内存不足，但交换区可用

Question

在/proc/meminfo你发现：

CommitLimit:    1551056920 kB
Committed_AS:   1549560424 kB

所以你已经达到了提交限制。

如果您禁用了内存过度使用（避免 OOM 杀手）经过：

echo 2 > /proc/sys/vm/overcommit_memory

然后提交限制计算如下：

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.

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

您可以通过以下方式使用完整内存：

echo 100 > /proc/sys/vm/overcommit_ratio

当物理 RAM 和交换区都被保留时，你就会出现内存不足的情况。

在这种情况下，这个名字overcommit_ratio有点误导：你没有过度承诺任何东西。

即使使用此设置，您也可能会在交换耗尽之前看到内存不足。 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);
  }
}

编译为：

gcc -o malloc malloc.c

运行方式（保留 1 GB 10 秒）：

./malloc 1073741824 10

如果运行此命令，即使有可用交换区，您也可能会看到 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，虽然free在实践中经常会做正确的事情，但查看 CommitLimit 和 Comfilled_AS 似乎更安全。

Answer 1