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RSS远小于容器内存限制，却收到了内存告警

news 来源：原创 2025/5/14 23:57:26

问题现象

在这里插入图片描述

早上收到容器实例内存超过80%的告警，对应的容器的内存的limits和request都是4GB。进入容器实例排查

top - 10:52:18 up 297 days, 17:34,  0 users,  load average: 19.01, 21.03, 21.43
Tasks:   7 total,   1 running,   6 sleeping,   0 stopped,   0 zombie
%Cpu(s): 17.0 us,  3.7 sy,  0.0 ni, 78.7 id,  0.2 wa,  0.0 hi,  0.5 si,  0.0 st
KiB Mem : 15844642+total, 55103968+free, 38557993+used, 64784454+buff/cache
KiB Swap:        0 total,        0 free,        0 used. 11079055+avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                                                                                      
    1 root      20   0   27.1g   1.2g  16716 S   4.0  0.1 128:36.94 java                                                                                                                                         
40291 root      20   0   15408   2132   1616 S   0.0  0.0   0:00.03 bash                                                                                                                                         
40708 root      20   0   59604   2120   1504 R   0.0  0.0   0:01.68 top                                                                                                                                          
57817 root      20   0   15408   2128   1616 S   0.0  0.0   0:00.04 bash                                                                                                                                         
59030 root      20   0   10048    984    820 S   0.0  0.0   0:00.00 more                                                                                                                                         
59705 root      20   0   15408   2128   1616 S   0.0  0.0   0:00.04 bash                                                                                                                                         
59833 root      20   0   10048    984    820 S   0.0  0.0   0:00.01 more

jvm进程仅仅使用了1.2GB物理内存，但是查看k8s的监控，显示我的容器的内存使用了3.5GB。rss远小于k8s监控的值，为什么会有这么大的差异？

排查过程

找运维了解情况，运维说我们的内存监控取的是/sys/fs/cgroup/memory/memory.usage_in_bytes文件的值。

[root@deploy-eis-ai-office-assistant-module-689565dbdf-gr6lq memory]# pwd
/sys/fs/cgroup/memory
[root@deploy-eis-ai-office-assistant-module-689565dbdf-gr6lq memory]# more memory.usage_in_bytes 
3786895360

的确是3.5个GB，那么这个指标的计算公式是怎样的？查看cgroup的官方定义

5.5 usage_in_bytes

For efficiency, as other kernel components, memory cgroup uses some optimization
to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
method and doesn’t show ‘exact’ value of memory (and swap) usage, it’s a fuzz
value for efficient access. (Of course, when necessary, it’s synchronized.)
If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
value in memory.stat(see 5.2).

memory.usage_in_bytes = rss + pagecache + swap
查看memory.stat文件

[root@deploy-eis-ai-office-assistant-module-689565dbdf-gr6lq memory]# more memory.stat 
cache 2529583104
rss 1271934976
rss_huge 2097152
mapped_file 36864
swap 0
pgpgin 2484647
pgpgout 1557053
pgfault 181876544
pgmajfault 1
inactive_anon 0
active_anon 1271922688
inactive_file 216563712
active_file 2313019392
unevictable 0
hierarchical_memory_limit 4294967296
hierarchical_memsw_limit 9223372036854771712
total_cache 2529583104
total_rss 1271934976
total_rss_huge 2097152
total_mapped_file 36864
total_swap 0
total_pgpgin 0
total_pgpgout 0
total_pgfault 0
total_pgmajfault 0
total_inactive_anon 0
total_active_anon 1271922688
total_inactive_file 216563712
total_active_file 2313019392
total_unevictable 0

rss + cache + swap 的内存正好等于 memory.usage_in_bytes。pagecache的容量在进程内存不够用的时候，会随时被操作系统的内存页面回收机制（page frame reclaim）回收。

那么pagecache具体什么时候回收呢？取决于下面这个参数

[root@deploy-eis-ai-office-assistant-module-689565dbdf-gr6lq vm]# pwd
/proc/sys/vm
[root@deploy-eis-ai-office-assistant-module-689565dbdf-gr6lq vm]# more min_free_kbytes 
159221

内存监控RSS + pageCache的和，并告警毫无意义，和运维沟通要求监控RSS的值。