Re: ~5x greater CPU load for a networked application when using 2.6.15-rt15-smp vs. 2.6.12-1.1390_FC4

[Nish Aravamudan]

On 2/23/06, Ingo Molnar <mingo@xxxxxxx> wrote:
>
> * Gautam H Thaker <gthaker@xxxxxxxxxxxx> wrote:
>
> > ::::::::::::::
> > top:  2.6.15-rt15-smp.out   # REAL_TIME KERNEL
> > ::::::::::::::
>
> >  2906 root     -66   0 18624 2244 1480 S 41.4  0.1  27:11.21 nalive.p
> >     6 root     -91   0     0    0    0 S 32.3  0.0  21:04.53 softirq-net-rx/
> >  1379 root     -40  -5     0    0    0 S 14.5  0.0   9:54.76 IRQ 23
>
> One effect of the -rt kernel is that it shows IRQ load explicitly -
> while the stock kernel can 'hide' it because there interrupts run
> 'atomically', making it hard to measure the true system overhead. The
> -rt kernel will likely show more overhead, but i'd not expect this
> amount of overhead.
>
> To figure out the true overhead of both kernels, could you try the
> attached loop_print_thread.c code, and run it on: an idle non-rt kernel,
> and idle -rt kernel, a busy non-rt kernel and a busy -rt kernel, and
> send me the typical/average loops/sec value you are getting?
>
> Furthermore, there have been some tasklet related fixes in 2.6.15-rt17,
> which maybe could improve this workload. Maybe ...

Would it make more sense to compare 2.6.15 and 2.6.15-rt17, as opposed
to 2.6.12-1.1390_FC4 and 2.6.15-rt17? Seems like the closer the two
kernels are, the easier it will be to isolate the differences.

Thanks,
Nish
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