From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <439071EF.3040004@domain.hid> Date: Fri, 02 Dec 2005 17:10:23 +0100 From: Wolfgang Grandegger MIME-Version: 1.0 Subject: Re: [Xenomai-help] i386 2.4 backport performing (too?) well References: <4390470F.8040503@domain.hid> In-Reply-To: <4390470F.8040503@domain.hid> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: xenomai@xenomai.org Jan Kiszka wrote: > Klaas Gadeyne wrote: > >>With the arrival of the 2.4 i386 adeos ipipe patch for xenomai [1], I >>decided to try to compile xenomai-trunk for a 2.4 kernel. This worked >>flawlessly, and moreover, I got excellent latency results: >> >>I used the "same" kernel config as for our 2.4.31 rtai3.0r5 kernel, >>which is based on Takis Issaris' liveCD config. >> >>This resulted in a maximal latency of 30 usec after a run of over 100 >>minutes under heavy load (tar and dd loops, compiling, keyboard >>interrupts and ping flood) [2]. >> >>For comparison, on the same hardware platform: - the RTAI lxrt-latency >>on rtai 3.0r5 (adeos oldgen r18c1 >> patch for 2.4.31 also) test reports 38 usec >>- the latency test of xenomai 2.01 running on a 2.6.14-ipipe-1.0-10 >> kernel resulted in a latency of 80 usec. >> >>This seems too good to be true? Can one simply compare the results of >>the former RTAI lxrt-latency test with the xenomai latency test? > > > Mmh, I tend to be sceptical as well, also remembering the results > Wolfgang posted about 2.6 vs. 2.4 on low-end PPC > (https://mail.gna.org/public/xenomai-core/2005-11/msg00131.html). > > Well, such worst-case improvement may be real if and only if there is > less *kernel* code in hard-irq-off sections with 2.4. The complexity of > adeos/ipipe and xenomai isn't changed between both scenarios. > > But I rather think that 2.4 just stresses the caches less than 2.6, thus > is may be more tricky to trigger the real worst-case path. So, what we > already saw with PPC: 2.4 and 2.6 may likely show similar RT > performances under Xenomai, but the overall system performance is much > better on low-end! However, I would be happy if this theory is too > pessimistic. In the meantime I have measured latencies a few PowerPC systems with Xenomai 2.1 under Linux 2.4 and 2.6 and RTAI 3.0r5 (using RTHAL) under 2.4. The latency figures for Xenomai compared with RTAI 3.0r5 are roughly a factor of two worse but I have not realized a significant difference between 2.4 and 2.6 (with Xenmomai, of course). Wolfgang.