[ros-dev] MP/UP HAL issues

[Timo Kreuzer]

Am 26.03.2011 11:45, schrieb Love Nystrom:
>
>     ---------- Forwarded message ----------
>     From: Timo Kreuzer <timo.kreuzer at web.de <mailto:timo.kreuzer at web.de>>\
>
> [abbreviated]
>
>
>     I'm against wasting precious compile time for an MP hal that
>     doesn't even work. And I would actually like to have the kernel
>     being compiled the same way. I bet the performance improvements of
>     inlining some spinlock code are really neglectable.
>
>
> I hate being a spoilsport, especially on an issue that may have gone 
> stale already,
> but *compile time* is not even 10% as important as *run time*.
> I dunno about the particulars in this case, it's just a general 
> priority opinion.
When I said neglectable runtime performance, I meant something that you 
wouldn't notice even if you were benchmarking the system.
We would need to examine this, since I can't say for sure if that's true.

>
> To me, performance is *everything* ..
> I gladly spend a *week* to gain significant performance,
> especially if it also makes the code clearer and more readable!
And what if it makes the code harder to read and maintain or if its 
likely to introduce bugs later? Performance is great, but you need to be 
reasonable. Saving a few clock cycles, saving you a microsecond per 
minute runtime is not worth downgrading code quality. Its not even worth 
massively increasing build time.

>
> Is it just I who think that software is getting slower and slower and 
> bigger and bigger these days?
> And I mean particularly the goo gaa that comes out of Redmond these 
> days :-/
If you look at Windows 7 improvements then you will notice that there 
are a lot of performance improvements. It generally runs faster then 
Windows XP. This is not achieved by saving a few clock cycles here and 
there, but by improving algorithms and adding additional features.

>     I'd even go as far as dropping UP support completely and
>     hotpatching spinlock functions.
>
>
> Dropping single core processor support sound like a bad idea to me.
Tell that MS, they did that with Windows Vista already.
Also I wasn't even talking about dropping the up kernel, I suggested 
compiling it in a different manner, that would not allow inlining 
spinlock functions as its done currently, but would cause them to be 
called. The function would be linked as UP or SMP depending on whether 
we are using UP or SMP kernel (can be done at link time or even at 
runtime). This way we save us the complete MP kernel build time (almost 
2 minutes on my machine).
IMO it would also make it easier to maintain the SMP kernel.

Regards,
Timo

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