On Wed, 16 Oct 2013, Mel Gorman wrote:
> It has been reported on very large machines that show_mem is taking almost
> 5 minutes to display information. This is a serious problem if there is
> an OOM storm. The bulk of the cost is in show_mem doing a very expensive
> PFN walk to give us the following information
>
> Total RAM: Also available as totalram_pages
> Highmem pages: Also available as totalhigh_pages
> Reserved pages: Can be inferred from the zone structure
> Shared pages: PFN walk required
> Unshared pages: PFN walk required
> Quick pages: Per-cpu walk required
>
> Only the shared/unshared pages requires a full PFN walk but that information
> is useless. It is also inaccurate as page pins of unshared pages would
> be accounted for as shared. Even if the information was accurate, I'm
> struggling to think how the shared/unshared information could be useful
> for debugging OOM conditions. Maybe it was useful before rmap existed when
> reclaiming shared pages was costly but it is less relevant today.
>
> The PFN walk could be optimised a bit but why bother as the information is
> useless. This patch deletes the PFN walker and infers the total RAM, highmem
> and reserved pages count from struct zone. It omits the shared/unshared page
> usage on the grounds that it is useless. It also corrects the reporting
> of HighMem as HighMem/MovableOnly as ZONE_MOVABLE has similar problems to
> HighMem with respect to lowmem/highmem exhaustion.
>
We haven't been hit by this for the oom killer, but we did get hit with
this for page allocation failure warnings as a result of having irqs
disabled and passing GFP_ATOMIC to the page allocator without GFP_NOWARN.
That was the intention of passing SHOW_MEM_FILTER_PAGE_COUNT into
show_mem() in 4b59e6c47309 ("mm, show_mem: suppress page counts in
non-blockable contexts").
With this, I assume we can just remove SHOW_MEM_FILTER_PAGE_COUNT
entirely?
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