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Each of the Multiple Cores keeps its own State
1 core, 1 state
(Count the Execution Units)
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2 cores, 2 states
4 cores, 4 states
• Registers
• Program Counter
• Stack Pointer
State Core
Computer Graphics
mjb – March 15, 2020
Computer Graphics
Hyperthreading and “Almost Amdahl”
hyperthreading.and.almost.amdahl.pptx
mjb – March 15, 2020
So, if that’s what Multicore is about, what is Hyperthreading?
1 core, 1 state 1 core, 2 states, with Hyperthreading
(Count the Execution Units)
2 cores, 4 states, with Hyperthreading
2 cores, 2 states
4 cores, 4 states
Computer Graphics
mjb – March 15, 2020
State Core
What is Hyperthreading and what can it Do?
State Core
Hyperthreading is when a CPU chip has more states than cores.
In this case, if one thread of execution blocks (waiting for a memory fetch, for instance), then the other thread can resume execution with its state.
If we let H be the fraction of a CPU’s capacity that one hyperthread can keep busy, then the remaining unused capacity is (1-H). If another hyperthread can keep H% of that capacity busy, then that leaves (1-H)*(1-H) remaining unused capacity and so on.
If we have n hyperthreads, then the final remaining unused capacity is (1-H)n. The capacity actually in use would then be 1-(1-H)n. If one thread can only keep the CPU H% busy, then the speed-up is potentially:
SU = ()
Computer Graphics
mjb – March 15, 2020
What is Hyperthreading and what can it Do?
If we have n hyperthreads, then the final remaining unused capacity is (1-H)n. The capacity actually in use would then be 1-(1-H)n. If one thread can only keep the CPU H% busy, then the speed-up is potentially:
SU = ()
# of hyperthreads on one core
Computer Graphics
mjb – March 15, 2020
H (the fraction of the CPU that one hyperthread can keep busy)
A Lidar Application:
Four Cores with Two Hyperthreads per Core
Source: Erzhuo Che
Note that this is upside-down from our usual convention. Sorry. I got this from someone else.
Computer Graphics
mjb – March 15, 2020
Expected Speedup
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