In a CPU with many cores, when some cores stall by waiting for memory loads, other cores can proceed by using data from their caches and this is even more likel

In a CPU with many cores, when some cores stall by waiting for memory loads, other cores can proceed by using data from their caches and this is even more likely to happen than for the SMT threads that share the same cache memory. When there are enough cores, they will keep the common memory interface busy all the time, so adding SMT is unlikely to increase the performance in a memory-throughput limited application when there already are enough cores. Keeping busy all the ALUs in a compute-limited application can usually be done well enough by out-of-order execution, because the modern CPUs have very big execution windows from which to choose instructions to be executed. So when there already are many cores, in many cases SMT may provide negligible advantages. On server computers there are much more opportunities for SMT to improve their efficiency, but on non-server computers I have encountered only one widespread application for which SMT is clearly beneficial, which is the compilation of big software projects (i.e. with thousands of source files). The big cores of Intel are optimized for single-thread performance. This optimization criterion results in bad multi-threaded performance. The reason is that the MT performance is limited by the maximum permissible chip area and by the maximum possible power consumption. A big core has very poor performance per area and performance per power ratios. Adding SMT to such a big core improves the multi-threaded performance, but it is not the best way of improving it, because in the same area and power consumption used by a big core one can implement 3 to 5 efficient cores, so such a replacement of a big core with multiple efficient cores will increase the multi-threaded performance much more than adding SMT. So unlike for the case of a CPU that uses only big cores, in hybrid CPUs, SMT does not make sense, because a better MT performance is obtained by keeping only a few big cores, to provide high single-thread performance, and by replacing the other big cores with smaller, more efficient cores.