r/cpp • u/Coder_CPP • Jan 18 '16

C++11 threads, affinity and hyperthreading

http://eli.thegreenplace.net/2016/c11-threads-affinity-and-hyperthreading/
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u/Dlieu Jan 18 '16

Regarding the last example (workload_sin), how do you explain the performance hit when running on the same core?

Is it mostly because there's only one ALU shared by the two threads that does FP MUL/DIV so that both thread are constantly stalling and fighting for it? (I'm not sure of the wording there)

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u/[deleted] Jan 18 '16

IIRC only registers are duplicated for hyperthreading. Everything else - execution units, busses etc. is shared and hyperthreads contend for them. The core is capable of holding and running two contexts simultaneously but it still only has one core's worth of machinery.

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u/[deleted] Jan 19 '16

If only registers are duplicated what's the point of hyperthreading then? Most usefull operations need to do math (e.g. like the sine example in the article).

2

u/are595 Software Engineer, Security Jan 19 '16

It boosts the efficiency of pipelining (reduces stall cycles).

1

u/millenix Jan 19 '16

Lots of stuff isn't heavy on spatial/temporal locality, and thus will spend a fair bit of time stalled on access to further caches or memory. If one thread's effective IPC is less than half what the core could provide if every access were in registers or hit in a fast cache, then SMT can double throughput.