a lot of time idle, not doing anything useful whenever a cache miss or pipeline stall occurs

In the T2000 (8-core, 32 hardware threads), if you have 100 processes, then you have at least 100 kernel threads (software threads). Optimally, 32 of them would be Running as "live hardware-threads" since each is assigned to a cpu core. In one cycle, 8 of them are executing, while the other 24 are in pipeline inside the cpu core.

http://en.wikipedia.org/wiki/UltraSPARC_T1 sheds more light.

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