Re: Simple question about CPUs and processes
From: Robert Watson (rwatson_at_FreeBSD.org)
Date: 01/31/05
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Date: Mon, 31 Jan 2005 15:00:24 +0000 (GMT) To: security@revolutionsp.com
On Mon, 31 Jan 2005 security@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
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