mm: memmap_init_zone() performance improvement

We have what we call an "architectural simulator".  It is a computer
program that pretends that it is a computer system.  We use it to test the
firmware before real hardware is available.  We have booted Linux on our
simulator.  As you would expect it takes longer to boot on the simulator
than it does on real hardware.

With my patch - boot time 41 minutes
Without patch - boot time 94 minutes

These numbers do not scale linearly to real hardware.  But indicate to me
a place where Linux can be improved.

memmap_init_zone() loops through every Page Frame Number (pfn), including
pfn values that are within the gaps between existing memory sections.  The
unneeded looping will become a boot performance issue when machines
configure larger memory ranges that will contain larger and more numerous
gaps.

The code will skip across invalid pfn values to reduce the number of loops
executed.

Signed-off-by: Mike Yoknis <mike.yoknis@hp.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 87a4af89e5a0baf5f4b60555fdd7beaf1d0a2f2e..cc14c7a64ba2d89f1cd8edf8592b4a269d00a766 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3907,8 +3907,11 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
                 * exist on hotplugged memory.
                 */
                if (context == MEMMAP_EARLY) {
-                       if (!early_pfn_valid(pfn))
+                       if (!early_pfn_valid(pfn)) {
+                               pfn = ALIGN(pfn + MAX_ORDER_NR_PAGES,
+                                               MAX_ORDER_NR_PAGES) - 1;
                                continue;
+                       }
                        if (!early_pfn_in_nid(pfn, nid))
                                continue;
                }