On Sat, 29 Aug 2009, Rafael J. Wysocki wrote: > On Friday 28 August 2009, Alan Stern wrote: > > On Fri, 28 Aug 2009, Rafael J. Wysocki wrote: > > > > > > Given this design, why bother to invoke device_resume() for the async > > > > devices? Why not just start up a bunch of async threads, each of which > > > > calls async_resume() repeatedly until everything is finished? (And > > > > rearrange async_resume() to scan the list first and do the actual > > > > resume second.) > > > > > > > > The same goes for the noirq versions. > > > > > > I thought about that, but there are a few things to figure out: > > > - how many threads to start > > > > That's a tough question. Right now you start roughly as many threads > > as there are async devices. That seems like overkill. > > In fact they are substantially fewer than that, for the following reasons. > > First, the async framework will not start more than MAX_THREADS threads, > which is 256 at the moment. This number is less than the number of async > devices to handle on an average system. Okay, but MAX_THREADS isn't under your control. Remember also that each thread takes up some memory, and during hibernation we are in a memory-constrained situation. > Second, no new async threads are started while the main thread is handling the > sync devices , so the existing threads have a chance to do their job. If > there's a "cluster" of sync devices in dpm_list, the number of async threads > running is likely to drop rapidly while those devices are being handled. > (BTW, if there were no sync devices, the whole thing would be much simpler, > but I don't think it's realistic to assume we'll be able to get rid of them any > time soon). Perhaps not, but it would be interesting to see what happens if every device is async. Maybe you can try it and get a meaningful result. > Finally, but not least importantly, async threads are not started for the > async devices that were previously handled "out of order" by the already > running async threads (or by async threads that have already finished). My > testing shows that there are quite a few of them on the average. For example, > on the HP nx6325 typically there are as many as 580 async devices handled "out > of order" during a _single_ suspend-resume cycle (including the "early" and > "late" phases), while only a few (below 10) devices are waited for by at least > one async thread. That is a difficult sort of thing to know in advance. It ought to be highly influenced by the percentage of async devices; that's another reason for wanting to know what happens when every device is async. > > I would expect that a reasonably small number of threads would suffice > > to achieve most of the possible time savings. Something on the order > > of 10 should work well. If the majority of the time is spent > > handling N devices then N+1 threads would be enough. Judging from some > > of the comments posted earlier, even 4 threads would give a big > > advantage. > > That unfortunately is not the case with the set of async devices including > PCI, ACPI and serio devices only. The average time savings are between 5% to > 14%, depending on the system and the phase of the cycle (the relative savings > are typically greater for suspend). Still, that amounts to .5 s in some cases. Without context it's hard to be sure, but I don't think your numbers contradict what I said. If you get between 5% and 14% time savings with 14 threads, then you might get between 4% and 10% savings with only 4 threads. I must agree, 14 threads isn't a lot. But at the moment that number is random, not under your control. > > > - when to start them > > > > You might as well start them at the beginning of dpm_resume and > > dpm_resume_noirq. That way they can overlap with the synchronous > > operations. > > In that case they would have to wait in the beginning, so I'd need a mechanism > to wake them up. You already have two such mechanisms: dpm_list_mtx and the embedded wait_queue_heads. Although in the scheme I'm proposing, no async threads would ever have to wait on a per-device waitqueue. A system-wide waitqueue might work out better (for use when a thread reaches the end of the list and then waits before starting over at the beginning). > Alternatively, there could be a limit to the number of async threads started > within the current design, but I'd prefer to leave that to the async framework > (namely, if MAX_THREADS makes sense for boot, it's also likely to make sense > for PM). Strictly speaking, a new thread should be started only when needed. That is, only when all the existing threads are busy running a callback. It shouldn't be too hard to keep track of when that happens. > > It comes down to this: Should there be many threads, each of which > > browses the list only once, or should there be a few threads, each of > > which browses the list many times? > > Well, quite obviously I prefer the many threads version. :-) Okay, clearly it's a matter of taste. To me the many-threads version seems less elegant and less well controlled. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
