[Xenomai-core] Re: 2.4 vs 2.6 in embedded space
Wolfgang Grandegger wrote: On 10/13/2005 11:11 AM Philippe Gerum wrote: Wolfgang Grandegger wrote: On 10/12/2005 04:39 PM Philippe Gerum wrote: Wolfgang Grandegger wrote: We have linux-2.4.14-rc3 running on all AMCC eval boards (see http://www.denx.de). But the kernel supported by RTAI/Fusion, linuxppc-2.6.10rc3, does not boot on Ebony. The main problem is the missing support for U-Boot but there might be others. And it's simply not worth the effort to port it, I think. Open question: to your opinion, is 2.6 on low-end embedded hw doomed "by design" and why, or do you think that part of the reluctance to move to 2.6 is mostly explained because 2.4 is just fine and up to the task, IOW it's kind of a "don't fix if it ain't broken" perception? As Wolfgang (Denk) already pointed out, 2.6 is less attractive on low end systems, because it's bigger, slower, ... This is also true for Xenomai (RTAI/fusion). It's difficult to beat the latency value of the old RTAI/RTHAL under 2.4. You need more CPU power and resources, that's how thing are going. Nevertheless, compared to the realtime preemption patch, Xenomai is _lightweight_ :-). I think so too; that's the problem with strictly native real-time support in the kernel: you must end up with some kind of SMPish structure which virtually exhibits an infinite number of processors (one per task basically), so it's not going to help reducing the cpu footprints and the various noisy artefacts implied by the generalized mutex approach (which is otherwise sound, that's no the issue). This is also why there is still space for real-time extensions, provided - I think - they run as symbiotically as possible with Linux, so that we don't end up telling people to ignore they have Linux while running their apps over it. I agree and I'm really interested to get the benchmark comparison tests http://www.opersys.com/lrtbf/index.html running on a low-end PowerPC system. Actually, those results are pretty bad compared to what we have now x86-wise: a dual 750 Mhz exhibits a worst-case latency of 42 us, and a dual 2.4 Ghz is under the 20 us thereshold, which includes a complete tasking in user-space, which was not accounted for in these tests. It's one reason more to have this benchmarking infrastructure, so that the numbers keep being updated regularly, whichever way they are progressing/regressing. As far as Xeno is concerned, we should be able to continue to reduce those footprints. From my window, I see two aspects we need to work on: - impact of the Adeos pipelining on cache especially for hw with sluggish memory bandwidth - a better placement of the hot data that are accessed inside the fast interrupt path (mainly those of the scheduler). That would be nice, indeed. I also understood, that iPIPE is already lighter than ADEOS. It is, yes. It has been alleviated from all the cruft needed to have it as a module on option, which was a genuinely BAD IDEA (tm, (C) 2002 rpm, ludicrous patent pending ). The arch that would benefit the most of the implied simplifications is x86 since this also solves a design issue there, but at least we now have a saner ground to build over and optimize it for other archs. Looking at the ppc figures since early 2005 or so, the raw latency has continuously been reduced, i.e. we went from ~120 us on a Freescale's Icecube running the user-space test, to 53 us as measured recently with 0.9.1+r8c4. I did not manage to check again on the Sandpoint (connection problem to the Vlab) which is very representative of the low-end hw issues we could face [and basically made me cry when I first looked at the latency reports], but I suspect that thing might have progressed there too. I've recently ported 0.9.1 over a Mvista kernel (experimental PREEMPT_RT-like stuff + other patches) on a mpc8541, and the figures for user-space are ~22 us worst-cast lat. Of course, this is not what one would call a sluggish low-end hw and I agree that a more structured design like Xeno can't beat a flat ISR-based design, but still, in any case, I'm optimistic enough to think that we likely have a margin of improvement there. When the iPIPE-Patch for PowerPC is available for a recent 2.6 kernel version, I could run benchmark tests on various PowerPC systems, e.g. on 4xx processors from AMCC, including a rather low-end 405 at 200 MHz. It mostly runs already, I just need to figure out why Xenomai's klatency test breaks my IceCube instead of quietly running like the latency one does... Furthermore I think, that part of the reluctance is also due to "development in progress" including features like the realtime preemption patch, especially on embedded PowerPC systems. People are waiting that things get available and stable. Well, we might all have the same problem here... Wolfgang. -- Philippe.
