Re: is hibernation usable?

[Luigi Semenzato <semenzato@google.com>]

On Thu, Feb 20, 2020 at 11:09 AM Chris Murphy <lists@colorremedies.com> wrote:
>
> On Thu, Feb 20, 2020 at 10:16 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.)
>
> What is vm.swappiness set to on your system? A fellow Fedora
> contributor who has consistently reproduced what you describe, has
> discovered he has vm.swappiness=0, and even if it's set to 1, the
> problem no longer happens. And this is not a documented consequence of
> using a value of 0.

I am using the default value of 60.

A zero value should cause all file pages to be discarded before any
anonymous pages are swapped.  I wonder if the fellow Fedora
contributor's workload has a lot of file pages, so that discarding
them is enough for the image allocator to succeed. In that case "sync;
echo 1 > /proc/sys/vm/drop_caches" would be a better way of achieving
the same result.  (By the way, in my experiments I do that just before
hibernating.)

> > 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.
>
> Fedora 31+ now uses cgroupsv2. In any case, my use case is making sure
> this works correctly, sanely, with mainline kernels because Fedora
> doesn't do custom things with the kernel.
>
>
>
> --
> Chris Murphy