Simple question about CPUs and processes

[security at revolutionsp.com]

Hey,

Thanks for the replies Robert and Ryan! That was insigthful.

I didn't know about the BP and the shutdown thingy, always learning :-)


>> On Mon, 31 Jan 2005 security at revolutionsp.com 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
>>
>>
>
>