[Xenomai-core] Re: 2.4 vs 2.6 in embedded space
On 10/13/2005 11:11 AM Philippe Gerum wrote: > Wolfgang Grandegger wrote: >> On 10/12/2005 04:39 PM Philippe Gerum wrote: >> >>>Wolfgang Grandegger wrote: >>> We have linux-2.4.14-rc3 running on all AMCC eval boards (see http://www.denx.de). But the kernel supported by RTAI/Fusion, linuxppc-2.6.10rc3, does not boot on Ebony. The main problem is the missing support for U-Boot but there might be others. And it's simply not worth the effort to port it, I think. >>> >>>Open question: to your opinion, is 2.6 on low-end embedded hw doomed "by >>>design" >>>and why, or do you think that part of the reluctance to move to 2.6 is >>>mostly >>>explained because 2.4 is just fine and up to the task, IOW it's kind of a >>>"don't >>>fix if it ain't broken" perception? >> >> >> As Wolfgang (Denk) already pointed out, 2.6 is less attractive on low >> end systems, because it's bigger, slower, ... This is also true for >> Xenomai (RTAI/fusion). It's difficult to beat the latency value of the >> old RTAI/RTHAL under 2.4. You need more CPU power and resources, that's >> how thing are going. Nevertheless, compared to the realtime preemption >> patch, Xenomai is _lightweight_ :-). > > I think so too; that's the problem with strictly native real-time support in > the > kernel: you must end up with some kind of SMPish structure which virtually > exhibits an infinite number of processors (one per task basically), so it's > not > going to help reducing the cpu footprints and the various noisy artefacts > implied by the generalized mutex approach (which is otherwise sound, that's > no > the issue). This is also why there is still space for real-time extensions, > provided - I think - they run as symbiotically as possible with Linux, so > that > we don't end up telling people to ignore they have Linux while running their > apps over it. I agree and I'm really interested to get the benchmark comparison tests http://www.opersys.com/lrtbf/index.html running on a low-end PowerPC system. > > As far as Xeno is concerned, we should be able to continue to reduce those > footprints. From my window, I see two aspects we need to work on: > - impact of the Adeos pipelining on cache especially for hw with sluggish > memory bandwidth > - a better placement of the hot data that are accessed inside the fast > interrupt > path (mainly those of the scheduler). That would be nice, indeed. I also understood, that iPIPE is already lighter than ADEOS. > Looking at the ppc figures since early 2005 or so, the raw latency has > continuously been reduced, i.e. we went from ~120 us on a Freescale's Icecube > running the user-space test, to 53 us as measured recently with 0.9.1+r8c4. I > did not manage to check again on the Sandpoint (connection problem to the > Vlab) > which is very representative of the low-end hw issues we could face [and > basically made me cry when I first looked at the latency reports], but I > suspect > that thing might have progressed there too. I've recently ported 0.9.1 over a > Mvista kernel (experimental PREEMPT_RT-like stuff + other patches) on a > mpc8541, > and the figures for user-space are ~22 us worst-cast lat. > Of course, this is not what one would call a sluggish low-end hw and I agree > that a more structured design like Xeno can't beat a flat ISR-based design, > but > still, in any case, I'm optimistic enough to think that we likely have a > margin > of improvement there. When the iPIPE-Patch for PowerPC is available for a recent 2.6 kernel version, I could run benchmark tests on various PowerPC systems, e.g. on 4xx processors from AMCC, including a rather low-end 405 at 200 MHz. >> Furthermore I think, that part of the reluctance is also due to >> "development in progress" including features like the realtime >> preemption patch, especially on embedded PowerPC systems. People are >> waiting that things get available and stable. >> > > Well, we might all have the same problem here... Wolfgang.
