Linux Swap – How to Test Swap Partition

linuxswap

I'm trying to diagnose some random segfaults on a headless server and one thing that seems curious is that they only seem to happen under memory pressure and my swap size will not go above 0.

How can I force my machine to swap to make sure that it is working properly?

orca ~ # free
             total       used       free     shared    buffers     cached
Mem:       1551140    1472392      78748          0     333920    1046368
-/+ buffers/cache:      92104    1459036
Swap:      1060280          0    1060280

orca ~ # swapon -s
Filename                                Type            Size    Used    Priority
/dev/sdb2                               partition       1060280 0       -1

Best Answer

Is this Linux? If so, you could try the following:

# sysctl vm.swappiness=100

(You might want to use sysctl vm.swappiness first to see the default value, on my system it was 10)

And then either use a program(s) that uses lots of RAM or write a small application that just eats up RAM. The following will do that (source: Experiments and fun with the Linux disk cache):

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <unistd.h>


int main(int argc, char** argv) {
    int max = -1;
    int mb = 0;
    int multiplier = 1; // allocate 1 MB every time unit. Increase this to e.g.100 to allocate 100 MB every time unit.
    char* buffer;

    if(argc > 1)
        max = atoi(argv[1]);

    while((buffer=malloc(multiplier * 1024*1024)) != NULL && mb != max) {
        memset(buffer, 1, multiplier * 1024*1024);
        mb++;
        printf("Allocated %d MB\n", multiplier * mb);
        sleep(1); // time unit: 1 second
    }      
    return 0;
}

Coded the memset line to initialise blocks with 1s rather than 0s, because the Linux virtual memory manager may be smart enough not to actually allocate any RAM otherwise.  I added the sleep(1) in order to give you more time to watch the processes as it gobbles up ram and swap. The OOM killer should kill this once you are out of RAM and SWAP to give to the program. You can compile it with

gcc filename.c -o memeater

where filename.c is the file you save the above program in. Then you can run it with ./memeater.

I wouldn't do this on a production machine.

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