我的老朋友 OOM 杀手来找我已经很久了。现在它每周来一次，我不知道它为什么会被触发。似乎不是我缺少内存。是什么原因造成的？可以做些什么来防止这种情况发生？
内核：3.13.0-49-通用#83-Ubuntu SMP
在触发 OOM killer 之前：
Fri Nov 16 01:45:01 EST 2018
COMMAND %MEM
mysqld 17.2
apache2 7.2
spamd child 7.0
apache2 7.0
apache2 6.9
apache2 6.8
apache2 6.8
apache2 6.6
apache2 6.6
apache2 6.4
apache2 6.4
/usr/sbin/spamd 6.4
spamd child 6.2
apache2 5.6
opendkim 0.6
horde-alarms 0.5
fail2ban-server 0.5
apache2 0.4
munin-node 0.2
pickup 0.1
cron 0.1
cron 0.1
cron 0.1
mem.sh 0.1
init 0.1
ps 0.1
config 0.0
tlsmgr 0.0
ntpd 0.0
qmgr 0.0
sh 0.0
sh 0.0
sh 0.0
systemd-logind 0.0
dovecot 0.0
auth 0.0
whoopsie 0.0
rsyslogd 0.0
log 0.0
vnstatd 0.0
master 0.0
dbus-daemon 0.0
anvil 0.0
cron 0.0
upstart-file-br 0.0
php5-fpm 0.0
upstart-socket- 0.0
upstart-udev-br 0.0
memcached 0.0
sshd 0.0
shellinaboxd 0.0
getty 0.0
getty 0.0
getty 0.0
getty 0.0
getty 0.0
php5-fpm 0.0
php5-fpm 0.0
/usr/sbin/postg 0.0
getty 0.0
systemd-udevd 0.0
acpid 0.0
opendkim 0.0
kthreadd 0.0
ksoftirqd/0 0.0
kworker/0:0H 0.0
rcu_sched 0.0
rcuos/0 0.0
rcu_bh 0.0
rcuob/0 0.0
migration/0 0.0
watchdog/0 0.0
khelper 0.0
kdevtmpfs 0.0
netns 0.0
writeback 0.0
kintegrityd 0.0
bioset 0.0
kworker/u3:0 0.0
kblockd 0.0
ata_sff 0.0
khubd 0.0
md 0.0
devfreq_wq 0.0
kworker/0:1 0.0
khungtaskd 0.0
kswapd0 0.0
ksmd 0.0
khugepaged 0.0
fsnotify_mark 0.0
ecryptfs-kthrea 0.0
crypto 0.0
kthrotld 0.0
vballoon 0.0
scsi_eh_0 0.0
scsi_eh_1 0.0
deferwq 0.0
charger_manager 0.0
scsi_eh_2 0.0
kpsmoused 0.0
kworker/0:2 0.0
ttm_swap 0.0
qxl_gc 0.0
jbd2/vda1-8 0.0
ext4-rsv-conver 0.0
kvm-irqfd-clean 0.0
atd 0.0
shellinaboxd 0.0
managesieve-log 0.0
kworker/u3:1 0.0
kauditd 0.0
kworker/u2:2 0.0
kworker/u2:0 0.0
Memory Space Details
Total Memory space : 2018 MB
Used Memory Space : 1055 MB
Free Memory : 962 MB
Swap memory Details
Total Swap space : 1023 MB
Used Swap Space : 164 MB
Free Swap : 859 MB
Node 0, zone DMA 9 12 6 8 7 4 1 2 2 0 0
Node 0, zone DMA32 2202 535 425 464 304 89 26 24 12 1 0
系统日志：
Nov 16 01:46:34 foobar kernel: [2269670.312226] apache2 invoked oom-killer: gfp_mask=0x200da, order=0, oom_score_adj=0
Nov 16 01:46:34 foobar kernel: [2269670.312231] apache2 cpuset=/ mems_allowed=0
Nov 16 01:46:34 foobar kernel: [2269670.312234] CPU: 0 PID: 14040 Comm: apache2 Not tainted 3.13.0-49-generic #83-Ubuntu
Nov 16 01:46:34 foobar kernel: [2269670.312235] Hardware name: DigitalOcean Droplet, BIOS 20171212 12/12/2017
Nov 16 01:46:34 foobar kernel: [2269670.312237] 0000000000000000 ffff880003f3b988 ffffffff81721b36 ffff8800041f1800
Nov 16 01:46:34 foobar kernel: [2269670.312240] ffff880003f3ba10 ffffffff8171c3f1 ffffffff81067996 ffff880003f3b9e8
Nov 16 01:46:34 foobar kernel: [2269670.312242] ffffffff810c75dc 0000000000000000 ffff88003fff7e28 0000000000000000
Nov 16 01:46:34 foobar kernel: [2269670.312244] Call Trace:
Nov 16 01:46:34 foobar kernel: [2269670.312250] [<ffffffff81721b36>] dump_stack+0x45/0x56
