Matthew Garrett (mjg59) wrote,

Hybrid suspend

One often requested feature for suspend support on Linux is hybrid suspend, or "suspend to both". This is where the system suspends to disk, but then puts the machine in S3 rather than powering down. If the user resumes without power having been removed they get the benefit of the fast S3 resume. If not, the system resumes from disk and no data is lost.

This is, clearly, the way suspend should work. We're not planning on adding it by default in Fedora, though, for a couple of reasons. The main reason right now is that the in-kernel suspend to disk is still slow. Triggering a suspend to disk on a machine with gigabytes of RAM (which is a basic laptop configuration these days) will leave you sitting there for an extended period of time when all you actually want to do is pick your machine up and leave. Fixing this properly is less than trivial. TuxOnIce improves the speed somewhat, at the expense of being a >500k patch against upstream that touches all sorts of interesting bits of the kernel such as the vm system. We're not supporting that for fairly obvious reasons. But even then, the suspend to disk process involves discarding some pages. Those need to be pulled in from disk again on resume. With the current implementation, suspend to both is fundamentally slower than suspend to RAM for both the suspend and resume paths.

So, what other approaches are there? One is to resume from RAM some period of time after suspending and then if the battery is low suspend to disk. Many recent machines will automatically resume when the battery level becomes critical. If the hardware doesn't support that, we can wake up after a set period and measure the battery consumption, set a new alarm and go to sleep again. The downside of this approach is that your system wakes up and does stuff without you being aware of it, which may be bad if it's inside a neoprane cover at the time. Cooking laptops is generally considered unhelpful.

Using the kexec approach to hibernation provides a more straightforward way of handling the problem. The fundamental problem with the existing approach is that it ties suspend into the vm system and involves making atomic copies of RAM into other bits of RAM. kexec would allow us to pre-allocate enough space on disk to save RAM as-is, and then simply kexec into a new kernel and dump RAM to disk without any of the tedious shrinking required first. Resuming from S3 would kexec back into the old kernel, whereas losing power would just fall back to reading off disk. The extra time taken on the S3 path would be minimal.

In an ideal world we'd adopt the Vista approach where "off" is synonymous with suspend. There's still more work to be done on enhancing reliability before that can be achieved, though.
Tags: advogato, fedora
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