作者：牧原
实战案例1
记一次内存充足但是java申请不到内存的排查
背景信息
客户8g的实例，java使用4g的内存申请，直接oom
排查如下
oom的记录显示为申请4g内存失败
4294967296 /1024 /1024 = 4096 M
1，第一反应是想起来之前的vm.min_free_kbytes & nr_hugepage导致的free大于available案例有关
centos7 memavailable 小于 memfree
二者的统计方式不一样
MemFree: The sum of LowFree+HighFree
+MemAvailable: An estimate of how much memory is available for starting new
+ applications, without swapping. Calculated from MemFree,
+ SReclaimable, the size of the file LRU lists, and the low
+ watermarks in each zone.
+ The estimate takes into account that the system needs some
+ page cache to function well, and that not all reclaimable
+ slab will be reclaimable, due to items being in use. The
+ impact of those factors will vary from system to system.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773
memfree统计的是所有内存的free内存，而memavailable统计的是可以拿来给程序用的内存，而客户设置了vm.min_free_kbytes（2.5G），这个内存在free统计，但是不在memavailable统计
nr_hugepage也会有这个问题2，跟客户要 free -m && sysctl -p && /proc/meminfo等信息分析问题
HugePages_Total 为0 说明没有设置nr_hugepage
MemAvailable: 7418172 kB 说明这么多内存可用
#sysctl -p
net.ipv4.ip_forward = 0
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 1
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 500000000
kernel.shmmni = 4096
kernel.shmall = 4000000000
kernel.sem = 250 512000 100 2048
net.ipv4.tcp_tw_recycle=1
net.ipv4.tcp_max_syn_backlog=4096
net.core.netdev_max_backlog=10000
vm.overcommit_memory=2
net.ipv4.conf.all.arp_filter = 1
net.ipv4.ip_local_port_range=1025 65535
kernel.msgmni = 2048
net.ipv6.conf.all.disable_ipv6=1
net.ipv4.tcp_max_tw_buckets = 5000
net.ipv4.tcp_max_syn_backlog = 8192
net.ipv4.tcp_keepalive_time = 600
#cat /proc/meminfo
MemTotal: 8009416 kB
MemFree: 7347684 kB
MemAvailable: 7418172 kB
Buffers: 18924 kB
Cached: 262836 kB
SwapCached: 0 kB
Active: 315188 kB
Inactive: 222364 kB
Active(anon): 256120 kB
Inactive(anon): 552 kB
Active(file): 59068 kB
Inactive(file): 221812 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 176 kB
Writeback: 0 kB
AnonPages: 255804 kB
Mapped: 85380 kB
Shmem: 880 kB
Slab: 40660 kB
SReclaimable: 22240 kB
SUnreclaim: 18420 kB
KernelStack: 4464 kB
PageTables: 6512 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 4004708 kB
Committed_AS: 2061568 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 21452 kB
VmallocChunk: 34359707388 kB
HardwareCorrupted: 0 kB
AnonHugePages: 126976 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 114560 kB
DirectMap2M: 4079616 kB
DirectMap1G: 6291456 kB3，实际上面的meminfo已经说明了问题，但是由于经验不足，一时没有看明白怎么回事，尝试自行测试
使用java命令，去申请超出我的测试机物理内存尝试，拿到报错
[root@test ~]# java -Xmx8192M -version
openjdk version "1.8.0_242"
OpenJDK Runtime Environment (build 1.8.0_242-b08)
OpenJDK 64-Bit Server VM (build 25.242-b08, mixed mode)
[root@test ~]# java -Xms8192M -version
OpenJDK 64-Bit Server VM warning: INFO: os::commit_memory(0x00000005c0000000, 5726797824, 0) failed; error='Cannot allocate memory' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 5726797824 bytes for committing reserved memory.
# An error report file with more information is saved as:
# /root/hs_err_pid8769.log
[root@test ~]# java -Xms4096M -version
openjdk version "1.8.0_242"
OpenJDK Runtime Environment (build 1.8.0_242-b08)
OpenJDK 64-Bit Server VM (build 25.242-b08, mixed mode)
[root@test ~]# java -Xms5000M -version
OpenJDK 64-Bit Server VM warning: INFO: os::commit_memory(0x0000000687800000, 3495428096, 0) failed; error='Cannot allocate memory' (errno=12)
......
