一、一些概念
1. OOM killer
顧名思義，OOM(out of memory) killer，是Linux作業系統發現記憶體不足時，它會強制殺死一些使用者程式（非核心程式），來保證系統有足夠的實體記憶體進行分配。
2. 記憶體overcommit
Linux對大部分申請記憶體的請求都回復”yes”，以便能跑更多更大的程式。因為申請記憶體後，並不會馬上使用記憶體。這種技術叫做Overcommit。
vm.overcommit_memory這個系統引數是用來設定記憶體分配策略的，它有三個可選值：
| vm.overcommit_memory | 含義 |
| 0 |
表示核心將檢查是否有足夠的可用記憶體供應用程式使用；如果有足夠的可用記憶體，記憶體申請允許；否則，記憶體申請失敗，並把錯誤返回給應用程式。 |
| 1 |
表示核心允許分配所有的可用記憶體，而不管當前的記憶體狀態如何。 |
| 2 |
表示核心允許分配超過所有實體記憶體和交換空間總和的記憶體 |
OOM killer與記憶體overcommit：兩者是相互作用的：
對於一臺16G的Linux伺服器。
如果使用overcommit_memory=0，如果沒有沒有足夠的記憶體分配，那麼應用想分配記憶體就會失敗。
如果使用overcommit_memory>0，每次的應用記憶體申請都會成功，這種成功是建立在OOM killer會殺掉部分程式來實現的。
二、試驗：
1. 環境：
(1) 記憶體：沒有開啟swap，可用記憶體12397M
total used free shared buffers cached Mem: 15948 3606 12342 0 10 45 -/+ buffers/cache: 3550 12397 Swap: 0 0 0
(2) overcommit_memory當前是0
cat /proc/sys/vm/overcommit_memory 0
2. 開啟應用強制吃記憶體。
我對於redis稍微熟悉一點，redis-server有個引數叫做–test-memory，可以吃掉系統的記憶體，單位是M
redis-server --test-memory 1024
(1) 開啟一個redis-server –test-memory 5120，吃掉5G記憶體，記憶體變為：
[@zw_53_159 fl]# free -m
total used free shared buffers cached
Mem: 15948 8704 7243 0 10 47
-/+ buffers/cache: 8646 7301
Swap: 0 0 0
(2) 再開啟一個redis-server –test-memory 5120，吃掉5G記憶體，記憶體變為：
[@zw_53_159 fl]# free -m
total used free shared buffers cached
Mem: 15948 13839 2109 0 10 47
-/+ buffers/cache: 13781 2166
Swap: 0 0 0
(3) 此時還有2166可用記憶體，如果在開啟3G的Redis會怎樣呢？
[@zw_53_159 ~]# redis-server --test-memory 3072 Unable to allocate 3072 megabytes: Cannot allocate memory
沒有足夠的記憶體了？為什麼呢：
- 沒有swap可用
- overcommit_memory=0，不會用overcommit機制。
(4) 修改overcommit_memory=1，
echo "vm.overcommit_memory = 1" >> /etc/sysctl.conf sysctl vm.overcommit_memory=1
(a) 當前系統只有一些Redis程式：
root 26738 21729 99 15:03 pts/5 00:06:10 redis-server --test-memory 5120 root 26910 22005 99 15:04 pts/7 00:05:34 redis-server --test-memory 5120
(b) 觀察OOM killer日誌：
tail -f /var/log/messages
繼續開啟3G的Redis: 開啟成功，但是OOM把之前一個redis-server給kill掉了，從兩個方面看
(a) 程式：程式26738不見了。
root 26910 22005 99 15:04 pts/7 00:08:46 redis-server --test-memory 5120 root 27805 21964 93 15:12 pts/6 00:00:27 redis-server --test-memory 3072
(b) OOM killer日誌：裡面涉及到細節很多，例如計算使用者程式的分數，這個分數（oom_score_adj）決定該使用者程式被殺掉的可能性，
最後算出：Out of memory: Kill process 26738 (redis-server) score 291 or sacrifice child Killed process 26738, UID 0, (redis-server) total-vm:5359844kB, anon-rss:5247132kB, file-rss:16kB
Feb 17 15:12:53 zw_53_159 kernel: redis-server invoked oom-killer: gfp_mask=0x201da, order=0, oom_adj=0, oom_score_adj=0 Feb 17 15:12:53 zw_53_159 kernel: redis-server cpuset=/ mems_allowed=0 Feb 17 15:12:53 zw_53_159 kernel: Pid: 32518, comm: redis-server Not tainted 2.6.32-279.el6.x86_64 #1 Feb 17 15:12:53 zw_53_159 kernel: Call Trace: Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff810c4971>] ? cpuset_print_task_mems_allowed+0x91/0xb0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff811170e0>] ? dump_header+0x90/0x1b0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff812146fc>] ? security_real_capable_noaudit+0x3c/0x70 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff81117562>] ? oom_kill_process+0x82/0x2a0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff811174a1>] ? select_bad_process+0xe1/0x120 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff811179a0>] ? out_of_memory+0x220/0x3c0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff811276be>] ? __alloc_pages_nodemask+0x89e/0x940 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8115c1da>] ? alloc_pages_current+0xaa/0x110 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff811144e7>] ? __page_cache_alloc+0x87/0x90 