On Mon, 31 Jan 2005 [EMAIL PROTECTED] wrote: > I'd like some insight on the following; Me and a friend were discussing > tech stuff and he said that, when using dual (or more) CPU systems, it > is the hardware itself (and alone) choosing which CPU will execute this > or that process. > > But I think it is the OS kernel (FreeBSD in this case) and the SMP > implementation that decide this process should live in CPU0 or CPU1, ie > it's FreeBSD itself choosing on what CPU a process will reside in.
This is basically the case on hardware supported by FreeBSD: the OS scheduler decides where to place processes in order, ideally, to maximize performance. The hardware will affect the performance, however, due to some systems supporting non-uniform access to chunks of memory depending on the CPU the code runs on, the contents of the cache, etc. Often, the job of the OS scheduler isn't simply to decide on fair or desirable run orders based on priority, but where best to run the process/thread based on where it's run recently, clusters of related CPUs, etc. These issues are becoming more important on i386-based systems with the advent of hyper-threading, dual-core chips, amd64 NUMA, etc. In the end, though, the decision (good or bad) will be made by the OS. There is two exceptions regarding execution: - The hardware may be involved in deciding which CPU will receive interrupts -- for example, it may round-robin deliver timer interrupts to CPUs to attempt to help manage interrupt load. This will cause an interrupt handler to run on the CPU selected by the hardware, at which point the OS can decide whether it wants to process the interrupt on that CPU, or forward it to another for processing. - There are some activities that must be performed on specific processors. For example, the boot necessarily starts out on the boot processor (BP). It turns out that many Intel or Intel-like systems get quite unhappy if system shutdown is initiated from a non-boot processor, so FreeBSD also arranges for the boot processor to issue the power down call into ACPI on platforms running ACPI. The hardware isn't choosing where to run this code per se, but if you pick the wrong one the hardware won't like you :-). Hope this is helpful, Robert N M Watson _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
