我有一台配备 8 个 domU 的 xen 服务器。服务器是 Xeon E31270，配备 16GB 内存。我认为它足够 8 台机器使用。有时 domU 会崩溃，我不知道原因。
崩溃后我可以连接到控制台，并且出现如下情况：
Oct 8 22:20:49 server kernel: [30892.320780] lowmem_reserve[]: 0 0 0 0
Oct 8 22:20:49 server kernel: [30892.320790] Node 0 DMA: 10*4kB 3*8kB 13*16kB 10*32kB 7*64kB 3*128kB 2*256kB 2*512kB 1*1024kB 2*2048kB 0*4096kB = 8080kB
Oct 8 22:20:49 server kernel: [30892.320817] Node 0 DMA32: 648*4kB 2*8kB 1*16kB 0*32kB 1*64kB 0*128kB 0*256kB 0*512kB 1*1024kB 1*2048kB 0*4096kB = 5760kB
Oct 8 22:20:49 server kernel: [30892.320842] 1491 total pagecache pages
Oct 8 22:20:49 server kernel: [30892.320847] 0 pages in swap cache
Oct 8 22:20:49 server kernel: [30892.320852] Swap cache stats: add 0, delete 0, find 0/0
Oct 8 22:20:49 server kernel: [30892.320858] Free swap = 0kB
Oct 8 22:20:49 server kernel: [30892.320862] Total swap = 0kB
Oct 8 22:20:49 server kernel: [30892.324024] 524288 pages RAM
Oct 8 22:20:49 server kernel: [30892.324024] 11010 pages reserved
Oct 8 22:20:49 server kernel: [30892.324024] 424467 pages shared
Oct 8 22:20:49 server kernel: [30892.324024] 503538 pages non-shared
Oct 8 22:20:49 server kernel: [30892.330308] apache2 invoked oom-killer: gfp_mask=0x200da, order=0, oom_adj=0
Oct 8 22:20:49 server kernel: [30892.330322] apache2 cpuset=/ mems_allowed=0
Oct 8 22:20:49 server kernel: [30892.330330] Pid: 23938, comm: apache2 Not tainted 2.6.32-5-xen-amd64 #1
Oct 8 22:20:49 server kernel: [30892.330337] Call Trace:
Oct 8 22:20:49 server kernel: [30892.330349] [<ffffffff810b7180>] ? oom_kill_process+0x7f/0x23f
Oct 8 22:20:49 server kernel: [30892.330358] [<ffffffff810b76a4>] ? __out_of_memory+0x12a/0x141
Oct 8 22:20:49 server kernel: [30892.330367] [<ffffffff810b77fb>] ? out_of_memory+0x140/0x172
Oct 8 22:20:49 server kernel: [30892.330376] [<ffffffff810bb59c>] ? __alloc_pages_nodemask+0x4e5/0x5f5
Oct 8 22:20:49 server kernel: [30892.330385] [<ffffffff810cc224>] ? do_wp_page+0x386/0x707
Oct 8 22:20:49 server kernel: [30892.330395] [<ffffffff8100c3a5>] ? __raw_callee_save_xen_pud_val+0x11/0x1e
Oct 8 22:20:49 server kernel: [30892.330404] [<ffffffff8100c369>] ? __raw_callee_save_xen_pmd_val+0x11/0x1e
Oct 8 22:20:49 server kernel: [30892.330412] [<ffffffff810cdfc7>] ? handle_mm_fault+0x7aa/0x80f
Oct 8 22:20:49 server kernel: [30892.330422] [<ffffffff8130f906>] ? do_page_fault+0x2e0/0x2fc
Oct 8 22:20:49 server kernel: [30892.330433] [<ffffffff8130d7a5>] ? page_fault+0x25/0x30
Oct 8 22:20:49 server kernel: [30892.330439] Mem-Info:
Oct 8 22:20:49 server kernel: [30892.330443] Node 0 DMA per-cpu:
Oct 8 22:20:49 server kernel: [30892.330450] CPU 0: hi: 0, btch: 1 usd: 0
Oct 8 22:20:49 server kernel: [30892.330463] CPU 1: hi: 0, btch: 1 usd: 0
Oct 8 22:20:49 server kernel: [30892.330466] Node 0 DMA32 per-cpu:
Oct 8 22:20:49 server kernel: [30892.330469] CPU 0: hi: 186, btch: 31 usd: 0
Oct 8 22:20:49 server kernel: [30892.330472] CPU 1: hi: 186, btch: 31 usd: 60
Oct 8 22:20:49 server kernel: [30892.330476] active_anon:342076 inactive_anon:115398 isolated_anon:0
