[ale] Why APIC?

[David Muse]

On Thu, 9 Dec 2004 13:33:40 -0500
Greg Freemyer <greg.freemyer at gmail.com> wrote:

> What is the advantage of APIC that it is typically enabled by default?
> 
> I do a lot of disk i/o intensive work, and I had assumed that using
> the APIC was a good thing for performance, but now I think I'm wrong.
> 
> ===> Why I ask
> 
> I have a machine based on an Intel P4 MB.
> 
> I have used it regularly with SUSE 9.2 for at least a month.
> 
> Yesterday it started complaining in /var/log/warn "Kernel: APIC error
> on CPU0: 60(60)"
> 
> Also the machine became unreliable and locked up a couple of times
> (ie. unresponsive to the keyboard.)
> 
> Based on a quick google search, one thing to try was "noapic" in the
> grub entry.
> 
> I've done that, and much to my surprise the disk i/o is even faster
> than before.
> 
> i.e. I just did a md5sum --check and per iostat I was reading 70,000
> 512-byte blocks per second on 2 ide channels simultaneously.
> 
> I don't remember ever getting more than 50,000 or so before.
> 
> Even better, when one of the md5sum --check commands finished, the
> other accelerated to 100,000 blocks per second.
> 
> Thanks
> Greg
> -- 
> Greg Freemyer
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://www.ale.org/mailman/listinfo/ale
> 

I believe that an APIC is an interrupt router for multiple CPU's.

If a device says "hey, look at me!" then without an APIC, all CPU's
would respond and then have to contend with each other for access to the
device using spinlocks.  One CPU would ultimately handle the device and
the others would go back to what they were doing before.  I believe an
APIC handles interrupt contention in hardware so that only 1 CPU gets
the interrupt.

On single CPU systems, I'm not sure why there'd even be an APIC, or if
there is one, why you'd use it.  Maybe if your system supports multiple
CPU's but you only have one plugged in or have the kernel compiled
without SMP support, then you could either use the APIC or not.  Maybe
some systems that support multiple CPU's require you to use the APIC
even if only 1 CPU is plugged in or if your kernel is built without SMP
support, so kernels are generally distributed with APIC support turned
on so the kernel will work on those systems.

I'm not sure, but I suspect that if I/O performance is suffering with
APIC support turned on, then it's either because your system has no APIC
and the kernel has to deal with errors each time the IDE controller
raises it's interrupt or because your system has an APIC which adds a
little extra overhead each time an interrupt is processed, which would
be fine on a multi-cpu machine because it's still better than
spinlocking, but on a single-cpu machine, it just adds latency.

It may always be better to disable the APIC on a single CPU
machine unless you know your system requires it to be enabled.

Dave Muse