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8112a10b>] ? __do_page_cache_readahead+0xdb/0x210 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8112a261>] ? ra_submit+0x21/0x30 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff81115813>] ? filemap_fault+0x4c3/0x500 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8113ec14>] ? __do_fault+0x54/0x510 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8113f1c7>] ? handle_pte_fault+0xf7/0xb50 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8113fe04>] ? handle_mm_fault+0x1e4/0x2b0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff81044479>] ? __do_page_fault+0x139/0x480 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff81039678>] ? pvclock_clocksource_read+0x58/0xd0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8103876c>] ? kvm_clock_read+0x1c/0x20 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff81038779>] ? kvm_clock_get_cycles+0x9/0x10 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8109c9a0>] ? getnstimeofday+0x60/0xf0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff8150326e>] ? do_page_fault+0x3e/0xa0 Feb 17 15:12:53 zw_53_159 kernel: [<ffffffff81500625>] ? page_fault+0x25/0x30 Feb 17 15:12:53 zw_53_159 kernel: Mem-Info: Feb 17 15:12:53 zw_53_159 kernel: Node 0 DMA per-cpu: Feb 17 15:12:53 zw_53_159 kernel: CPU 0: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 1: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 2: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 3: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 4: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 5: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 6: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 7: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 8: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 9: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 10: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 11: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 12: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 13: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 14: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 15: hi: 0, btch: 1 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: Node 0 DMA32 per-cpu: Feb 17 15:12:53 zw_53_159 kernel: CPU 0: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 1: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 2: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 3: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 4: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 5: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 6: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 7: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 8: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 9: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 10: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 11: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 12: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 13: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 14: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 15: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: Node 0 Normal per-cpu: Feb 17 15:12:53 zw_53_159 kernel: CPU 0: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 1: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 2: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 3: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 