Re: voluntary-preempt T3 latency spikes with fan speed change

Lee Revell
2004-10-11T22:08:11Z

On Mon, 2004-10-11 at 13:03, Andrew Rodland wrote:
> torbenh@gmx.de wrote:
>
> >
> > hi...
> >
> > i am seeing latency spikes (ie jack xruns) when the fan of my
> > asus l3d laptop changes speed.
> >
> > is there any chance to fix this ?
> > i have turned off acpi in the kernel, as this gives me latency spikes
> > all over.
> >
> > i am quite new to the VP patches, and want to help where i can.
> >
> > i also got a quite strange latency trace here:
> >
> > could someone sched some light on this please ?
> >
>
> I can't say for certain, but I'm guessing that your laptop has a deeply
> broken BIOS that implements ACPI and suchlike by using SMM, which blocks
> out interrupts, and there's nothing, I believe, you can do about it.
> Disabling ACPI seems sensible; at least you can avoid causing these delays
> intentionally, but if some sensor interrupt triggers a flip into SMM to
> enable the fan, you're just screwed for a number of milliseconds.
>

Many, many people are seeing this problem (weird, often periodic latency
spikes on laptops that go away when ACPI is disabled). This would
explain a lot of weird bug reports. So are most laptops just
incompatible with low latency applications, or are we talking about a
small minority of broken hardware?

Is there any way to tell a priori whether a machine will have this
problem?

Lee