If, for example, there is a let's say embedded application which run on unicore CPU. And then that application would be ported on multi core CPU. Would that app run on single or multiple cores? To be more specific I am interested in ARM CPU (but not only) and toolchain specifics e. g. standard C/C++ libraries. The intention of this question is this: is it CPU's responsibility to "decide" to execute on multiple cores or compiler toolchain, developer and standard platfor specific libraries? And again, I am interested also in other systems' tendencies out there. There are plenty of applications and RTOS (for example Linux) that run on different CPUs but the same architecture, so does that mean that they are compiled differently?
3 Answers
1
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Generally speaking single-threaded code will always run on one core. To take advantage of multiple cores you need to have either multiple processes, multiple threads, or both.
There's nothing your compiler can do to help you here. This is an architectural consideration.
If you have multiple threads, for example, most multi-core systems will run them on whatever cores are available if the operating system you're running is properly compiled to support that. Running an OS that's been compiled single-core only will obviously limit your options here.
1
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A single threaded program will run in one thread. It is theoretically possible for the thread to be scheduled to move to a different core, but the scheduler cannot turn a single thread into multiple threads and give you any parallel processing.
EDIT I misunderstood your question. If there are multiple threads in the application, and that application is binary compatible with the new multicore CPU, the threads will indeed be scheduled to run on different CPUs, if the OS scheduler deems it appropriate.
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Well it all depends on the software that if it wants to utilize other cores or not (if present). Lets take an example of Linux on ARM's cortexA53.
Initially a vendor provided boot loader runs on, FSBL (First state bootloader). It then passes control to Arm trusted firmware. ATF then runs uboot. All these run on a single core. Then uboot loads linux kernel and passes control to it. Linux then initializes some stuff and looks into some option, first in the bootargs for smp or nosmp flags. if smp it will get the number of CPUs assigned to it from dtb and then using SMC calls to ATF it will start other cores and then assign work to those cores to provide true feel of multiprocessing environment. This is normally called load balancing and in linux it is mostly done in fair.c file.
