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

[James Hartley]

On 06/01/17 22:43, Andrew Morton wrote:
> On Fri, 25 Nov 2016 18:55:18 +0000 Paul Burton <paul.burton@xxxxxxxxxx> 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. 

James