[PATCH] allocation looping + kswapd CPU cycles

Mark Hemment
2001-05-08T12:45:13Z

In 2.4.3pre6, code in page_alloc.c:__alloc_pages(), changed from;

try_to_free_pages(gfp_mask);
wakeup_bdflush();
if (!order)
goto try_again;
to
try_to_free_pages(gfp_mask);
wakeup_bdflush();
goto try_again;

This introduced the effect of a non-zero order, __GFP_WAIT allocation
(without PF_MEMALLOC set), never returning failure. The allocation keeps
looping in __alloc_pages(), kicking kswapd, until the allocation succeeds.

If there is plenty of memory in the free-pools and inactive-lists
free_shortage() will return false, causing the state of these
free-pools/inactive-lists not to be 'improved' by kswapd.

If there is nothing else changing/improving the free-pools or
inactive-lists, the allocation loops forever (kicking kswapd).

Does anyone know why the 2.4.3pre6 change was made?

The attached patch (against 2.4.5-pre1) fixes the looping symptom, by
adding a counter and looping only twice for non-zero order allocations.

The real fix is to measure fragmentation and the progress of kswapd, but
that is too drastic for 2.4.x.

Mark

diff -ur linux-2.4.5-pre1/mm/page_alloc.c markhe-2.4.5-pre1/mm/page_alloc.c
--- linux-2.4.5-pre1/mm/page_alloc.c Fri Apr 27 22:18:08 2001
+++ markhe-2.4.5-pre1/mm/page_alloc.c Tue May 8 13:42:12 2001
@@ -275,6 +275,7 @@
{
zone_t **zone;
int direct_reclaim = 0;
+ int loop;
unsigned int gfp_mask = zonelist->gfp_mask;
struct page * page;

@@ -313,6 +314,7 @@
&& nr_inactive_dirty_pages >= freepages.high)
wakeup_bdflush(0);

+ loop = 0;
try_again:
/*
* First, see if we have any zones with lots of free memory.
@@ -453,7 +455,8 @@
if (gfp_mask & __GFP_WAIT) {
memory_pressure++;
try_to_free_pages(gfp_mask);
- goto try_again;
+ if (!order || loop++ < 2)
+ goto try_again;
}
}