Re: is hibernation usable?

[Luigi Semenzato <semenzato@google.com>]

I was forgetting: forcing swap by eating up memory is dangerous
because it can lead to unexpected OOM kills, but you can mitigate that
by giving the memory-eaters a higher OOM kill score.  Still, some way
of calling try_to_free_pages() directly from user-level would be
preferable.  I wonder if such API has been discussed.


On Thu, Feb 20, 2020 at 9:16 AM Luigi Semenzato <semenzato@google.com> wrote:
>
> I think this is the right group for the memory issues.
>
> I suspect that the problem with failed allocations (ENOMEM) boils down
> to the unreliability of the page allocator.  In my experience, under
> pressure (i.e. pages must be swapped out to be reclaimed) allocations
> can fail even when in theory they should succeed.  (I wish I were
> wrong and that someone would convincingly correct me.)
>
> I have a workaround in which I use memcgroups to free pages before
> starting hibernation.  The cgroup request "echo $limit >
> .../memory.limit_in_bytes"  blocks until memory usage in the chosen
> cgroup is below $limit.  However, I have seen this request fail even
> when there is extra available swap space.
>
> The callback for the operation is mem_cgroup_resize_limit() (BTW I am
> looking at kernel version 4.3.5) and that code has a loop where
> try_to_free_pages() is called up to retry_count, which is at least 5.
> Why 5?  One suspects that the writer of that code must have also
> realized that the page freeing request is unreliable and it's worth
> trying multiple times.
>
> So you could try something similar.  I don't know if there are
> interfaces to try_to_free_pages() other than those in cgroups.  If
> not, and you aren't using cgroups, one way might be to start several
> memory-eating processes (such as "dd if=/dev/zero bs=1G count=1 |
> sleep infinity") and monitor allocation, then when they use more than
> 50% of RAM kill them and immediately hibernate before the freed pages
> are reused.  If you can build your custom kernel, maybe it's worth
> adding a sysfs entry to invoke try_to_free_pages().  You could also
> change the hibernation code to do that, but having the user-level hook
> may be more flexible.
>
>
> On Wed, Feb 19, 2020 at 6:56 PM Chris Murphy <lists@colorremedies.com> wrote:
> >
> > Also, is this the correct list for hibernation/swap discussion? Or linux-pm@?
> >
> > Thanks,
> >
> > Chris Murphy