Re: hibernation memory usage

[Luigi Semenzato <semenzato@google.com>]

On Thu, Oct 17, 2019 at 3:55 PM Luigi Semenzato <semenzato@google.com> wrote:
>
> To make hibernation work, I have been playing with this suggestion:
>
> > Whatever doesn't fit into 50% of RAM needs to be swapped out before
> > hibernation.  The efficiency of that depends on the swap handling code
> > and the underlying hardware.  If that is efficient enough overall,
> > trying to avoid it altogether isn't going to make much of a
> > difference.
>
> What's a good way of swapping out 50% of RAM?  I have tried playing
> with /proc/sys/vm/min_free_kbytes.  Lowering it below MemFree forces
> reclaim and gets swapping started.  Unfortunately the reclaim also
> hits file pages, so badly that the system thrashes to a grinding halt.
> Swappiness is already set to 100.  Internally it seems that values up
> to 200 are valid, and I wish that the entire range was allowed, but I
> am not sure it would help.  Right now I am playing with the production
> kernel, but similar situations triggered the same behavior in the
> Chrome OS kernels, even after internally setting swappiness to values
> close to 200.

One more data point: playing with min_free_kbytes is flakey: it can
cause OOM-kills even when I increase it slowly (so that I don't
completely destroy the file RSS) and there is plenty of swap space
available.  Not sure why.

Is there perhaps a way of achieving this using the userland suspend
API?  If there is, I am not able to see it.

Thanks!



> On Tue, Oct 8, 2019 at 1:10 PM Luigi Semenzato <semenzato@google.com> wrote:
> >
> > Actually, I think we only need to change the MM watermarks before
> > hibernation and after resume.  There's a patch that will do just that:
> >
> > https://lkml.org/lkml/2013/2/17/210
> >
> > It didn't make it into mainline (which seems kind of unreasonable,
> > since the watermarks themselves are based on heuristics) but shouldn't
> > be difficult to apply.  Or are there simpler solutions?
> >
> > On Tue, Oct 8, 2019 at 9:18 AM Luigi Semenzato <semenzato@google.com> wrote:
> > >
> > > Yes, that makes sense, thank you.  Use separate partitions for swap
> > > and hibernation.
> > >
> > > Normally the kernel starts swapping out when there's no reclaimable
> > > memory, so anon usage will be high.  Do you think cranking up
> > > /proc/vm/swappiness would be enough to ensure that file pages stay
> > > over 50%?  Or would you use some tricks, such as running a
> > > high-priority process which allocates >50% of RAM, thus forcing other
> > > anon pages to be swapped out, then killing that process and quickly
> > > hibernating before too many pages are brought back in?  Or changing
> > > the kernel so that in the first part of hibernation we'll just swap
> > > stuff out?
> > >
> > > On Tue, Oct 8, 2019 at 8:39 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > > >
> > > > On Tue, Oct 8, 2019 at 5:26 PM Luigi Semenzato <semenzato@google.com> wrote:
> > > > >
> > > > > Thank you for your reply!
> > > > >
> > > > > I understand the need for saving all state, not just process/task
> > > > > state.  But for many of the systems that could benefit from
> > > > > hibernation, the majority of RAM is taken by user processes (I am
> > > > > thinking laptops).  It should be possible to copy their anonymous
> > > > > pages to disk more or less directly, without making an extra copy like
> > > > > it's done for all other pages.  I am not sure what happens with kernel
> > > > > tasks, but they don't have anonymous pages (that I know).
> > > > >
> > > > > I am curious to know how/if hibernation is currently used in practice.
> > > > > It doesn't seem practical to require that user processes take less
> > > > > than 50% of RAM at all times.  There may be special cases in which the
> > > > > restriction can be achieved by terminating non-essential processes
> > > > > before hibernating, but I don't know of any.
> > > > >
> > > > > I would also like to know how much work it might take to avoid the
> > > > > extra copy of the anonymous pages of frozen processes.
> > > >
> > > > Whatever doesn't fit into 50% of RAM needs to be swapped out before
> > > > hibernation.  The efficiency of that depends on the swap handling code
> > > > and the underlying hardware.  If that is efficient enough overall,
> > > > trying to avoid it altogether isn't going to make much of a
> > > > difference.