Hi. On Sunday 15 July 2007 22:33:32 Rafael J. Wysocki wrote: > Hi, > > Since many alternative approaches to hibernation are now being considered and > discussed, I thought it might be a good idea to list some things that in my not > so humble opinion should be taken care of by any hibernation framework. They > are listed below, not in any particular order, because I think they all are > important. Still, I might have forgotten something, so everyone with > experience in implementing hibernation, especially Pavel and Nigel, please > check if the list is complete. > > (1) Filesystems mounted before the hibernation are untouchable > > When there's a memory snapshot, either in the form of a hibernation image, > or in the form of the "old" kernel and processes available to the "new" > kexeced kernel responsible for saving their memory, the filesystems mounted > before the hibernation should not be accessed, even for reading, because > that would cause their on-disk state to be inconsistent with the snapshot > and might lead to a filesystem corruption. > > (2) Swap space in use before the hibernation must be handled with care > > If swap space is used for saving the memory snapshot, the snapshot-saving > application (or kernel) must be careful enough not to overwrite swap pages > that contain valid memory contents stored in there before the hibernation. > > (3) There are memory regions that must not be saved or restored > > Some memory regions contain data that shouldn't be overwritten during the > restore, because that might lead to the system not working correctly > afterwards. Also, on some systems there are valid 'struct pages' > structures that in fact corresond to memory holes and we should not attempt > to save those pages. > > (4) The user should be able to limit the size of a hibernation image > > There are a couple of reasons of that. For example, the storage space > used for saving the image may be smaller than the entire RAM or the user > may want the image to be saved quickier. > > (5) Hibernation should be transparent from the applications' point of view > > Generally, applications should not notice that hibernation took place. > [Note that I don't regard all processes as applications and I think that > there may be processes which need to handle the hibernation in a special > way.] Ideally, for example, if some audio is being played when a > hibernation starts, the audio player should be able to continue playing the > same audio after the restore from the point in which it has been > interrupted by the hibernation. Also, the CPU affinities and similar > settings requested by the applications before a hibernation should be > binding after the restore. > > (6) State of devices from before hibernation should be restored, if possible > > If possible, during a restore devices should be brought back to the same > state in which they were before the corresponding hibernation. Of course > in some situations it might be impossible to do that (eg. the user > connected the hibernated system to a different IP subnet and then > restored), but as a general rule, we should do our best to restore the > state of devices, which is directly related to point (5) above. > > (7) On ACPI systems special platform-related actions have to be carried out at > the right points, so that the platform works correctly after the restore > > The ACPI specification requires us to invoke some global ACPI methods > during the hibernation and during the restore. Moreover, the ordering of > code related to these ACPI methods may not be arbitrary (eg. some of > them have to be executed after devices are put into low power states etc.). > > (8) Hibernation and restore should not be too slow > > In my opinion, if more than one minute is needed to hibernate the system > with the help of certain hibernation framework, then this framework is not > very useful in practice. It might be useful to perform some special tasks > (eg. moving a server to another place without taking it down), but it is > not very useful, for example, to notebook users. > > (9) Hibernation framework should not be too difficult to set up > > It follows from my experience that if the users are required to do too much > work to set up a hibernation framework, they will not use it as long as > there are simpler alternatives (some of them will not use hibernation at > all if it's too difficult to get to work). On the other hand, if the users > are provided with a working hibernation framework by their distribution > and they find it useful, they are not likely to use kernel.org kernels if > t's too difficult to replace the distribution kernel with a generic one due > to the hibernation framework's requirements. > > All of the existing hibernation frameworks have been written with the above > points in mind and that's why they are what they are. In particular, the > existence of the tasks freezer, hated by some people to the point of insanity, > follows directly from points (1), (4) and (5). > > In my opinion any hibernation framework that doesn't take the above > requirements into account in any way will be a failure. Moreover, the existing > frameworks fail to follow some of them too, so I consider all of these > frameworks as a work in progress. For this reason, I will much more appreciate > ideas allowing us to improve the existing frameworks in a more or less > evolutionary way, then attempts to replace them all with something entirely > new. Sounds good to me. Nothing extra occurs immediately. Regards, Nigel -- See http://www.tuxonice.net for Howtos, FAQs, mailing lists, wiki and bugzilla info.
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