[Xenomai-core] Re: 2.4 vs 2.6 in embedded space
Wolfgang Grandegger wrote: On 10/12/2005 04:39 PM Philippe Gerum wrote: Wolfgang Grandegger wrote: We have linux-2.4.14-rc3 running on all AMCC eval boards (see http://www.denx.de). But the kernel supported by RTAI/Fusion, linuxppc-2.6.10rc3, does not boot on Ebony. The main problem is the missing support for U-Boot but there might be others. And it's simply not worth the effort to port it, I think. Open question: to your opinion, is 2.6 on low-end embedded hw doomed "by design" and why, or do you think that part of the reluctance to move to 2.6 is mostly explained because 2.4 is just fine and up to the task, IOW it's kind of a "don't fix if it ain't broken" perception? As Wolfgang (Denk) already pointed out, 2.6 is less attractive on low end systems, because it's bigger, slower, ... This is also true for Xenomai (RTAI/fusion). It's difficult to beat the latency value of the old RTAI/RTHAL under 2.4. You need more CPU power and resources, that's how thing are going. Nevertheless, compared to the realtime preemption patch, Xenomai is _lightweight_ :-). I think so too; that's the problem with strictly native real-time support in the kernel: you must end up with some kind of SMPish structure which virtually exhibits an infinite number of processors (one per task basically), so it's not going to help reducing the cpu footprints and the various noisy artefacts implied by the generalized mutex approach (which is otherwise sound, that's no the issue). This is also why there is still space for real-time extensions, provided - I think - they run as symbiotically as possible with Linux, so that we don't end up telling people to ignore they have Linux while running their apps over it. As far as Xeno is concerned, we should be able to continue to reduce those footprints. From my window, I see two aspects we need to work on: - impact of the Adeos pipelining on cache especially for hw with sluggish memory bandwidth - a better placement of the hot data that are accessed inside the fast interrupt path (mainly those of the scheduler). Looking at the ppc figures since early 2005 or so, the raw latency has continuously been reduced, i.e. we went from ~120 us on a Freescale's Icecube running the user-space test, to 53 us as measured recently with 0.9.1+r8c4. I did not manage to check again on the Sandpoint (connection problem to the Vlab) which is very representative of the low-end hw issues we could face [and basically made me cry when I first looked at the latency reports], but I suspect that thing might have progressed there too. I've recently ported 0.9.1 over a Mvista kernel (experimental PREEMPT_RT-like stuff + other patches) on a mpc8541, and the figures for user-space are ~22 us worst-cast lat. Of course, this is not what one would call a sluggish low-end hw and I agree that a more structured design like Xeno can't beat a flat ISR-based design, but still, in any case, I'm optimistic enough to think that we likely have a margin of improvement there. Furthermore I think, that part of the reluctance is also due to "development in progress" including features like the realtime preemption patch, especially on embedded PowerPC systems. People are waiting that things get available and stable. Well, we might all have the same problem here... -- Philippe.
[Xenomai-core] Re: 2.4 vs 2.6 in embedded space
On 10/12/2005 04:39 PM Philippe Gerum wrote: > Wolfgang Grandegger wrote: >> We have linux-2.4.14-rc3 running on all AMCC eval boards (see >> http://www.denx.de). But the kernel supported by RTAI/Fusion, >> linuxppc-2.6.10rc3, does not boot on Ebony. The main problem is the >> missing support for U-Boot but there might be others. And it's simply >> not worth the effort to port it, I think. > > Open question: to your opinion, is 2.6 on low-end embedded hw doomed "by > design" > and why, or do you think that part of the reluctance to move to 2.6 is mostly > explained because 2.4 is just fine and up to the task, IOW it's kind of a > "don't > fix if it ain't broken" perception? As Wolfgang (Denk) already pointed out, 2.6 is less attractive on low end systems, because it's bigger, slower, ... This is also true for Xenomai (RTAI/fusion). It's difficult to beat the latency value of the old RTAI/RTHAL under 2.4. You need more CPU power and resources, that's how thing are going. Nevertheless, compared to the realtime preemption patch, Xenomai is _lightweight_ :-). Furthermore I think, that part of the reluctance is also due to "development in progress" including features like the realtime preemption patch, especially on embedded PowerPC systems. People are waiting that things get available and stable. Wolfgang.
