Re: [PATCH 1/4] timer: Added usleep[_range] timer

[Arjan van de Ven]

On 7/28/2010 2:22 PM, Andrew Morton wrote:
> On Wed, 28 Jul 2010 14:04:47 -0700
> Arjan van de Ven<arjan@xxxxxxxxxxxxxxx>  wrote:
>
>    
> > On 7/28/2010 1:58 PM, Andrew Morton wrote:
> >      
> > > My main concern is that someone will type usleep(50) and won't realise
> > > that it goes and sleeps for 100 usecs and their code gets slow as a
> > > result.  This sort of thing takes *years* to discover and fix.  If we'd
> > > forced them to type usleep_range() instead, it would never have happened.
> > >
> > >
> > >
> > > Another question: what is the typical overhead of a usleep()?  IOW, at
> > > what delay value does it make more sense to use udelay()?  Another way
> > > of asking that would be "how long does a usleep(1) take"?  If it
> > > reliably consumes 2us CPU time then we shouldn't do it.
> > >
> > > But it's not just CPU time, is it?  A smart udelay() should put the CPU
> > > into a lower power state, so a udelay(3) might consume less energy than
> > > a usleep(2), because the usleep() does much more work in schedule() and
> > > friends?
> > >
> > >        
> > for very low values of udelay() you're likely right.... but we could and
> > should catch that inside usleep imo and fall back to a udelay
> > it'll likely be 10 usec or so where we'd cut off.
> >
> > now there is no such thing as a "low power udelay", not really anyway....
> >      
> Yup.  I can't find any arch which tries to do anything fancy.
>
> x86's rep_nop() tries to save a bit of juice, doesn't it?  Should we be
> using that?
>    

it doesn't save juice so much as it is a "yield to my hyperthreading 
brother"
(there is some power saved as well potentially...)

afaik we already use this in udelay()    (cpu_relax is rep_nop after all)

> Because we use udelay() in many places - it wouldn't surprise me if
> some people's machines were consuming significant amounts of
> time/energy in there, if they have suitably broken hardware or drivers.
>    

the only real place I've seen it used (based on profiles) is in libata 
in the intel piix sata driver
(the non-AHCI one)... and those are completly wrong, Alan Cox had 
patches to fix it but those somehow went nowhere


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