Re: help me understand why oom-killer engages with lots of freememory left

From: KAMEZAWA Hiroyuki
Date: Mon Jun 22 2009 - 21:51:59 EST

On Mon, 22 Jun 2009 17:59:43 +0200
Daniel Kabs <daniel.kabs@xxxxxx> wrote:

> Hi there,
> I'd like some help in researching why oom-killer slashes processes although there seems to be plenty of RAM left.
> I am talking about an embedded system using kernel and 256 MByte of RAM, no swap space and the root filesystem residing in an tmpfs. When the
> system is up and running the regular workload, /proc/meminfo shows more than 22 MByte of free RAM - this is after I free pagecache, dentries and
> inodes using
> echo 3 > /proc/sys/vm/drop_caches
> Now sometimes executing a new process triggers OOM-Killer. With "new process" I mean something small like a shell or perl script, nothing that would
> consume MBytes of memory. Nevertheless, OOM-Killer starts to kill processes.
> In the output of the oom-killer (see example below), 20396kB of free memory is mentioned. So I see no need for oom-killer to bring complete
> pandemonium. Aside from that I fail to put the output of oom-killer to good use.
> I hope someone here would help me interpret the kernel output, or tell me what could possibly have caused the oom-killer to kick in with so much free
> memory left.

At quick glance,

> Quote of 1st oom-killer output:
> checkd invoked oom-killer: gfp_mask=0x44d0, order=2, oomkilladj=0

order=2 requires 16kb page.
checkd invoked oom-killer: gfp_mask=0x44d0, order=2, oomkilladj=0
> checkd invoked oom-killer: gfp_mask=0x44d0, order=2, oomkilladj=0
> [<c00328c8>] (dump_stack+0x0/0x14) from [<c006baf0>] (oom_kill_process+0x104/0x1cc)
> [<c006b9ec>] (oom_kill_process+0x0/0x1cc) from [<c006bf44>] (out_of_memory+0x1b8/0x200)
> [<c006bd8c>] (out_of_memory+0x0/0x200) from [<c006ea34>] (__alloc_pages_internal+0x2e8/0x3d4)
> [<c006e74c>] (__alloc_pages_internal+0x0/0x3d4) from [<c006eb40>] (__get_free_pages+0x20/0x54)
> [<c006eb20>] (__get_free_pages+0x0/0x54) from [<c008bee8>] (__kmalloc_track_caller+0xb8/0xd8)
> [<c008be30>] (__kmalloc_track_caller+0x0/0xd8) from [<c0214064>] (__alloc_skb+0x5c/0x100)
> r8:c020f610 r7:c0354128 r6:00003ec0 r5:00003ec0 r4:cf1c26c0
> [<c0214008>] (__alloc_skb+0x0/0x100) from [<c020f610>] (sock_alloc_send_skb+0x1e4/0x260)
> [<c020f42c>] (sock_alloc_send_skb+0x0/0x260) from [<c0271488>] (unix_stream_sendmsg+0x1ec/0x2f4)
> [<c027129c>] (unix_stream_sendmsg+0x0/0x2f4) from [<c020c5c8>] (sock_aio_write+0xf8/0xfc)
> [<c020c4d0>] (sock_aio_write+0x0/0xfc) from [<c008dfe4>] (do_sync_write+0xc4/0x108)
> [<c008df20>] (do_sync_write+0x0/0x108) from [<c008e974>] (vfs_write+0x13c/0x144)
> r8:c002f004 r7:cf093f78 r6:00007b8e r5:bee9db40 r4:c69ad980
> [<c008e838>] (vfs_write+0x0/0x144) from [<c008edc0>] (sys_write+0x44/0x74)
> r7:00000000 r6:00000000 r5:fffffff7 r4:c69ad980
> [<c008ed7c>] (sys_write+0x0/0x74) from [<c002ee80>] (ret_fast_syscall+0x0/0x2c)
> r7:00000004 r6:bee9db40 r5:00000016 r4:00007b8e
> Mem-info:
> Normal per-cpu:
> CPU 0: hi: 90, btch: 15 usd: 0
> Active_anon:8449 active_file:0 inactive_anon:10986
> inactive_file:14 unevictable:32228 dirty:0 writeback:14 unstable:0

Almost all used pages is for anon and this system has no swap.

> free:5099 slab:1535 mapped:1381 pagetables:140 bounce:0
> Normal free:20396kB min:1996kB low:2492kB high:2992kB active_anon:33796kB inactive_anon:43944kB active_file:0kB inactive_file:56kB
> unevictable:128912kB present:249936kB pages_scanned:0 all_unreclaimable? no
> handle_end_of_frame: 880 remained in px DMA-desc
> lowmem_reserve[]: 0 0
> Normal: 1445*4kB 1781*8kB 15*16kB 2*32kB 1*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 20396kB

Here, almost all free pages are for low-order ones.

Considering zone_watermark_ok()'s page-order check, (used internal in alloc_pages())
1242 for (o = 0; o < order; o++) {
1243 /* At the next order, this order's pages become unavailable */
1244 free_pages -= z->free_area[o].nr_free << o;
1246 /* Require fewer higher order pages to be free */
1247 min >>= 1;
1249 if (free_pages <= min)
1250 return 0;
1251 }
return 1
Assume free_pages=5099.
At order-0,
free_pages = 5099 - 1445*1 = 3654 > (min/2)=998
free_pages = 3654 - 1781*2 = 92 < (min/4)=499

Then, zone_watermark_ok() fails
=> go into try_to_free_page()
=> but almost all pages are anon and there are no swap at all.

Then, I think.
1st reason is fragmentation.
2nd reason is noswap.
3rd reason is high-order allocation for socket.

One easy workaround I can think of is making UNIX domain socket's SNDBUF
size smaller. This can be modified by sysctl, IIUC.

But, hmm, order=2 is not very high. So, reducing memory usage may be a
choice if noswap.


> 48566 total pagecache pages
> 62976 pages of RAM
> 5256 free pages
> 1487 reserved pages
> 1388 slab pages
> 6670 pages shared
> 0 pages swap cached
> Out of memory: kill process 995 (httpd) score 2646 or a child
> Killed process 2491 (stream.cgi)
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