* Kristian Benoit <[email protected]> wrote:
[...]
> "plain" run:
>
> Measurements | Vanilla | preempt_rt | ipipe
> ---------------+-------------+----------------+-------------
> fork | 97us | 91us (-6%) | 101us (+4%)
> mmap | 776us | 629us (-19%) | 794us (+2%)
some of you have wondered how it's possible that the PREEMPT_RT kernel
is _faster_ than the vanilla kernel in these two metrics.
I've done some more profiling, and one reason is kmap_atomic(). As i
pointed out in an earlier mail, in your tests you not only had HIGHMEM64
enabled, but also HIGHPTE, which is a heavy kmap_atomic() user. [and
which is an option meant for systems with 8GB or more RAM, not the
typical embedded target.]
kmap_atomic() is a pretty preemption-unfriendly per-CPU construct, which
under PREEMPT_RT had to be changed and was mapped into kmap(). The
performance advantage comes from the caching built into kmap() and not
having to do per-page invlpg calls. (which can be pretty slow,
expecially on highmem64) The 'mapping kmap_atomic into kmap' technique
is perfectly fine under PREEMPT_RT because all kernel code is
preemptible, but it's not really possible in the vanilla kernel due to
the fundamental non-preemptability of interrupts, the preempt-off-ness
of the mmu_gather mechanism, the atomicity of the ->page_table_lock
spinlock, etc.
so this is a case of 'fully preemptible beats non-preemptible due to
flexibility', but it should be more of an exception than the rule,
because generally the fully preemptible kernel tries to be 1:1 identical
to the vanilla kernel. But it's an interesting phenomenon from a
conceptual angle nevertheless.
Ingo
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