Re: [PATCH RT 00/02] SLOB optimizations

Ingo Molnar
2005-12-22T21:12:41Z

* Eric Dumazet <dada1@cosmosbay.com> wrote:

> >in any case, on sane platforms (i386, x86_64) an irq-disable is
> >well-optimized in hardware, and is just as fast as a preempt_disable().
>
> I'm afraid its not the case on current hardware.
>
> The irq enable/disable pair count for more than 50% the cpu time spent
> in kmem_cache_alloc()/kmem_cache_free()/kfree()

because you are not using NMI based profiling?

> oprofile results on a dual Opteron 246 :
>
> You can see the high profile numbers right after cli and popf(sti)
> instructions, popf being VERY expensive.

that's just the profiling interrupt hitting them. You should not analyze
irq-safe code with a non-NMI profiling interrupt.

CLI/STI is extremely fast. (In fact in the -rt tree i'm using them
within mutexes instead of preempt_enable()/preempt_disable(), because
they are faster and generate less register side-effect.)

Ingo