From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5112945F.8080102@xenomai.org> Date: Wed, 06 Feb 2013 18:35:27 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <51128CE4.4020303@siemens.com> <51128E3E.808@xenomai.org> <511293EB.1080502@siemens.com> In-Reply-To: <511293EB.1080502@siemens.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] ipipe/x86: do not restore during context switch List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai On 02/06/2013 06:33 PM, Jan Kiszka wrote: > 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. Without xsave, we never restore fpu if the RT task never used it. This changes with xsave? -- Gilles.