On 06/01/17 22:43, Andrew Morton wrote:

On Fri, 25 Nov 2016 18:55:18 +0000 Paul Burton [off-list ref] wrote:

quoted

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

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