From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Wed, 06 Feb 2013 18:33:31 +0100
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: Re: [Xenomai] ipipe/x86: do not restore during context switch
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: Xenomai <xenomai@xenomai.org>

On 2013-02-06 18:09, Gilles Chanteperdrix wrote:
> On 02/06/2013 06:03 PM, Jan Kiszka wrote:
> 
>> Gilles,
>>
>> do you remember if this core-3.4 change was a performance optimization
>> or a necessary fix? Also, I'm not yet understanding why we need all the
>> #ifdefs except for the first one which forces fpu.preload to 0.
> 
> 
> It is a performance optimization, without it, we systematically hit the
> maximum latency when the timer would tick during a context switch which
> restores the FPU. Note that if you change that, you will probably break
> -forge.

According to the Intel folks who introduced eagerfpu, xsave, or at least
xsaveopt (which I didn't implemented yet) is now faster than serializing
clts/stts. On the other hand, the worst case is a full SSE + AVX restore
while the target RT task is not depending on the FPU.

Jan

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