Re: Help understanding slow down

Andrew Morton
2004-05-25T09:59:00Z

Ingo Molnar <mingo@elte.hu> wrote:
>
>
> * Andrew Morton <akpm@osdl.org> wrote:
>
> > Ingo Molnar <mingo@elte.hu> wrote:
> > >
> > > with the patch below we will print a big fat warning. (I did not want to
> > > deny idle=poll altogether - future HT implementations might work fine
> > > with polling idle.)
> >
> > idle=poll is handy when profiling the kernel with oprofile
> > clock-unhalted events. Because if you use the normal halt-based idle
> > loop no profile "ticks" are accounted to idle time at all and the
> > results are really hard to understand.
>
> it makes it a bit more plausible, but kernel profiling based on ticks in
> a HT environment is still quite unreliable, even with idle=poll. The HT
> cores will yield to each other on various occasions - like spinlock
> loops. This disproportionatly increases the hits of various looping
> functions, creating false impressions of lock contention where there's
> only little contention. Plus idle=poll is a constant ~20% performance
> drain on the non-idle HT core, further distorting the profile. HT makes
> profiling really hard, no matter what.

But often one is looking for relativities rather than real absolute
numbers. (In which case the absent idle time doesn't matter, but it freaks
me out...)

> but ... we agree on the warning printk, right?

yup.