--------------- S Y S T E M ---------------
OS:CentOS Linux release 7.4.1708 (Core)
uname:Linux 3.10.0-693.2.2.el7.x86_64 #1 SMP Tue Sep 12 22:26:13 UTC 2017 x86_64
libc:glibc 2.17 NPTL 2.17
rlimit: STACK 8192k, CORE 0k, NPROC 15088, NOFILE 65535, AS infinity
load average:0.05 0.05 0.05
/proc/meminfo:
MemTotal: 3881692 kB
MemFree: 2567724 kB
MemAvailable: 2968640 kB
Buffers: 69016 kB
Cached: 536116 kB
SwapCached: 0 kB
Active: 355280 kB
Inactive: 326020 kB
Active(anon): 87864 kB
Inactive(anon): 13296 kB
Active(file): 267416 kB
Inactive(file): 312724 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 72 kB
Writeback: 0 kB
AnonPages: 72200 kB
Mapped: 31232 kB
Shmem: 24996 kB
Slab: 63032 kB
SReclaimable: 51080 kB
SUnreclaim: 11952 kB
KernelStack: 1664 kB
PageTables: 4044 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 1678700 kB
Committed_AS: 2282236 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 14280 kB
VmallocChunk: 34359715580 kB
HardwareCorrupted: 0 kB
AnonHugePages: 30720 kB
HugePages_Total: 256
HugePages_Free: 256
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 57216 kB
DirectMap2M: 3088384 kB
DirectMap1G: 3145728 kB
container (cgroup) information:
container_type: cgroupv1
cpu_cpuset_cpus: 0-1
cpu_memory_nodes: 0
active_processor_count: 2
cpu_quota: -1
cpu_period: 100000
cpu_shares: -1
memory_limit_in_bytes: -1
memory_and_swap_limit_in_bytes: -1
memory_soft_limit_in_bytes: -1
memory_usage_in_bytes: 697741312
memory_max_usage_in_bytes: 0
CPU:total 2 (initial active 2) (1 cores per cpu, 2 threads per core) family 6 model 79 stepping 1, cmov, cx8, fxsr, mmx, sse, sse2, sse3, ssse3, sse4.1, sse4.2, popcnt, avx, avx2
, aes, clmul, erms, rtm, 3dnowpref, lzcnt, ht, tsc, bmi1, bmi2, adx
/proc/cpuinfo:
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 79
model name : Intel(R) Xeon(R) CPU E5-2682 v4 @ 2.50GHz
stepping : 1
microcode : 0x1
cpu MHz : 2500.036
cache size : 40960 KB
physical id : 0
siblings : 2
core id : 0
cpu cores : 1
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl
eagerfpu pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch fsgsbase tsc_adjust
bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt
bogomips : 5000.07
clflush size : 64
cache_alignment : 64
address sizes : 46 bits physical, 48 bits virtual
power management:
processor : 1
vendor_id : GenuineIntel
cpu family : 6
model : 79
model name : Intel(R) Xeon(R) CPU E5-2682 v4 @ 2.50GHz
stepping : 1
microcode : 0x1
cpu MHz : 2500.036
cache size : 40960 KB
physical id : 0
siblings : 2
core id : 0
cpu cores : 1
apicid : 1
initial apicid : 1
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl
eagerfpu pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch fsgsbase tsc_adjust
bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt
bogomips : 5000.07
clflush size : 64
cache_alignment : 64
address sizes : 46 bits physical, 48 bits virtual
power management:
Memory: 4k page, physical 3881692k(2567600k free), swap 0k(0k free)
vm_info: OpenJDK 64-Bit Server VM (25.242-b08) for linux-amd64 JRE (1.8.0_242-b08), built on Jan 28 2020 14:28:22 by "mockbuild" with gcc 4.8.5 20150623 (Red Hat 4.8.5-39)
time: Thu Feb 20 15:13:30 2020
timezone: CST
elapsed time: 0 seconds (0d 0h 0m 0s)4， java测试证明正常申请内存不会有问题，超额的内存才会oom，那么为什么超额呢，视线回归到
sysctl -p有所发现
vm.overcommit_memory=2
overcommit_memory
0 — 默认设置。：当应用进程尝试申请内存时，内核会做一个检测。内核将检查是否有足够的
可用内存供应用进程使用；如果有足够的可用内存，内存申请允许；否则，内存申请失败，并把错误返回给应用进程。
举个例子，比如1G的机器，A进程已经使用了500M，当有另外进程尝试malloc 500M的内存时，内核就会进行check，
发现超出剩余可用内存，就会提示失败。
1 — 对于内存的申请请求，内核不会做任何check，直到物理内存用完，触发OOM杀用户态进程。同样是上面的例子，
1G的机器，A进程500M，B进程尝试malloc 500M，会成功，但是一旦kernel发现内存使用率接近1个G(内核有策略)，
就触发OOM，杀掉一些用户态的进程(有策略的杀)。
2 — 当 请求申请的内存 >= SWAP内存大小 + 物理内存 * N，则拒绝此次内存申请。解释下这个N：N是一个百分比，
根据overcommit_ratio/100来确定，比如overcommit_ratio=50（我的测试机默认50%），那么N就是50%。
vm.overcommit_ratio
只有当vm.overcommit_memory = 2的时候才会生效，内存可申请内存为
SWAP内存大小 + 物理内存 * overcommit_ratio/100
看看上面日志的overcommit信息
CommitLimit: 4004708 kB 小于客户申请的4096M
Committed_AS: 2061568 kB
CommitLimit：最大能分配的内存(测试下来在vm.overcommit_memory=2时候生效)，具体的值是
SWAP内存大小（ecs均未开启） + 物理内存 * overcommit_ratio / 100
Committed_AS：当前已经分配的内存大小5，两相对照，说明客户设置的vm.overcommit_memory在生效，建议改回0再试试
vm.overcommit_memory = 2
[root@test ~]# grep -i commit /proc/meminfo
CommitLimit: 1940844 kB
Committed_AS: 480352 kB
# java -Xms2048M -version 失败了
OpenJDK 64-Bit Server VM warning: INFO: os::commit_memory(0x0000000080000000, 1431830528, 0) failed; error='Cannot allocate memory' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 1431830528 bytes for committing reserved memory.
# An error report file with more information is saved as:
# /root/hs_err_pid1267.log
改回0 恢复
vm.overcommit_memory = 0
vm.overcommit_ratio = 50
[root@test ~]# java -Xms2048M -version
openjdk version "1.8.0_242"
OpenJDK Runtime Environment (build 1.8.0_242-b08)
OpenJDK 64-Bit Server VM (build 25.242-b08, mixed mode)