4: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 5: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 6: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 7: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 8: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 9: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 10: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 11: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 12: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 13: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 14: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: CPU 15: hi: 186, btch: 31 usd: 0 Feb 17 15:12:53 zw_53_159 kernel: active_anon:3223340 inactive_anon:756981 isolated_anon:9 Feb 17 15:12:53 zw_53_159 kernel: active_file:234 inactive_file:91 isolated_file:0 Feb 17 15:12:53 zw_53_159 kernel: unevictable:0 dirty:0 writeback:0 unstable:0 Feb 17 15:12:53 zw_53_159 kernel: free:33241 slab_reclaimable:2879 slab_unreclaimable:8604 Feb 17 15:12:53 zw_53_159 kernel: mapped:139 shmem:55 pagetables:10616 bounce:0 Feb 17 15:12:53 zw_53_159 kernel: Node 0 DMA free:15676kB min:60kB low:72kB high:88kB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:15272kB mlocked:0kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? yes Feb 17 15:12:53 zw_53_159 kernel: lowmem_reserve[]: 0 3512 16137 16137 Feb 17 15:12:53 zw_53_159 kernel: Node 0 DMA32 free:65068kB min:14692kB low:18364kB high:22036kB active_anon:3141792kB inactive_anon:404kB active_file:48kB inactive_file:32kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:3596496kB mlocked:0kB dirty:0kB writeback:0kB mapped:80kB shmem:4kB slab_reclaimable:204kB slab_unreclaimable:48kB kernel_stack:16kB pagetables:276kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no Feb 17 15:12:53 zw_53_159 kernel: lowmem_reserve[]: 0 0 12625 12625 Feb 17 15:12:53 zw_53_159 kernel: Node 0 Normal free:52220kB min:52824kB low:66028kB high:79236kB active_anon:9751568kB inactive_anon:3027520kB active_file:888kB inactive_file:332kB unevictable:0kB isolated(anon):36kB isolated(file):0kB present:12928000kB mlocked:0kB dirty:0kB writeback:0kB mapped:476kB shmem:216kB slab_reclaimable:11312kB slab_unreclaimable:34368kB kernel_stack:3472kB pagetables:42188kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:146 all_unreclaimable? no Feb 17 15:12:53 zw_53_159 kernel: lowmem_reserve[]: 0 0 0 0 Feb 17 15:12:53 zw_53_159 kernel: Node 0 DMA: 3*4kB 2*8kB 0*16kB 1*32kB 2*64kB 1*128kB 0*256kB 0*512kB 1*1024kB 1*2048kB 3*4096kB = 15676kB Feb 17 15:12:53 zw_53_159 kernel: Node 0 DMA32: 81*4kB 73*8kB 64*16kB 48*32kB 36*64kB 31*128kB 29*256kB 30*512kB 30*1024kB 1*2048kB 0*4096kB = 65292kB Feb 17 15:12:53 zw_53_159 kernel: Node 0 Normal: 753*4kB 473*8kB 314*16kB 216*32kB 162*64kB 86*128kB 31*256kB 10*512kB 0*1024kB 0*2048kB 0*4096kB = 53164kB Feb 17 15:12:53 zw_53_159 kernel: 335 total pagecache pages Feb 17 15:12:53 zw_53_159 kernel: 0 pages in swap cache Feb 17 15:12:53 zw_53_159 kernel: Swap cache stats: add 53710499, delete 53710499, find 25961598/29739317 Feb 17 15:12:53 zw_53_159 kernel: Free swap = 0kB Feb 17 15:12:53 zw_53_159 kernel: Total swap = 0kB Feb 17 15:12:53 zw_53_159 kernel: 4194303 pages RAM Feb 17 15:12:53 zw_53_159 kernel: 111523 pages reserved Feb 17 15:12:53 zw_53_159 kernel: 1456 pages shared Feb 17 15:12:53 zw_53_159 kernel: 4044970 pages non-shared Feb 17 15:12:53 zw_53_159 kernel: [ pid ] uid tgid total_vm rss cpu oom_adj oom_score_adj name Feb 17 15:12:53 zw_53_159 kernel: [ 651] 0 651 2750 188 0 -17 -1000 