https://community.intel.com/t5/Software-Tuning-Performance/The-effect-of-hyperthreads-on-IPC/m-p/1403367#M8089 <P>I’ve been experimenting with a 4-socket Skylake based server with 24 cores per socket. I’ve run a number of workloads, each utilizing all of the cores, with both hyperthreads turned on and off. The workloads ranged from running queries on a on a highly parallel database to artificially constructed arithmetic operations. What I noticed in all cases was that when hyperthreads were turned on, workload throughputs improved by anywhere from 5 to 15%. This is was to be expected and is well-known. What surprised me, was that in all cases, the improvement in throughput was accompanied by a decrease in IPC (instruction per cycle). In others words with hyperthreads turned, although throughput improved, the system-wide IPC decreased by about 20% when compared to hyperthreads turned off.</P> <P>Given that a hyper thread kicks in when a pipeline is stalled, I would have expected hyperthreading would increase the IPC, not decrease it. What am I missing?</P> <P>Thanks</P> Tue, 26 Jul 2022 05:45:45 GMT gostanian__richard 2022-07-26T05:45:45Z https://community.intel.com/t5/Software-Tuning-Performance/The-effect-of-hyperthreads-on-IPC/m-p/1403367#M8089 <P>I’ve been experimenting with a 4-socket Skylake based server with 24 cores per socket. I’ve run a number of workloads, each utilizing all of the cores, with both hyperthreads turned on and off. The workloads ranged from running queries on a on a highly parallel database to artificially constructed arithmetic operations. What I noticed in all cases was that when hyperthreads were turned on, workload throughputs improved by anywhere from 5 to 15%. This is was to be expected and is well-known. What surprised me, was that in all cases, the improvement in throughput was accompanied by a decrease in IPC (instruction per cycle). In others words with hyperthreads turned, although throughput improved, the system-wide IPC decreased by about 20% when compared to hyperthreads turned off.</P> <P>Given that a hyper thread kicks in when a pipeline is stalled, I would have expected hyperthreading would increase the IPC, not decrease it. What am I missing?</P> <P>Thanks</P> Tue, 26 Jul 2022 05:45:45 GMT https://community.intel.com/t5/Software-Tuning-Performance/The-effect-of-hyperthreads-on-IPC/m-p/1403367#M8089 gostanian__richard 2022-07-26T05:45:45Z