>On Sat 08. Apr - 02:42:12, Brown, Len wrote: >> Timo, Holger, >> Andi pointed me to your FOSDEM Linux Power Management presentation: >> >> http://en.opensuse.org/FOSDEM2006 >> >> http://files.opensuse.org/opensuse/en/b/b5/One_step_opendesign.pdf >> >> And I'm glad to see you working on Linux Power Management. >> >> But I'm a little concerned that user-space and the kernel are >> a little out of sync on a few things. >> >> I'm happy to see that the userspace p-state governor >> is no longer enabled by default on SuSE systems. >> While it was passable on servers with steady-state >> workloads, it was very bad for laptops where the >> machine spends a lot of time idle, but has short >> bursts of processing need which userspace could >> not detect. These laptops would spend virtually >> all their time in Pn when using the userspace governor. > >To be honest, this observation suprises me a little bit. We did some >measurements with userspace agains ondemand governor some time >ago and did not notice any big differences in the results between them. >Well, these tests are about 1 1/2 years ago, though, and there went some >changes into the kernel until now ;-) Yes, measurements show that ondemand as improved considerably since its initial implementation. It continues to improve today, though there is now smaller room for improvement. Also, the other important thing to meausre here is *response time* -- not throughput. This will expose the benefits of switching quickly via ondemand vs. slowly via userspace. This is particularly important on interarctive workloads. No, you'll not notice much, if any, difference for course grain things like doing a kernel build or running a steady-state server workload. >Nevertheless, we adjust the sampling rate in any case and >currently set it to 333 milliseconds (that's configurable). >We noticed if we use the >default ondemand setting, the ondemand governor increases the frequency >too often although there is not much to do which is also not >helpful. I have not observed the ondemand governor today switching up more often than is helpful. I speak for intel hardware, of course. It might be that other hardware, which can not switch up and down very quickly, not not benefit from ondemand and may be better suited to userspace. >But 333 milliseconds is maybe a bit too high, it's taken because >of historical reasons. >This value _was_ the default interval of our main event loop. >I think I will lower it a bit. Go ahead and tune userspace to work optimally on systems that can't run ondemand. Systems that are able to run ondemand should not be running userspace at all. >Furthermore, we had some problems on multiprocessor systems in the past >(about 1/2 year ago) with the ondemand governor. After some time the >system was running (even some hours or even days) the machine locked up >hard. Thus, we set the userspace governor by default on those systems >where we never experienced such problems. At the moment I did >only get one similar report where the root cause is not clear. It is important that this failure be root caused and this doubt be put behind us. Got a bug URL? >So I stick to the >ondemand governor in any case in newer releases. And such lockups are >really hard to reproduce and to debug. > >Another argument was that speedstep_ich was not yet ready for ondemand >which it is now IIRC. speedstep-centrino and acpi-cpufreq support real p-states and can can support ondemand. (indeed, these two drivers need to be merged into a single driver) While older systems will use speedstep-ich, I don't expect to see much use for it on modern systems. p4clockmod is just t-states, and one could argue that it should not exist at all. I don't know if the amd-specific drivers would work or not. Last I heard their latency was too high, but maybe they've fixed that. There is a cpufreq architecture issue here here, of course. the drivers make all the different states look the same to the governors. But P-states and T-states are not the same, they are very different. >> The next step is to delete the userspace governor >> as a valid governor selection entirely. If somebody >> really wants manual control, they can still set the >> limits within which "ondemand" will stay. > >In current code, I always try to use the ondemand governor at >first and if that fails we automatically switch to the userspace >implementation at runtime. > >This way has the advantage that we always get a working cpu >frequency scaling support.. But it also has one big disadvantage, we do >not get reports about not working ondemand governor so maybe >we simply did fot notice the improvements in this area. For our stable >releases, I will keep the current inplementation. For the unstable one, >I will disable the >autoswitching code and if it still works good then for a few >month, I will remove the userspace implementation completely. >It should not hurt to let >the code in for some time and remove the visible configuration option, >just to have fallback under strange circumstances. Would this >be ok with you? I think you'll need to keep the userspace backup scheme for systems which have switching latency too high to load and run ondemand. However, systems which can run ondemand, should never run userspace, and providing userspace as an option on such systems is probably not the right knob to present to administrators on those boxes. >> I'm happy to see that clock throttling is not enabled by >> default in recent SuSE release, at least on my laptop >> which supports P-states. >> >> I'd like to see no option to enable clock-throttling on >> systems that support real p-states. > >Yes, this is reasonable, indeen. Will do that. With p-states in this >context, you mean cpufreq here? throttling is always T-states. cpufreq is usually p-states, but in the case of p4clockmod, it is T-states also. As I mentioned above, cpufreq is doing you a dis-service by hiding the difference from you and really need to be enhanced to know (and export) the difference. >> It is useful only for workloads which have an infinite >> amount of non-idle computing which you don't care how >> slow it computes. For the vast majority of workloads >> it just slows down the machine and delays the processor >> from getting into idle where it can save a non-linear >> amount of power. Further, there exist today systems which >> will consume MORE power in deep C-states when throttled >> vs. when not throttled. >> >> The other major topic is the user/kernel interface >> for power management policy. there needs to be in-kernel >> state for this, else the device drivers will have no low-latency >> way to get the answer to the simple policy question of how >they should >> optimize for performance vs power at any given instant when they >> recognize their device is idle.. this state should be controlled >> by user space, but I think it is most practical for it to >> be kernel resident. > >I'm not sure if I completely understand what you mean here. Do you mean >the so called "runtime device power management"? yes. >If so, I fully agree with you. But I do not set a specific >policy in the powersave code explicitely for that feature. >If the policy information >will go into the kernel, I will use and set this one, of course. okay, great. Yes, the kernel folks have known for years that this has to be done. Hopefully progress will be made soon... thanks, -Len - To unsubscribe from this list: send the line "unsubscribe linux-laptop" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