Nov 16 01:46:34 foobar kernel: [2269670.312257] [<ffffffff8171c3f1>] dump_header+0x7f/0x1f1
Nov 16 01:46:34 foobar kernel: [2269670.312262] [<ffffffff81067996>] ? put_online_cpus+0x56/0x80
Nov 16 01:46:34 foobar kernel: [2269670.312273] [<ffffffff810c75dc>] ? rcu_oom_notify+0xcc/0xf0
Nov 16 01:46:34 foobar kernel: [2269670.312276] [<ffffffff811529ce>] oom_kill_process+0x1ce/0x330
Nov 16 01:46:34 foobar kernel: [2269670.312280] [<ffffffff812d75f5>] ? security_capable_noaudit+0x15/0x20
Nov 16 01:46:34 foobar kernel: [2269670.312282] [<ffffffff81153104>] out_of_memory+0x414/0x450
Nov 16 01:46:34 foobar kernel: [2269670.312285] [<ffffffff81159470>] __alloc_pages_nodemask+0xa60/0xb80
Nov 16 01:46:34 foobar kernel: [2269670.312289] [<ffffffff81199dda>] alloc_pages_vma+0x9a/0x140
Nov 16 01:46:34 foobar kernel: [2269670.312292] [<ffffffff8118c5ab>] read_swap_cache_async+0xeb/0x160
Nov 16 01:46:34 foobar kernel: [2269670.312294] [<ffffffff8118c6b8>] swapin_readahead+0x98/0xe0
Nov 16 01:46:34 foobar kernel: [2269670.312297] [<ffffffff8117a536>] handle_mm_fault+0xa76/0xf10
Nov 16 01:46:34 foobar kernel: [2269670.312301] [<ffffffff8172db74>] __do_page_fault+0x184/0x570
Nov 16 01:46:34 foobar kernel: [2269670.312305] [<ffffffff811ceb6f>] ? user_path_at_empty+0x5f/0x90
Nov 16 01:46:34 foobar kernel: [2269670.312308] [<ffffffff810a02c5>] ? set_next_entity+0x95/0xb0
Nov 16 01:46:34 foobar kernel: [2269670.312312] [<ffffffff8101260b>] ? __switch_to+0x16b/0x4d0
Nov 16 01:46:34 foobar kernel: [2269670.312314] [<ffffffff8172df7a>] do_page_fault+0x1a/0x70
Nov 16 01:46:34 foobar kernel: [2269670.312316] [<ffffffff8172d5e9>] do_async_page_fault+0x29/0xe0
Nov 16 01:46:34 foobar kernel: [2269670.312320] Mem-Info:
Nov 16 01:46:34 foobar kernel: [2269670.312321] Node 0 DMA per-cpu:
Nov 16 01:46:34 foobar kernel: [2269670.312323] CPU 0: hi: 0, btch: 1 usd: 0
Nov 16 01:46:34 foobar kernel: [2269670.312324] Node 0 DMA32 per-cpu:
Nov 16 01:46:34 foobar kernel: [2269670.312325] CPU 0: hi: 186, btch: 31 usd: 93
Nov 16 01:46:34 foobar kernel: [2269670.312329] active_anon:101755 inactive_anon:101941 isolated_anon:208
Nov 16 01:46:34 foobar kernel: [2269670.312329] active_file:294 inactive_file:309 isolated_file:64
Nov 16 01:46:34 foobar kernel: [2269670.312329] unevictable:0 dirty:0 writeback:63 unstable:0
Nov 16 01:46:34 foobar kernel: [2269670.312329] free:12210 slab_reclaimable:2743 slab_unreclaimable:8287
Nov 16 01:46:34 foobar kernel: [2269670.312329] mapped:2580 shmem:6561 pagetables:20656 bounce:0
Nov 16 01:46:34 foobar kernel: [2269670.312329] free_cma:0
Nov 16 01:46:34 foobar kernel: [2269670.312331] Node 0 DMA free:4596kB min:704kB low:880kB high:1056kB active_anon:4844kB i
nactive_anon:5424kB active_file:0kB inactive_file:0kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:15992kB ma
