On Wed, Dec 8, 2010 at 11:01 AM, Jamie Lokier <ja...@shareable.org> wrote: > 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.
Agreed -- either could be the case. It's something you can never be certain about without doing some measurements... Cheers ---Dave _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
