Re: Help understanding slow down

Ingo Molnar
2004-05-25T09:51:37Z

* 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 ... we agree on the warning printk, right?

Ingo