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

[KOSAKI Motohiro]

(10/16/13 6:42 AM), 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.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

That's ok. I haven't used such information on my long oom debugging history.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>


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