linux-kernel - Re: [PATCH] mm: page_alloc: Skip over regions of invalid pfns where possible

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Date:   Tue, 10 Jan 2017 15:48:44 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     James Hartley <james.hartley@...tec.com>
Cc:     Paul Burton <paul.burton@...tec.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: Skip over regions of invalid pfns where
 possible

On Tue, 10 Jan 2017 23:37:53 +0000 James Hartley <james.hartley@...tec.com> wrote:

> 
> On 06/01/17 22:43, Andrew Morton wrote:
> > On Fri, 25 Nov 2016 18:55:18 +0000 Paul Burton <paul.burton@...tec.com> wrote:
> >
> >> When using a sparse memory model memmap_init_zone() when invoked with
> >> the MEMMAP_EARLY context will skip over pages which aren't valid - ie.
> >> which aren't in a populated region of the sparse memory map. However if
> >> the memory map is extremely sparse then it can spend a long time
> >> linearly checking each PFN in a large non-populated region of the memory
> >> map & skipping it in turn.
> >>
> >> When CONFIG_HAVE_MEMBLOCK_NODE_MAP is enabled, we have sufficient
> >> information to quickly discover the next valid PFN given an invalid one
> >> by searching through the list of memory regions & skipping forwards to
> >> the first PFN covered by the memory region to the right of the
> >> non-populated region. Implement this in order to speed up
> >> memmap_init_zone() for systems with extremely sparse memory maps.
> > Could we have a changelog which includes some timing measurements? 
> > That permits others to understand the value of this patch.
> >
> I have tested this patch on a virtual model of a Samurai CPU with a
> sparse memory map.  The kernel boot time drops from 109 to 62 seconds. 

Thanks.  Nice.  I updated the changelog.