Re: is hibernation usable?

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 [off-list ref] wrote:

Also, is this the correct list for hibernation/swap discussion? Or linux-pm@?

Thanks,

Chris Murphy