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SubjectRe: hackbench: 2.6.12-rc6 vs. 2.6.12-rc6-RT-V0.7.48-06

* Esben Nielsen <simlo@phys.au.dk> wrote:

> Depends on the configuration options. If you used the same configs
> they should be the same but if you activated PREEMPT_RT it should slow
> things down due to the overhead of irq-threading and mutexes instead
> of spinlocks - although I am surprised the factor is _that_ big!

one has to make sure the default debug options are disabled.
(DEADLOCK_DETECT, PREEMPT_DEBUG, etc.) But even then there will be
considerable overhead. Until recently, the overhead used to be even
bigger. Note that hackbench is pretty much the worst-case for
PREEMPT_RT: lots of locking and scheduling done and we compare the UP
kernel to the SMP kernel in essence. (in fact PREEMPT_RT's locking is
currently higher overhead than SMP)

Ingo
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