Borttaget

Many factors contribute to scalability. Good scalability means that twice the CPU cores would get the job done in about half the time. That can be true for 2/4/6 core machines for well written software, but going beyond 6 CPU cores, many factors conspire to make further improvements incremental.

• As more CPU cores are used, Intel Turbo Boost drops the clock speed down to the base clock speed—each CPU core slows down as more are used. See the chart of turbo boost clock speeds for Intel Xeon E series (as this was written, data was not available for the Intel Xeon W processors).

• Contention for on-chip cache memory and contention for main memory—CPU cores are forced to wait on molasses-slow main memory (far slower than CPU registers and on-chip memory caches).

• Contention for shared application data structures (“thread safety”)—CPU cores are forced to queue up to access a necessary resource. The must “take turns” and thus go idle while waiting.

• Inappropriate algorithm selection for many CPU cores and/or poor multi-threading implementation. This is a software engineering problem. Some workloads can only be done in a sequential series of steps; this is called serialization and it precludes using many CPU cores (or even two). The workload can be a series of single-threaded and multi-threaded tasks. How much more CPUs helps depends heavily on whether the task can be split up for each CPU core to handle simultaneously. This is Hard To Do in software development, even if the task is amenable to it, and attempts to parallelize often lead to difficult to track down sporadic bugs. Few software engineers have the skills to write correct multi-threaded code.