udevd Feb 17 15:12:53 zw_53_159 kernel: [ 1494] 0 1494 1538 27 1 0 0 portreserve Feb 17 15:12:53 zw_53_159 kernel: [ 1501] 0 1501 62992 1755 0 0 0 rsyslogd Feb 17 15:12:53 zw_53_159 kernel: [ 1518] 0 1518 2763 194 0 -17 -1000 udevd Feb 17 15:12:53 zw_53_159 kernel: [ 1543] 0 1543 2285 55 2 0 0 irqbalance Feb 17 15:12:53 zw_53_159 kernel: [ 1561] 32 1561 4742 58 0 0 0 rpcbind Feb 17 15:12:53 zw_53_159 kernel: [ 1746] 0 1746 19667 210 4 0 0 master Feb 17 15:12:53 zw_53_159 kernel: [ 1766] 89 1766 19729 213 0 0 0 qmgr Feb 17 15:12:53 zw_53_159 kernel: [ 1778] 0 1778 27541 36 0 0 0 abrtd Feb 17 15:12:53 zw_53_159 kernel: [ 1786] 0 1786 27016 53 6 0 0 abrt-dump-oops Feb 17 15:12:53 zw_53_159 kernel: [ 1810] 0 1810 1754 27 8 0 0 rhsmcertd Feb 17 15:12:53 zw_53_159 kernel: [ 1855] 0 1855 1014 21 5 0 0 mingetty Feb 17 15:12:53 zw_53_159 kernel: [ 1857] 0 1857 1014 22 2 0 0 mingetty Feb 17 15:12:53 zw_53_159 kernel: [ 1859] 0 1859 1014 22 2 0 0 mingetty Feb 17 15:12:53 zw_53_159 kernel: [ 1861] 0 1861 1014 21 5 0 0 mingetty Feb 17 15:12:53 zw_53_159 kernel: [ 1862] 0 1862 2749 187 9 -17 -1000 udevd Feb 17 15:12:53 zw_53_159 kernel: [ 1864] 0 1864 1014 22 3 0 0 mingetty Feb 17 15:12:53 zw_53_159 kernel: [ 1865] 0 1865 1017 22 0 0 0 agetty Feb 17 15:12:53 zw_53_159 kernel: [ 1867] 0 1867 1014 22 13 0 0 mingetty Feb 17 15:12:53 zw_53_159 kernel: [ 2502] 0 2502 9198 108 12 0 0 keepalived Feb 17 15:12:53 zw_53_159 kernel: [ 2503] 0 2503 9724 140 4 0 0 keepalived Feb 17 15:12:53 zw_53_159 kernel: [ 2504] 0 2504 9724 127 6 0 0 keepalived Feb 17 15:12:53 zw_53_159 kernel: [ 2695] 0 2695 9198 108 8 0 0 keepalived Feb 17 15:12:53 zw_53_159 kernel: [ 2696] 0 2696 9724 141 8 0 0 keepalived Feb 17 15:12:53 zw_53_159 kernel: [ 2697] 0 2697 9724 128 12 0 0 keepalived Feb 17 15:12:53 zw_53_159 kernel: [ 2515] 0 2515 5066 127 0 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [31653] 0 31653 1561 24 0 0 0 oscmd Feb 17 15:12:53 zw_53_159 kernel: [16865] 99 16865 5166 225 8 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16866] 99 16866 5166 225 0 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16867] 99 16867 5166 225 12 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16868] 99 16868 5166 225 2 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16869] 99 16869 5166 225 15 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16870] 99 16870 5166 225 5 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16871] 99 16871 5166 225 0 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [16872] 99 16872 5166 225 2 0 0 nginx Feb 17 15:12:53 zw_53_159 kernel: [26947] 0 26947 16018 166 0 -17 -1000 sshd Feb 17 15:12:53 zw_53_159 kernel: [ 2739] 0 2739 29303 159 10 0 0 crond Feb 17 15:12:53 zw_53_159 kernel: [ 6394] 0 6394 19966 172 0 0 0 monitor Feb 17 15:12:53 zw_53_159 kernel: [ 6401] 0 6401 64605 3298 14 0 0 monitor_cron Feb 17 15:12:53 zw_53_159 kernel: [ 6402] 0 6402 19529 117 0 0 0 monitor_agent Feb 17 15:12:53 zw_53_159 kernel: [31724] 0 31724 4816 40 2 0 0 nutcracker Feb 17 15:12:53 zw_53_159 kernel: [ 7144] 0 7144 34895 1490 4 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [31641] 0 31641 34361 1517 10 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [31649] 0 31649 34361 2098 14 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [32512] 0 32512 34361 1614 7 0 0 redis-sentinel Feb 17 15:12:53 zw_53_159 kernel: [32518] 0 32518 34361 1614 0 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [32524] 0 32524 34361 1614 13 0 0 redis-sentinel Feb 17 15:12:53 zw_53_159 kernel: [ 