Commit:     9ef9acb05a741ec10a5e9122717736de12adced9
Parent:     e010487dbe09d63cf916fd1b119d17abd0f48207
Author:     Mel Gorman <[EMAIL PROTECTED]>
AuthorDate: Tue Oct 16 01:25:54 2007 -0700
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Tue Oct 16 09:43:00 2007 -0700

    Do not group pages by mobility type on low memory systems
    Grouping pages by mobility can only successfully operate when there are more
    MAX_ORDER_NR_PAGES areas than mobility types.  When there are insufficient
    areas, fallbacks cannot be avoided.  This has noticeable performance impacts
    on machines with small amounts of memory in comparison to 
    For example, on IA64 with a configuration including huge pages spans 1GiB 
    MAX_ORDER_NR_PAGES so would need at least 4GiB of RAM before grouping pages 
    mobility would be useful.  In comparison, an x86 would need 16MB.
    This patch checks the size of vm_total_pages in build_all_zonelists(). If
    there are not enough areas,  mobility is effectivly disabled by considering
    all allocations as the same type (UNMOVABLE).  This is achived via a
    __read_mostly flag.
    With this patch, performance is comparable to disabling grouping pages
    by mobility at compile-time on a test machine with insufficient memory.
    With this patch, it is reasonable to get rid of grouping pages by mobility
    a compile-time option.
    Signed-off-by: Mel Gorman <[EMAIL PROTECTED]>
    Acked-by: Andy Whitcroft <[EMAIL PROTECTED]>
    Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
 mm/page_alloc.c |   24 +++++++++++++++++++++++-
 1 files changed, 23 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 03fef8d..676aec9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -159,8 +159,13 @@ EXPORT_SYMBOL(nr_node_ids);
+int page_group_by_mobility_disabled __read_mostly;
 static inline int get_pageblock_migratetype(struct page *page)
+       if (unlikely(page_group_by_mobility_disabled))
+               return MIGRATE_UNMOVABLE;
        return get_pageblock_flags_group(page, PB_migrate, PB_migrate_end);
@@ -174,6 +179,9 @@ static inline int allocflags_to_migratetype(gfp_t 
gfp_flags, int order)
        WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK);
+       if (unlikely(page_group_by_mobility_disabled))
+               return MIGRATE_UNMOVABLE;
        /* Cluster high-order atomic allocations together */
        if (unlikely(order > 0) &&
                        (!(gfp_flags & __GFP_WAIT) || in_interrupt()))
@@ -2375,9 +2383,23 @@ void build_all_zonelists(void)
                /* cpuset refresh routine should be here */
        vm_total_pages = nr_free_pagecache_pages();
-       printk("Built %i zonelists in %s order.  Total pages: %ld\n",
+       /*
+        * Disable grouping by mobility if the number of pages in the
+        * system is too low to allow the mechanism to work. It would be
+        * more accurate, but expensive to check per-zone. This check is
+        * made on memory-hotadd so a system can start with mobility
+        * disabled and enable it later
+        */
+       if (vm_total_pages < (MAX_ORDER_NR_PAGES * MIGRATE_TYPES))
+               page_group_by_mobility_disabled = 1;
+       else
+               page_group_by_mobility_disabled = 0;
+       printk("Built %i zonelists in %s order, mobility grouping %s.  "
+               "Total pages: %ld\n",
+                       page_group_by_mobility_disabled ? "off" : "on",
        printk("Policy zone: %s\n", zone_names[policy_zone]);
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