Messages in this thread | | | Date | Tue, 25 May 2004 13:42:58 +0200 | From | Ingo Molnar <> | Subject | Re: Help understanding slow down |
| |
* 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |