Re: [Adeos-main] [RFC][PATCH] Reduce pipeline_head access overhead

[Philippe Gerum <rpm@xenomai.org>]

Jan Kiszka wrote:
> This patch is in fact part of my xnlock rework (updates on that one will
> follow the next days). I noticed that the complexity of
> ipipe_cpudom_var(head_domain, *), normally inlined, is fairly relevant
> for the text size of Xenomai and may thus impact the WCET as well.
> 
> So I applied the same pattern as already used for ipipe_root_cpudom_var
> also to the head domain, ie. assigned a fixed slot, the topmost, in
> ipipe_percpu_daddr/darray to the head domain. This gives a nice text
> size reduction as we no longer need to go via
> __ipipe_pipeline.next->slot (2 memory accesses, even 3 on x867/SMP +
> some arithmetics) to look up the address of per-cpu domain variables
> like "status".
> 
> The drawback is that this introduces an inconsistency to the
> ipipe_*_pipeline_head API: It no longer works for head == root. While
> this is most probably irrelevant in practice, it remains ugly. I already
> thought about renaming the modified services (and deprecating the
> original ones), but for now I would like to focus the discussion on the
> optimized services.
> 
> The patch already survived quite some testing on various real and
> virtual boxes with Xenomai. Latency reduction is hard to estimate as
> such comparably small optimizations are within the noise range of fast
> test boxes. Maybe someone could give this a try on a smaller device (UP
> should benefit from this patch as well!).
>

The root domain cannot be an invariant pipeline head, by design, so I don't see
any showstopper regarding this approach. This looks good.

> Jan
> 
> 
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-- 
Philippe.