Re: about definition of iowait

[Cao jin]

On 06/07/2016 08:01 PM, Peter Zijlstra wrote:
> On Tue, Jun 07, 2016 at 07:24:46PM +0800, Cao jin wrote:
> > Hi,
> >
> > I noticed some problems about iowait entry of /proc/stat: it seems not
> > accurate, and sometimes will decrease in SMP.
> >
> > For UP, we have a simple definition about iowait, which is:
> > The time that the processor is idle, during which there is a task waiting
> > for I/O.
> >
> > This definition seems don't fit well when step into SMP era, so I think
> > maybe it can be extended like:
> > For a given CPU, the I/O wait time is the time during which that CPU was
> > idle, and there is at least one outstanding disk I/O operation requested by
> > a task scheduled on that CPU.
> >
> > But tasks can migrate among cpus, so this definition maybe not accurate.
> >
> > I also saw some suggestions that it should be a global value, not per cpu,
> > but don`t see clear definition, so anyone have suggestions about it?
> > (personally guessing, maybe: there is task waiting for outstanding I/O while
> > all cpus are idle? But I don't think it is good...)
>
> It is possible to come up with definition for a single global number;
> per-cpu iowait numbers are doomed.
>
> But I don't see it matters one way or the other. Personally I would just
> make the kernel return 0 and be done with it.

I see. Thanks peter

--
Yours Sincerely,

Cao jin