Oct 8 22:20:49 server kernel: [30892.330477] active_file:268 inactive_file:481 isolated_file:0
Oct 8 22:20:49 server kernel: [30892.330477] unevictable:1125 dirty:2 writeback:13 unstable:0
Oct 8 22:20:49 server kernel: [30892.330478] free:3410 slab_reclaimable:1718 slab_unreclaimable:6946
Oct 8 22:20:49 server kernel: [30892.330478] mapped:899 shmem:113 pagetables:35697 bounce:0
Oct 8 22:20:49 server kernel: [30892.330502] Node 0 DMA free:8036kB min:32kB low:40kB high:48kB active_anon:1144kB inactive_anon:1268kB active_file:8kB inactive_file:8kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:11792kB mlocked:0kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:0kB slab_unreclaimable:224kB kernel_stack:16kB pagetables:1228kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
Oct 8 22:20:49 server kernel: [30892.330518] lowmem_reserve[]: 0 2004 2004 2004
Oct 8 22:20:49 server kernel: [30892.330523] Node 0 DMA32 free:5604kB min:5708kB low:7132kB high:8560kB active_anon:1367160kB inactive_anon:460324kB active_file:1064kB inactive_file:1916kB unevictable:4500kB isolated(anon):0kB isolated(file):0kB present:2052320kB mlocked:4500kB dirty:8kB writeback:52kB mapped:3600kB shmem:452kB slab_reclaimable:6872kB slab_unreclaimable:27560kB kernel_stack:3528kB pagetables:141560kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:992 all_unreclaimable? no
Oct 8 22:20:49 server kernel: [30892.330539] lowmem_reserve[]: 0 0 0 0
Oct 8 22:20:49 server kernel: [30892.330544] Node 0 DMA: 1*4kB 2*8kB 13*16kB 10*32kB 7*64kB 3*128kB 2*256kB 2*512kB 1*1024kB 2*2048kB 0*4096kB = 8036kB
Oct 8 22:20:49 server kernel: [30892.330579] Node 0 DMA32: 609*4kB 2*8kB 1*16kB 0*32kB 1*64kB 0*128kB 0*256kB 0*512kB 1*1024kB 1*2048kB 0*4096kB = 5604kB
Oct 8 22:20:49 server kernel: [30892.330605] 1522 total pagecache pages
Oct 8 22:20:49 server kernel: [30892.330610] 0 pages in swap cache
Oct 8 22:20:49 server kernel: [30892.330615] Swap cache stats: add 0, delete 0, find 0/0
Oct 8 22:20:49 server kernel: [30892.330621] Free swap = 0kB
Oct 8 22:20:49 server kernel: [30892.330625] Total swap = 0kB
Oct 8 22:20:49 server kernel: [30892.333018] 524288 pages RAM
Oct 8 22:20:49 server kernel: [30892.333018] 11010 pages reserved
Oct 8 22:20:49 server kernel: [30892.333018] 424367 pages shared
Oct 8 22:20:49 server kernel: [30892.333018] 503658 pages non-shared
看起来这个 domU 内存不足。但是 munin 监控中没有报告任何内存问题：
内存使用情况http://xmages.net/storage/10/1/0/1/c/upload/d9a9f7b8.png
如您所见，系统使用了大约 0.2G，还有 1G 可用。
我的问题是：
这是 xen 特有的问题吗，实际内存使用情况和 munin 显示的内存使用情况不同（我在真实的硬件机器上从未见过这样的问题）？
或者这可能只是监控问题，无法捕捉到异常高负载和 domU 关闭的时刻？
我该如何解决这个问题呢？在电子邮件中收到 domU 瘫痪的消息真的很烦人。
顺便说一句，这种情况是当 domU 有 2G 内存时出现的。
答案1
看来您的 DomU 配置为没有交换。根据我的经验，完全缺乏交换空间会导致各种奇怪的问题。给您的 DomU 一些交换空间，看看它是否会表现得更好。
dd if=/dev/zero of=/path/with/enough/diskspace/swapfile bs=1M count=1024
mkswap /path/with/enough/diskspace/swapfile
swapon /path/with/enough/diskspace/swapfile
这将为您创建一个 1 GB 的交换文件，直到下次重新启动。如果您想永久使用该交换，请将其添加到/etc/fstab：
/path/with/enough/diskspace/swapfile swap swap defaults 0 0
我并不是说服务器应该进行大量交换或者根本不进行交换，但是如果 Linux 知道它有可用的交换空间，它就会更加高兴，OOM 触发的几率就会降低。