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Date: Tue, 18 May 2010 22:23:24 +0200
From: Wolfgang Mauerer <wolfgang.mauerer@domain.hid>
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Subject: Re: [Xenomai-core] Question about getting system time
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: "Kiszka, Jan" <jan.kiszka@domain.hid>, Andreas Glatz <AndreasGlatz@domain.hid>, "xenomai@xenomai.org" <xenomai@xenomai.org>

Gilles Chanteperdrix wrote:
> Wolfgang Mauerer wrote:
>>> On the one hand you make complicated code (which will be costly on low
>>> end hardware) to avoid shutting interrupts around a few assignments, but
>>> on the other hand you leave an architecture specific function pointer
>>> call where we want a fast behaviour on average (remember, we do all this
>>> to avoid a system call, which is only a few hundreds nanoseconds on your
>>> big iron x86), and where we have a generic fast replacement. Sometimes,
>>> I do not understand your logic.
>> But using the same argument, you could get rid of Linux vsyscall based
>> gettimeofday()...
> 
> I do not see your point, the Linux code does not go a long way to make
> lockless code, it simply turns off interrupts around the gtod data
> update, which is really reasonable given the size of the masking
> section. The reading is lockless, the writing is not.
> 
I was referring to the argument that system calls are so fast that
replacing gtod with a syscall-less version that uses function
pointer dereferencing instead does not make much of a difference.

Be it as it may, I need to check how far our budget can cover
the (much more comprehensive) modifications required for the
solution suggested by you. Let's see.

Regards, Wolfgang