linux-kernel - Re: [PATCH V2 0/6] VA to numa node information

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Date:   Mon, 24 Sep 2018 19:14:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     "prakash.sangappa" <prakash.sangappa@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dave.hansen@...el.com, nao.horiguchi@...il.com,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        khandual@...ux.vnet.ibm.com
Subject: Re: [PATCH V2 0/6] VA to numa node information

On Fri 14-09-18 12:01:18, Steven Sistare wrote:
> On 9/14/2018 1:56 AM, Michal Hocko wrote:
[...]
> > Why does this matter for something that is for analysis purposes.
> > Reading the file for the whole address space is far from a free
> > operation. Is the page walk optimization really essential for usability?
> > Moreover what prevents move_pages implementation to be clever for the
> > page walk itself? In other words why would we want to add a new API
> > rather than make the existing one faster for everybody.
> 
> One could optimize move pages.  If the caller passes a consecutive range
> of small pages, and the page walk sees that a VA is mapped by a huge page, 
> then it can return the same numa node for each of the following VA's that fall 
> into the huge page range. It would be faster than 55 nsec per small page, but 
> hard to say how much faster, and the cost is still driven by the number of 
> small pages. 

This is exactly what I was arguing for. There is some room for
improvements for the existing interface. I yet have to hear the explicit
usecase which would required even better performance that cannot be
achieved by the existing API.

-- 
Michal Hocko
SUSE Labs