From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <43907C35.90403@domain.hid> Date: Fri, 02 Dec 2005 18:54:13 +0200 From: Heikki Lindholm MIME-Version: 1.0 Subject: Re: [Xenomai-help] i386 2.4 backport performing (too?) well References: <4390470F.8040503@domain.hid> <439071EF.3040004@domain.hid> <439077A7.6000903@domain.hid> In-Reply-To: <439077A7.6000903@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: Philippe Gerum Cc: xenomai@xenomai.org Philippe Gerum kirjoitti: > Wolfgang Grandegger wrote: > >> 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). > > > We still need to compare pure kernel-space executions since RTAI does > not support user-space for PPC, but v2.1 does not provide the kernel > space test yet, due to the build system refactoring. Still pondering how > to introduce this back into the codebase. As a silly off-hand remark, it would be nice to get rid of the "making a module" for the kernel space tasks and be able to use the same .c for both kernel and user space. Maybe a module loader stub as init and some linker magic would do the trick. Don't know about main(), though.. -- Heikki Lindholm