記憶體不足，但交換可用

即使有可用的交換，我的伺服器也會耗盡記憶體。

為什麼？

我可以這樣重現它：

eat_20GB_RAM() {
 perl -e '$a="c"x10000000000;print "OK\n";sleep 10000';
}
export -f eat_20GB_RAM
parallel -j0 eat_20GB_RAM ::: {1..25} &

當它穩定下來（即所有程序進入睡眠狀態）時，我再執行一些：

parallel --delay 5 -j0 eat_20GB_RAM ::: {1..25} &

當穩定（即所有程序進入睡眠狀態）時，使用大約 800 GB RAM/swap：

$ free -m
             total        used        free      shared  buff/cache   available
Mem:         515966      440676       74514           1         775       73392
Swap:       1256720      341124      915596

當我再跑幾個時：

parallel --delay 15 -j0 eat_20GB_RAM ::: {1..50} &

我開始得到：

Out of memory!

即使有明顯的交換可用。

$ free
             total        used        free      shared  buff/cache   available
Mem:      528349276   518336524     7675784       14128     2336968     7316984
Swap:    1286882284  1017746244   269136040

為什麼？

$ cat /proc/meminfo 
MemTotal:       528349276 kB
MemFree:         7647352 kB
MemAvailable:    7281164 kB
Buffers:           70616 kB
Cached:          1503044 kB
SwapCached:        10404 kB
Active:         476833404 kB
Inactive:       20837620 kB
Active(anon):   476445828 kB
Inactive(anon): 19673864 kB
Active(file):     387576 kB
Inactive(file):  1163756 kB
Unevictable:       18776 kB
Mlocked:           18776 kB
SwapTotal:      1286882284 kB
SwapFree:       269134804 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:      496106244 kB
Mapped:           190524 kB
Shmem:             14128 kB
KReclaimable:     753204 kB
Slab:           15772584 kB
SReclaimable:     753204 kB
SUnreclaim:     15019380 kB
KernelStack:       46640 kB
PageTables:      3081488 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    1551056920 kB
Committed_AS:   1549560424 kB
VmallocTotal:   34359738367 kB
VmallocUsed:     1682132 kB
VmallocChunk:          0 kB
Percpu:           202752 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
FileHugePages:         0 kB
FilePmdMapped:         0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
Hugetlb:               0 kB
DirectMap4k:    12251620 kB
DirectMap2M:    522496000 kB
DirectMap1G:     3145728 kB

在/proc/meminfo你發現：

CommitLimit:    1551056920 kB
Committed_AS:   1549560424 kB

所以你處於送出限制。

如果您通過以下方式禁用了記憶體過度使用（以避免OOM-killer）：

echo 2 > /proc/sys/vm/overcommit_memory

然後送出限制計算為：

2   -   Don't overcommit. The total address space commit
       for the system is not permitted to exceed swap + a
       configurable amount (default is 50%) of physical RAM.
       Depending on the amount you use, in most situations
       this means a process will not be killed while accessing
       pages but will receive errors on memory allocation as
       appropriate.

（來自：https ://www.kernel.org/doc/Documentation/vm/overcommit-accounting ）

您可以通過以下方式使用全部記憶體：

echo 100 > /proc/sys/vm/overcommit_ratio

然後，當物理 RAM 和交換都被保留時，您將出現記憶體不足。

在這種情況下，這個名字overcommit_ratio有點誤導：你沒有過度使用任何東西。

即使使用此設置，您也可能會在交換耗盡之前看到記憶體不足。malloc.c:

#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <unistd.h>

void main(int argc, char **argv) {
 long bytes, sleep_sec;
 if(argc != 3) {
   printf("Usage: malloc bytes sleep_sec\n");
   exit(1);
 }
 sscanf(argv[1],"%ld",&bytes);
 sscanf(argv[2],"%ld",&sleep_sec);
 printf("Bytes: %ld Sleep: %ld\n",bytes,sleep_sec);
 if(malloc(bytes)) {
   sleep(sleep_sec);
 } else {
   printf("Out of memory\n");
   exit(1);
 }
}

編譯為：

gcc -o malloc malloc.c

執行方式（保留 1 GB 10 秒）：

./malloc 1073741824 10

如果你執行它，即使有免費交換，你也可能會看到 OOM：

# Plenty of ram+swap free before we start
$ free -m
             total        used        free      shared  buff/cache   available
Mem:         515966        2824      512361          16         780      511234
Swap:       1256720           0     1256720

# Reserve 1.8 TB
$ ./malloc 1800000000000 100 &
Bytes: 1800000000000 Sleep: 100

# It looks as if there is plenty of ram+swap free
$ free -m
             total        used        free      shared  buff/cache   available
Mem:         515966        2824      512361          16         780      511234
Swap:       1256720           0     1256720

# But there isn't: It is all reserved (just not used yet)
$ cat /proc/meminfo |grep omm
CommitLimit:    1815231560 kB
Committed_AS:   1761680484 kB

# Thus this fails (as you would expect)
$ ./malloc 180000000000 100
Bytes: 180000000000 Sleep: 100
Out of memory

因此，雖然free在實踐中經常會做正確的事情，但查看 CommitLimit 和 Committed_AS 似乎更防彈。

引用自：https://unix.stackexchange.com/questions/599180