7333] 0 7333 34361 1507 4 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [17108] 0 17108 180793 1783 13 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [21470] 0 21470 851513 785994 7 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [ 5708] 0 5708 34361 2125 6 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [ 5716] 0 5716 34361 2061 4 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [ 5724] 0 5724 34361 2122 10 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [ 5732] 0 5732 34361 2124 7 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [ 6214] 0 6214 34361 2091 6 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [29556] 0 29556 34361 1563 4 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [32104] 0 32104 34361 1614 13 0 0 redis-sentinel Feb 17 15:12:53 zw_53_159 kernel: [32539] 0 32539 34361 1614 11 0 0 redis-sentinel Feb 17 15:12:53 zw_53_159 kernel: [14316] 0 14316 442937 8430 4 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [32038] 0 32038 24454 234 0 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [32042] 0 32042 27119 128 1 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [ 3901] 0 3901 24454 231 0 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [ 3905] 0 3905 27118 117 5 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [19921] 89 19921 19691 206 6 0 0 pickup Feb 17 15:12:53 zw_53_159 kernel: [20382] 0 20382 24454 235 0 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [20386] 0 20386 27119 116 10 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [20412] 0 20412 25235 22 6 0 0 tail Feb 17 15:12:53 zw_53_159 kernel: [20413] 0 20413 25813 34 6 0 0 grep Feb 17 15:12:53 zw_53_159 kernel: [21515] 0 21515 24454 236 0 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [21519] 0 21519 27119 119 0 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [21725] 0 21725 24454 237 13 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [21729] 0 21729 27119 117 1 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [21960] 0 21960 24454 234 10 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [21964] 0 21964 27119 125 5 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [22001] 0 22001 24454 247 13 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [22005] 0 22005 27119 118 0 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [22039] 0 22039 24454 234 10 0 0 sshd Feb 17 15:12:53 zw_53_159 kernel: [22043] 0 22043 27119 127 8 0 0 bash Feb 17 15:12:53 zw_53_159 kernel: [26738] 0 26738 1339961 1311787 1 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [26910] 0 26910 1339961 1311787 14 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: [27566] 0 27566 25235 22 10 0 0 tail Feb 17 15:12:53 zw_53_159 kernel: [27567] 0 27567 25813 32 10 0 0 grep Feb 17 15:12:53 zw_53_159 kernel: [27805] 0 27805 815673 522185 13 0 0 redis-server Feb 17 15:12:53 zw_53_159 kernel: Out of memory: Kill process 26738 (redis-server) score 291 or sacrifice child Feb 17 15:12:53 zw_53_159 kernel: Killed process 26738, UID 0, (redis-server) total-vm:5359844kB, anon-rss:5247132kB, file-rss:16kB ^C
三、結論：
overcommit機制雖然能夠保證，每次申請記憶體都能成功，但是也存在使用者程式被OOM killer的可能性，如果設定也會存在很大問題。
具體要設定成0或者>0，還是要取決於場景，注意本實驗沒有開啟swap。
測試伺服器關閉swap的方法是swapoff -a，未線上上測試過會有什麼影響，請小心使用。
四、後續測試：
1. 新增了swap之後，記憶體概括如下：
[@zw_53_159 ~]# free -m
total used free shared buffers cached
Mem: 15948 361 15587 0 3 13
-/+ buffers/cache: 344 15604
Swap: 16383 0 16383
2. 使用第二節進行測試，發現OOM killer是針對可用記憶體（實體記憶體+swap）才生效，不像網上有些文章說的只要實體記憶體不夠，就執行OOM killer
五、參考文獻
有關swap開關可以參考：Linux服務端swap配置和監控