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SubjectRe: System freezes after OOM
On Wed 13-07-16 10:35:01, Jerome Marchand wrote:
> On 07/13/2016 01:44 AM, Mikulas Patocka wrote:
> > The problem of swapping to dm-crypt is this.
> >
> > The free memory goes low, kswapd decides that some page should be swapped
> > out. However, when you swap to an ecrypted device, writeback of each page
> > requires another page to hold the encrypted data. dm-crypt uses mempools
> > for all its structures and pages, so that it can make forward progress
> > even if there is no memory free. However, the mempool code first allocates
> > from general memory allocator and resorts to the mempool only if the
> > memory is below limit.
> >
> > So every attempt to swap out some page allocates another page.
> >
> > As long as swapping is in progress, the free memory is below the limit
> > (because the swapping activity itself consumes any memory over the limit).
> > And that triggered the OOM killer prematurely.
>
> There is a quite recent sysctl vm knob that I believe can help in this
> case: watermark_scale_factor. If you increase this value, kswapd will
> start paging out earlier, when there might still be enough free memory.
>
> Ondrej, have you tried to increase /proc/sys/vm/watermark_scale_factor?

I suspect this would just change the timing or the real problem gets
hidden.
--
Michal Hocko
SUSE Labs

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