Re: [PATCH] mm: Do not walk all of system memory during show_mem

[Mel Gorman <mgorman@suse.de>]

On Wed, Oct 16, 2013 at 06:11:56PM -0700, David Rientjes wrote:
> 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?

We could once all the per-arch show_mem functions were updated similar
to lib/show_mem.c. I've added a todo item to do just that. Thanks.


-- 
Mel Gorman
SUSE Labs

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