[Xen-devel] [Fwd: [patch] first boot allocator to not scan hole on every allocation]

[Jes Sorensen <jes@xxxxxxx>]

Hi,

Alex suggested I send this one straight to xen-devel given it's
generic code.

I would claim it's harmless to architectures which have memory
starting 0x00 and it's absolutely vital for those architectures that
don't.

Cheers,
Jes

--- Begin Message ---

• To: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
• From: Jes Sorensen <jes@xxxxxxx>
• Date: Mon, 11 Dec 2006 16:01:35 +0100
• Cc: Alex Williamson <alex.williamson@xxxxxx>

Hi,

Ok, I finally figured out why the boot allocator took .0172 seconds
for each single page allocation on a small Altix (a partitioned system
would be even worse). I posted a patch for this earlier, but it was
wrong (by a factor of 0x40 :-( ).

This patch goes into common/page_alloc.c but it should be obviously
correct.

It's really vital for the SN2 port to get this one in.

Cheers,
Jes

Keep track of which page is the first available to the boot allocator
to avoid scanning the hole "0 ... first_pg" on every allocation, on
platforms that do not have physical memory starting at address 0x00.

This reduces the Xen boot time by roughly 150 seconds on a small
Altix with 6GB of RAM.

Signed-off-by: Jes Sorensen <jes@xxxxxxx>

diff -r 66609699c9d0 xen/common/page_alloc.c
--- a/xen/common/page_alloc.c   Fri Dec 08 14:29:42 2006 +0100
+++ b/xen/common/page_alloc.c   Mon Dec 11 15:54:10 2006 +0100
@@ -137,6 +137,8 @@ static void map_alloc(unsigned long firs
 }
 
 
+static unsigned long first_pg = -1UL;
+
 static void map_free(unsigned long first_page, unsigned long nr_pages)
 {
     unsigned long start_off, end_off, curr_idx, end_idx;
@@ -152,6 +154,13 @@ static void map_free(unsigned long first
     start_off = first_page & (PAGES_PER_MAPWORD-1);
     end_idx   = (first_page + nr_pages) / PAGES_PER_MAPWORD;
     end_off   = (first_page + nr_pages) & (PAGES_PER_MAPWORD-1);
+
+    /*
+     * Set first_pg to the first entry initialized so the boot
+     * allocator can avoid scanning the hole from 0 for no reason
+     */
+    if (first_page < first_pg)
+        first_pg = first_page;
 
     if ( curr_idx == end_idx )
     {
@@ -258,7 +267,7 @@ unsigned long alloc_boot_pages(unsigned 
 {
     unsigned long pg, i = 0;
 
-    for ( pg = 0; (pg + nr_pfns) < max_page; pg += pfn_align )
+    for ( pg = first_pg; (pg + nr_pfns) < max_page; pg += pfn_align )
     {
         i = alloc_boot_pages_at(nr_pfns, pg);
         if (i != 0)
@@ -301,7 +310,7 @@ void end_boot_allocator(void)
                 INIT_LIST_HEAD(&heap[i][j][k]);
 
     /* Pages that are free now go to the domain sub-allocator. */
-    for ( i = 0; i < max_page; i++ )
+    for ( i = first_pg; i < max_page; i++ )
     {
         curr_free = next_free;
         next_free = !allocated_in_map(i+1);
@@ -482,7 +491,7 @@ void scrub_heap_pages(void)
 
     printk("Scrubbing Free RAM: ");
 
-    for ( pfn = 0; pfn < max_page; pfn++ )
+    for ( pfn = first_pg; pfn < max_page; pfn++ )
     {
         /* Every 100MB, print a progress dot. */
         if ( (pfn % ((100*1024*1024)/PAGE_SIZE)) == 0 )

--- End Message ---

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