From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4F80C10E.1000804@domain.hid> Date: Sun, 08 Apr 2012 00:34:54 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] Disadvantages of preempt_rt List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anisha Kaul Cc: xenomai@xenomai.org On 04/07/2012 07:33 AM, Anisha Kaul wrote: > Greetings, > > From here: http://www.xenomai.org/index.php/Xenomai:Roadmap#Xenomai_3_FAQ > > Quote: >> the target hardware platform has limited horsepower, and/or you want the real-time job to put the smallest possible overhead on your system. This is where dual kernels are usually better than a native preemption system. > > > Preempt_rt does preempt the whole Linux. In what way does preempting > Linux put load on the system? I think you are misunderstanding what preempt_rt is, preempt_rt does not preempt linux, preempt_rt is simply linux modified, when a linux kernel patched with preempt_rt runs, nothing else than linux runs, so nothing preempts the whole linux. This is achieved by means which have a global overhead on the whole system. The exact means are described on the rt wiki FAQ: https://rt.wiki.kernel.org/articles/f/r/e/Frequently_Asked_Questions_7407.html Note the answer to FAQ 3, which explains that improving latencies is a trade off, and means degrading the average performances of the system. That is the overhead we are talking about. -- Gilles.