naged:15908kB mlocked:0kB dirty:0kB writeback:0kB mapped:92kB shmem:96kB slab_reclaimable:64kB slab_unreclaimable:232kB kernel
_stack:40kB pagetables:684kB unstable:0kB bounce:0kB free_cma:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? yes
Nov 16 01:46:34 foobar kernel: [2269670.312336] lowmem_reserve[]: 0 975 975 975
Nov 16 01:46:34 foobar kernel: [2269670.312338] Node 0 DMA32 free:44244kB min:44348kB low:55432kB high:66520kB active_anon:
402176kB inactive_anon:402340kB active_file:1176kB inactive_file:1236kB unevictable:0kB isolated(anon):832kB isolated(file):25
6kB present:1032172kB managed:1001956kB mlocked:0kB dirty:0kB writeback:252kB mapped:10228kB shmem:26148kB slab_reclaimable:10
908kB slab_unreclaimable:32916kB kernel_stack:2520kB pagetables:81940kB unstable:0kB bounce:0kB free_cma:0kB writeback_tmp:0kB
pages_scanned:10612 all_unreclaimable? yes
Nov 16 01:46:34 foobar kernel: [2269670.312342] lowmem_reserve[]: 0 0 0 0
Nov 16 01:46:34 foobar kernel: [2269670.312343] Node 0 DMA: 3*4kB (EM) 3*8kB (EM) 3*16kB (UE) 3*32kB (EM) 3*64kB (UMR) 3*12
8kB (UE) 3*256kB (UEM) 2*512kB (EM) 2*1024kB (EM) 0*2048kB 0*4096kB = 4596kB
Nov 16 01:46:34 foobar kernel: [2269670.312351] Node 0 DMA32: 297*4kB (EM) 610*8kB (UEM) 406*16kB (UEM) 234*32kB (UEM) 130*
64kB (UEM) 56*128kB (UEM) 20*256kB (UEM) 3*512kB (E) 0*1024kB 1*2048kB (R) 0*4096kB = 44244kB
Nov 16 01:46:34 foobar kernel: [2269670.312362] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2
048kB
Nov 16 01:46:34 foobar kernel: [2269670.312363] 14604 total pagecache pages
Nov 16 01:46:34 foobar kernel: [2269670.312365] 7377 pages in swap cache
Nov 16 01:46:34 foobar kernel: [2269670.312366] Swap cache stats: add 2208965, delete 2201588, find 55272636/55445769
Nov 16 01:46:34 foobar kernel: [2269670.312367] Free swap = 0kB
Nov 16 01:46:34 foobar kernel: [2269670.312367] Total swap = 1048572kB
Nov 16 01:46:34 foobar kernel: [2269670.312368] 262041 pages RAM
Nov 16 01:46:34 foobar kernel: [2269670.312369] 0 pages HighMem/MovableOnly
Nov 16 01:46:34 foobar kernel: [2269670.312370] 7554 pages reserved
Nov 16 01:46:34 foobar kernel: [2269670.312610] Out of memory: Kill process 1047 (mysqld) score 115 or sacrifice child
Nov 16 01:46:34 foobar kernel: [2269670.312637] Killed process 1047 (mysqld) total-vm:896532kB, anon-rss:103356kB, file-rss
:0kB
Nov 16 01:46:34 foobar kernel: [2269670.554632] init: mysql main process (1047) killed by KILL signal
Nov 16 01:46:34 foobar kernel: [2269670.554653] init: mysql main process ended, respawning
Nov 16 01:46:34 foobar kernel: [2269670.602286] audit_printk_skb: 9 callbacks suppressed
Nov 16 01:46:34 foobar kernel: [2269670.602290] type=1400 audit(1542350794.977:14): apparmor="STATUS" operation="profile_re
place" profile="unconfined" name="/usr/sbin/mysqld" pid=14117 comm="apparmor_parser"
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14200]: Upgrading MySQL tables if necessary.
