4 ms·

Hey, noob question guy here. Can anyone explain why the last graph shows a slight performance drop going from 48 to 96 threads?

• kovacs_x 3y ago
  96 processes on 24 cores, not 96 cores. apparently 2 process per core is more efficient than 4.
• frankreyes 3y ago
  It can be many things, my guess is likely memory bandwidth. There's so much MB/s the RAM can handle. Also, above 48 cores, those are hyper threading and for CPU bound tasks, hyper threading is known to be slower. Example: https://ieeexplore.ieee.org/document/7804711 https://ieeexplore.ieee.org/document/7804711 Edit: looks like there's only 12 cores per CPU so that's 24 physical cores. 48 HT cores. So the drop must be cache trashing?
  • hinkley 3y ago
    Higher parallelism is usually about timeliness for interactive workloads, not throughput. Unless it’s just the classic fallacy that parallelism = speed. For throughput tasks it’s often the case that you go with less parallelism to reduce Amdahl’s law a few percent, and instead investing in keeping the pipeline saturated, so that the variance in concurrent tasks is lower. Work stealing being one of the more notable tricks.