[PATCH] oom-killer trigger

[PATCH] oom-killer trigger

[Rik van Riel]

Hi,

the OOM killer in Linux 2.4 has a rather embarrasing bug.

1. the OOM killer never triggers if we have > freepages.min
   of free memory
2. __alloc_pages() never allocates pages to < freepages.min
   for user allocations

==> the OOM killer never gets triggered under some workloads;
    the system just sits around with nr_free_pages == freepages.min

The patch below trivially fixes this by upping the OOM kill limit
by a really small number of pages ...

Now lets hope it won't trigger too early (but since it'll only
trigger when we're completely out of swap, etc...).

regards,

Rik
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Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

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--- mm/oom_kill.c.orig	Thu Mar  1 18:57:11 2001
+++ mm/oom_kill.c	Thu Mar  1 18:58:23 2001
@@ -188,13 +188,17 @@
  *
  * Returns 0 if there is still enough memory left,
  * 1 when we are out of memory (otherwise).
+ *
+ * Note that since __alloc_pages() never lets user
+ * allocations go below freepages.min, we have to
+ * use a slightly higher threshold here...
  */
 int out_of_memory(void)
 {
 	struct sysinfo swp_info;

 	/* Enough free memory?  Not OOM. */
-	if (nr_free_pages() > freepages.min)
+	if (nr_free_pages() > freepages.min + 4)
 		return 0;

 	if (nr_free_pages() + nr_inactive_clean_pages() > freepages.low)

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Re: [PATCH] oom-killer trigger

[David Mansfield]

> 
> 1. the OOM killer never triggers if we have > freepages.min
>    of free memory
> 2. __alloc_pages() never allocates pages to < freepages.min
>    for user allocations
> 
> ==> the OOM killer never gets triggered under some workloads;
>     the system just sits around with nr_free_pages == freepages.min
> 
> The patch below trivially fixes this by upping the OOM kill limit
> by a really small number of pages ...

> +       if (nr_free_pages() > freepages.min + 4)


Call me stupid, but why not just change the > to >= (or < to <=) rather
than introducing a magic number (4).  Or at least make the magic number
interesting, like:

+       if (nr_free_pages() > freepages.min + 42)

:-)

Thanks for the bugfix,
David

-- 
David Mansfield                                           (718) 963-2020
david@ultramaster.com
Ultramaster Group, LLC                               www.ultramaster.com
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