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Date:   Thu, 13 Sep 2018 17:10:16 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     prakash.sangappa@oracle.com
Cc:     Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
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        kirill.shutemov@linux.intel.com, khandual@linux.vnet.ibm.com,
        steven.sistare@oracle.com
Subject: Re: [PATCH V2 0/6] VA to numa node information
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On Thu, 13 Sep 2018 15:32:25 -0700 "prakash.sangappa" <prakash.sangappa@oracle.com> wrote:

> >> https://marc.info/?t=152524073400001&r=1&w=2
> > It would be really great to give a short summary of the previous
> > discussion. E.g. why do we need a proc interface in the first place when
> > we already have an API to query for the information you are proposing to
> > export [1]
> >
> > [1] http://lkml.kernel.org/r/20180503085741.GD4535@dhcp22.suse.cz
> 
> The proc interface provides an efficient way to export address range
> to numa node id mapping information compared to using the API.
> For example, for sparsely populated mappings, if a VMA has large portions
> not have any physical pages mapped, the page walk done thru the /proc file
> interface can skip over non existent PMDs / ptes. Whereas using the
> API the application would have to scan the entire VMA in page size units.
> 
> Also, VMAs having THP pages can have a mix of 4k pages and hugepages.
> The page walks would be efficient in scanning and determining if it is
> a THP huge page and step over it. Whereas using the API, the application
> would not know what page size mapping is used for a given VA and so would
> have to again scan the VMA in units of 4k page size.
> 
> If this sounds reasonable, I can add it to the commit / patch description.

Preferably with some runtime measurements, please.  How much faster is
this interface in real-world situations?  And why does that performance
matter?

It would also be useful to see more details on how this info helps
operators understand/tune/etc their applications and workloads.  In
other words, I'm trying to get an understanding of how useful this code
might be to our users in general.