Re: [PATCH 0/7] psi: pressure stall information for CPU, memory, and IO

[Bart Van Assche]

On 05/14/18 08:39, Christopher Lameter wrote:
> On Mon, 7 May 2018, Johannes Weiner wrote:
> > What to make of this number? If CPU utilization is at 100% and CPU
> > pressure is 0, it means the system is perfectly utilized, with one
> > runnable thread per CPU and nobody waiting. At two or more runnable
> > tasks per CPU, the system is 100% overcommitted and the pressure
> > average will indicate as much. From a utilization perspective this is
> > a great state of course: no CPU cycles are being wasted, even when 50%
> > of the threads were to go idle (and most workloads do vary). From the
> > perspective of the individual job it's not great, however, and they
> > might do better with more resources. Depending on what your priority
> > is, an elevated "some" number may or may not require action.
>
> This looks awfully similar to loadavg. Problem is that loadavg gets
> screwed up by tasks blocked waiting for I/O. Isnt there some way to fix
> loadavg instead?

The following article explains why it probably made sense in 1993 to 
include TASK_UNINTERRUPTIBLE in loadavg and also why this no longer 
makes sense today:

http://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html

Bart.