Dave Martin wrote: > On Sun, Dec 5, 2010 at 2:12 PM, Thomas Petazzoni > <thomas.petazz...@free-electrons.com> wrote: > > Hi, > > > > On Fri, 3 Dec 2010 16:28:27 +0000 > > Dave Martin <dave.mar...@linaro.org> wrote: > > > >> This allows for more active power management of such functional > >> blocks: if the CPU is not fully loaded, you can turn them off -- the > >> kernel can spot when there is significant idle time and do this. If > >> the CPU becomes fully loaded, applications which have soft-realtime > >> constraints can notice this and switch to their accelerated code > >> (which will cause the kernel to switch the functional unit(s) on). > >> Or, the kernel can react to increasing CPU load by speculatively turn > >> it on instead. This is analogous to the behaviour of other power > >> governors in the system. Non-aware applications will still work > >> seamlessly -- these may simply run accelerated code if the hardware > >> supports it, causing the kernel to turn the affected functional > >> block(s) on. > > > > From a power management perspective, is it really useful to load the > > CPU instead of using specialized units which usually provide more > > computing power per watt consumed ? > > No--- but you can't in general just exchange cycles on one functional > unit for cycles on another in the same way as you > > Suppose 90% if your code (by execution time) can take advantage of a > specialised functional unit. Should you turn that unit on? > > Now, suppose only 5% of the code can take advantage, but the platform > is not completely busy. Turning on a special functional unit consumes > extra power and will provide no speedup to the user -- is it still > worth turning it on? What if the CPU is fully loaded doing other work > and your program is close to missing its realtime deadlines -- should > you turn on the separate unit now?
I think Thomas's point is that doing the 5% on the CPU may consume more power than turning on the special functional unit - even when the system is not busy and the user doesn't see a time difference. I don't know if that's true for available hardware, but it seems like it's worth investigating before taking the idea further. -- Jamie _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
