On Sun, 13 Dec 2009, Alan Stern wrote: > > Namely that there's no apparent sane way to say "don't wait for children". > > > > PCI bridges that don't suspend at all - or any other device that only > > suspends in the 'suspend_late()' thing, for that matter - don't have any > > reason what-so-ever to wait for children, since they aren't actually > > suspending in the first place. But you make them wait regardless, which > > then serializes things unnecessarily (for example, two unrelated USB > > controllers). > In short, allowing devices to suspend before their children would be > dangerous and probably would not save a significant amount of time. There's more to be said. Even without this "don't wait for children" thing, there can be bad interactions causing unnecessary delays. For example, suppose A (async) is the parent of B (sync), B comes before C (sync) in dpm_list, and C is the parent of D (async). Even if A & B are unrelated to C & D, they will be forced to wait for them. It doesn't matter that A and D are unrelated and so could suspend concurrently. In essence, every synchonrous device is treated as though it depends on all the synchronous devices preceding it in dpm_list. That's a lot of unnecessary constraints. At the moment we have no choice, because we have to assume that some of those constraints actually are necessary -- and we don't know which ones. It's an inescapable fact: If there are unnecessary ordering constraints then you generally can't be 100% efficient in carrying out parallel operations. Compared with all these extra "synchronous" constraints, the relatively small number of "don't need to wait for children" constraints is harmless. I bet that if we got rid of all unnecessary constraints except for making parents always wait for their children, we'd attain more than 95% of the ideal speedup. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