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14204]: /usr/bin/mysql_upgrade: the '--basedir' option is always ignored
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14204]: Looking for 'mysql' as: /usr/bin/mysql
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14204]: Looking for 'mysqlcheck' as: /usr/bin/mysqlcheck
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14204]: This installation of MySQL is already upgraded to 5.5.61, use --forc
e if you still need to run mysql_upgrade
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14217]: Checking for insecure root accounts.
Nov 16 01:46:37 foobar /etc/mysql/debian-start[14223]: Triggering myisam-recover for all MyISAM tables
Nov 16 01:46:51 foobar kernel: [2269686.979583] apache2 invoked oom-killer: gfp_mask=0x200da, order=0, oom_score_adj=0
Nov 16 01:46:51 foobar kernel: [2269686.979588] apache2 cpuset=/ mems_allowed=0
答案1
这里有很多事情你需要做。
首先，OOM killer 是由阿帕奇2请求的内存超出了可用内存，这不是 mysqld 请求的。但由于 mysqld 进程当时占用的内存最多，因此该进程被终止。
考虑设置 sysctl vm.oom_kill_allocating_task，这将导致请求内存的进程被终止。
其次，您很可能已使用 mpm_prefork 配置了 Apache，并将 PHP 作为 Apache 模块运行。此设置速度慢、性能不佳、线程不安全，并且当 PHP 失败时，失败会被归咎于其他组件，因为 PHP 很好地隐藏在 Apache 内部。现代 Web 服务器设置使用 php-fpm 和 nginx 或带有 mpm_event 的 Apache。这将为您提供更好的性能和对 PHP 正在做什么的可见性（因为它很可能正在消耗您的所有内存）。
最后，您的 droplet 似乎有 1GB RAM 和 1GB 交换空间。如果您经常用完，那么您的 VM 可能太小了。考虑添加 RAM。
答案2
从 syslog 中我们可以看到，OOM killer 杀死了 mysql 服务器进程 mysqld
11 月 16 日 01:46:34 foobar 内核：[2269670.312610] 内存不足：终止进程 1047 (mysqld) 得分 115 或牺牲子进程
11 月 16 日 01:46:34 foobar 内核：[2269670.312637] 已终止进程 1047 (mysqld) total-vm：896532kB，anon-rss：103356kB，file-rss：0kB
正如您所说，以前没有发生过这种情况，但现在开始发生了。因此，我怀疑您的数据库已经增长，一些表也已经增长，现在当一些带有连接和/或子选择的 SELECT 发出时，所有内容都无法放入内存中，进程会被 OOM Killer 杀死。您应该检查您的 mysql 数据库：表大小、索引存在/正确性、查询计划以确定问题的原因。如果您使用的是 innodb 存储，请检查内存和缓冲区相关参数，因为它们通常需要精确调整才能使用最大内存并且不会同时调用 OOM Killer。